Risks and Opportunities of Pb-free Solder Terminals and ...cache.freescale.com/files/product/doc/ENV_RisksOppPBFreeSolder... · Risks and Opportunities of Pb-free Solder Terminals

Freescale Semiconductor GmbH EMEA Quality 1/17 FWW, September 2004

Risks and Opportunities of Pb-free Solder Terminals and Packaging for Processability at Elevated Temperatures

Dr.Friedrich-Wilhelm Wulfert

Freescale Halbleiter Deutschland GmbH EMEA Quality Technologies & Standards Schatzbogen 7, 81829 München, Germany

[email protected] tel: *49-(0)89-92103-470, fax: *49-(0)89-92103-610

Introduction The European Directives WEEE (1), RoHS (2) and ELV (3) challenge the whole industry into major effort for environmental improvement actions. In meeting the directives the semiconductor manufacturers will contribute their share to a sustainable world by adjusting their materials and processes in their production lines. Obligations are to deliver products compliant with legal regulations and to satisfy customers’ “green” expectations all along the value chain. Development and implementation have to take advantage of state-of-the-art materials and processes while carefully considering the related technical and commercial aspects. As Pb-free soldering processes require higher temperatures than the conventional SnPb soldering, tremendous work is put into components’ readiness for adequate processability during board soldering. This is expressed by an improved Resistance against Soldering Heat (RaSH) and superior Moisture Level Sensitivity (MSL) for real life Package Peak Temperatures (PPT). Environmental goals and robust packages of integrated circuits are achieved by development and introduction of new materials and processes. Environmentally Preferred Products Legislation is banning the use of Pb, Cd, Hg, Cr-VI and polybrominated biphenyle (PBB) as well as diphenylether (PBDE) in production. Thus the semiconductor industry has to eliminate Pb from terminal solder finishes, Br (and pro-actively also Sb) from mold compounds and substrate boards where glycolether has to be removed out of the solder mask layer, too. New materials are being introduced to cope with the elevated process temperatures. Freescale Semiconductor formerly named Motorola, Inc.’s Semiconductor Products Sector (SPS) actively supports the need for environmentally safe products, http://www.freescale.com/greenproducts . Freescale Semiconductor has an Environmentally Preferred Product (EPP) program to help ensure to meet customer and legislative requirements. The Freescale Semiconductor EPP development program includes:

• Review of current product portfolio for compliance to high-temperature soldering processes (245°C–260°C) needed to facilitate the use of Pb (lead)-free solder. • Implementation of robust Pb-free plating finishes across multiple factories and subcontractors • Qualification of Pb-free solder balls for area array packages • Implementation of Pb-free solder balls for flip-chip packages • Evaluation of Br-free encapsulants and organic substrates • Incorporation of Pb-free terminations and high-temperature reflow capability into new product designs


Processability of Integrated Circuits The JEDEC/IPC Joint Standard J-STD-020 (4) is a free standard and –with “Pb-free” appearing on the horizon– it belongs to the most frequently visited and downloaded documents from http://www.jedec.org. This standard takes care of the reliable processing of moisture sensitive surface mount components and has to be followed to maintain package integrity of components during heat exposure of board soldering. The relevant temperature is measured at the top of the parts and is defined as Package Peak Temperature (PPT). This package temperature is often also named Peak Reflow Temperature (PRT) which –because of the ‘reflow’ in the technical term– can be (and has been) misleading to take the temperature in the solderjoint where the material reflow happens. It is important to note that the package temperature is the reference temperature for the parts’ Moisture Sensitivity Level (MSL) and it must not be confused with the solder joint reflow temperature. The MSL reflects the robustness of semiconductor components for board soldering and tells how long the parts are allowed to be exposed to a controlled environment before it is necessary to dry-bake them again before any soldering step. Absorption of water has to be kept at a tolerable level so that no ‚pop-corn’ effects compromise parts’ reliable performance later on. Table 1 is a partial list of J-STD-020B MSL categories showing simulation and processing rules for correct storage and handling prior to soldering. The standard is important for double-sided reflow, i.e. for top- and bottom-side board assemblies where it is mandatory to prevent excess moisture take-up of the plastic components during storage before they will see a second exposure to soldering heat. This advice of best-practice is also applicable for re-work, service and repair soldering.

Table 1: Moisture Sensitivity Levels per J-STD-020C As indicated above, the technical justification for this J-STD-020 standard and its importance for the industry is based upon the inherent behaviour of components where plastics are used for encapsulation, glue, seal or underfill which all absorb more or less water at slower or faster speed. Existing voids and gaps fill with water, in addition the material properties change and the adhesion at interfaces is weakened.

unlimited1 year

4 weeks168 hours72 hours48 hours24 hours

Time on Label(TOL)

NOYESYESYESYESYESYESYES

122a3455a6

F l o o r L i f eMoisture Sensitivity

Level (MSL)

</= 30°C/85%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH

Dry-PackingRequired Time Conditions

unlimited1 year

4 weeks168 hours72 hours48 hours24 hours

Time on Label(TOL)

NOYESYESYESYESYESYESYES

122a3455a6

F l o o r L i f eMoisture Sensitivity

Level (MSL)

</= 30°C/85%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH</= 30°C/60%RH

Dry-PackingRequired Time Conditions


During soldering the different thermal expansion coefficients of the package materials result in thermo-mechanical stress that can exceed the adhesion and cohesion limits. These forces together with the pressure of superboiling water steam can result in delamination and cracks. In the best case scenario the failures occur directly after soldering and –for example– then are found to be due to lifted 1st or 2nd wire bonds or due to sheared bond wires. However and more annoying are latent or intermittent failures as result of the reduced package integrity. Delamination at the top of the chip surface or at the bottom of the leadframe can cause chip overheating as the thermal resistance is increased. Imperfect package integrity thus can lead to ingress of moisture and contamination which together can cause corrosive attack with all of its states from leakage to opens. There is a conflict of interest between good solder joint formation of hot and fast soldering and maintenance of good package integrity by keeping the package temperature low and also by using slow temperature gradients. This basic problem is getting into the foreground again with required minimum solder joint temperatures of 225°C - 235°C for Pb-free SnAgCu solders which have liquidus between 221°C - 227°C. That is higher than the usual solder joint temperatures of nearly eutectic SnPb solders with 205°C - 220°C whith the liquidus between 183°C - 210°C. Now it is paramount to take note of the fact that board assemblies use a mix of package types of different materials and dimensions which results in a spread of thermal mass and heat conduction on the boards. Uneven heat distribution plus oven and process tolerances are reflected by a delta-T on the various boards ranging in size, component size, arrangement and density. Detailed investigations were performed to characterize the thermal conditions at components during reflow soldering where the process window is narrowed for Pb-free (5). Figure 1 shows the relevant temperatures and where to measure them for a reliable board production.

PPT 2PPT 1

SJT 2SJT 1

Package Peak Temperature (PPT)as measured at the top package surface

Solder Joint Temperature (SJT)as measured in the solderjoint

PPT 2PPT 1

SJT 2SJT 1



PPT 2PPT 1

SJT 2SJT 1



Figure 1: Solder Joint Temperature and Package Peak Temperature have to be determined for critical components on the boards. In production the minimum SJT has to be reached and the maximum PPT must not be exceeded for any component on the printed circuit board. A rule-of-thumb says “Small components get hotter than large parts”. However, looking into this with more detail one recognizes that the component temperature is more ruled by its materials’ effective thermal mass of leadframe and plastic volumes and also the thermal conduction of the components to the board is more important than simply the part’s outside dimension. It is obvious that externally similar components can exhibit a totally different thermal behaviour when heatsinks are embedded and/or exposed to the outside when compared to standard surface mount devices.


Table 2 outlines the changed thermal stress scenario on the package bodies during the conventional SnPb process compared to Pb-free soldering. The shown temperature classes base upon J-STD-020C and depend on package volumes and thicknesses giving guidelines for product classification which, however, should be verified in the real board production environment to prevent excess package temperatures can affect parts’ mechanical integrity.

SnPb Eutectic Process - Package Peak Temperature

Pb-free Process - Package Peak Temperature

SnPb Eutectic Process - Package Peak Temperature

Pb-free Process - Package Peak Temperature

Table 2: Package Peak Temperature (PPT) of surface mount devices. MSL/PPT is a product characteristic. Freescale Semiconductor is determining the Moisture Sensitivity Level of the components using the Package Peak Temperature Profile (Figure 2) as measured with a thermo couple at the package top surface. This profile builds on JEDEC/IPC J-STD-020C and reflects several customers’ requirements and their production processes which were evaluated for standardization.

Picture 2: Package Peak Temperature (PPT) profile for determination of components’ Moisture Sensitivity Level (MSL) and the parts’ processability per J-STD-020

including customer board soldering requirements.

50

100

150

200

250

25 50 75 100 125 150

90 to 180 sec.150°C to 200°C

Peak (large/small)245°C/260°C Pb-free225°C/245°C SnPb

0

Time (seconds)

Tem

pera

ture

(°C

)

175 200 225 250

60-90 sec.>217°C Pb-free>183°C SnPb

3°C/sec. Max. Ramp50°C to 120°C

3°C/sec. Max. Ramp183°C or 217°C to peak

Time within 5°C of Peak20-40 sec. Pb-free, small10-30 sec. Pb-free, large

10-30 sec. SnPb

Thermal couple on package body

Target 75 sec

Target 130 sec

Target 30 sec small

20 sec small& Pb

Target 2C/sec.

Target2.2°C/sec. Minimum.

183°C or 217°C to peak

6°C/sec. Max.

50

100

150

200

250

25 50 75 100 125 150

90 to 180 sec.150°C to 200°C

Peak (large/small)245°C/260°C Pb-free225°C/245°C SnPb

0

Time (seconds)

Tem

pera

ture

(°C

)

175 200 225 250

60-90 sec.>217°C Pb-free>183°C SnPb

3°C/sec. Max. Ramp50°C to 120°C

3°C/sec. Max. Ramp183°C or 217°C to peak

Time within 5°C of Peak20-40 sec. Pb-free, small10-30 sec. Pb-free, large

10-30 sec. SnPb

Thermal couple on package body

Target 75 sec

Target 130 sec

Target 30 sec small

20 sec small& Pb

Target 2C/sec.

Target2.2°C/sec. Minimum.

183°C or 217°C to peak

6°C/sec. Max.


Development towards Pb-free There are 3 activities in the development process to reduce Pb from electronic components or systems:

- Activity 1 is the evaluation of moisture performance in applications of Pb-free solders requiring high temperature soldering for customer board assembly. Evaluations are performed on existing Pb-bearing components to validate moisture sensitivity levels at the higher reflow temperatures based on customer request.

- Activity 2 is the evaluation of Pb-free terminals and terminal plating finishes for components. Evaluations are performed on Pb-free components to validate reliability of Pb-free plating finishes for Peripheral Packages and Pb-free spheres for Area Array Packages. Pb-free packages must also be compliant to the new high temperature reflow moisture sensitivity levels (MSL) conditions.

- Activity 3 is the development of solutions to Pb reduction to the flip chip packages and to evaluate high temperature die attach soldering solutions for power packages that require high thermal conductivity heat sinks integral to the package.

Pb-free component terminals and MSL/PPT Future plating finishes on the external pins of the leadframe or the solder balls of integrated circuits will be Pb-free. The lead finish is dependent on product, assembly site and package type. In order to be classified as “Pb-free”, the component will have a Pb-free finish and will be high temperature reflow capable with a minimum processability of MSL3/245°C for “large” components respectively MSL3/260°C for “small” units. For component size classifications refer to J-STD-020 (latest version). Freescale Semiconductor developed matte tin (Sn) and tin bismuth (SnBi) as Sn-based and Pb-free plating finishes besides the already long existing and marketed option of nickel palladium (NiPd) or nickel palladium gold (NiPdAu) for its leadframe product. The solderability of Pb-free finishes is inferior to conventional SnPb, however, it satisfies standard requirements of wettability and also of nowadays board production (8). Freescale will focus on matte Sn for electroplating of Cu leadframe devices. Still required Alloy42 leadframe parts have to be converted to Cu leadframes (6) and customers are asked to express their future needs of such Alloy42 based legacy parts. In course of the necessary changes new mold compounds will be evaluated and introduced for appropriate MSL/PPT performance of the product. The goal of MSL1/260°C for parts’ processability will not always be possible for older generation components. Area array packages will have Sn4.0Ag0.5Cu solder spheres (7) and will be manufactured with a processability goal of at least MSL3/245°C. They usually belong to the large component category in J-STD-020. The directive allows high-Pb solders greater than 85% and is based upon technical justification. This applies to high-Pb content solders for which there is no technological or cost effective solution. Flip-chip BGA will be available with Pb-free solder balls for external soldering, however, it will continue to have its high content Pb internal bumps. Heat Sink Small Outline Packages (HSOP) or other thermally enhanced components with exposed heat sinks (TEQFP, TEQFN) for better power dissipation will continue or even change to high-Pb solder die attach now, in order to establish a hierarchy from inside to outside solder melting points of internally high and externally lower liquidus temperatures or ranges. Modern mold compounds do not use Br- or Sb-formulars anymore for flame retardents since there are new halogen-free solutions. Other new additives are of aromatic nature and help to become compliant with environmental and safety regulations. The new materials have to be


evaluated for effectiveness in accident and also their chemical stability during normal operation. No short-term or long-term reactions between package and chip materials are allowed in production, assembly and use of the components in their field application. For improved package integrity there are additional cleaning steps in evaluation where plasma or UV-ozone are applied prior to wire bonding and/or molding. The intent is to create fresh and clean surfaces, to reduce the contamination level of foreign or residual elements at the interfaces where different materials are brought together and expected to adhere and connect for a reliable life. Thus the extrinsic thermo-mechanical stress during components’ use is addressed by an improved intrinsic robustness of the interfaces. Board Soldering with Pb-free Components During the change-over from conventional SnPb to the coming Pb-free board soldering not all components will be available with the required solder finish. There will be conventional SnPb parts on Pb-free boards and already converted Pb-free components will land on printed circuit boards which are still run through traditional SnPb soldering processes. Attention is required when soldering SnPb components under Pb-free conditions that the parts’ MSL/PPT is adequate. Dry-baking prior to soldering might be necessary. SnPb solders melt without problems in both air and nitrogen atmospheres of Pb-free solder ovens and at normal Pb-free temperatures. Pb-free leadframe parts can be put on boards with SnPb solders and no changes have to be done to the SnPb process. The SnPb solder finish or paste on the board and/or from the wave rule the solder system and are not influenced by the minute amount of Pb-free solder on the leads. Good solder joints form, the component reliability is unaffected when its MSL/PPT was followed during the board soldering and related handling. The situation is different when soldering Pb-free solder balls of BGAs on to boards with SnPb solder paste. Care to completely melt and mix of both solders has to be taken where now the large volume of the solder balls determines the necessary temperatures and soldering kinetics. The process is well set when the “dual collapse” of the BGA towards the board can be observed, the BGA has to sink into the paste and further moves towards the board when the solder balls melt. Then enough time has to be given to the molten solders to form a homogeneous connection. Experience shows that solder joint temperatures of >225°C yield good and reliable solder joints between SnPb pastes and SnAgCu balls. That is higher than at the upper end of the normally established 205°C - 220°C in the joints of SnPb soldering. The conditional up- and downwards compatibility and related areas for required special attention are sketched in Figure 3.

Figure 3: Conditional up- and downward compatiblity of Pb-free in SnPb and vice versa.

SnPb TerminalComponent

Pb-free TerminalComponent

SnPb Solder BoardAttach Process

SnAgCu Pb-free SolderBoard Attach ProcessPb-free

Conventional

OK, components require robustness for higherpackage temperature compliance (MSL/PPT)

OK, must set up profile to be >225°Cfor array package solderjoint temperature

Inhibit Bi and Pb „contamination“in HighTemperature applications!

SnPb TerminalComponent

Pb-free TerminalComponent

SnPb Solder BoardAttach Process

SnAgCu Pb-free SolderBoard Attach ProcessPb-free

Conventional

OK, components require robustness for higherpackage temperature compliance (MSL/PPT)

OK, must set up profile to be >225°Cfor array package solderjoint temperature

Inhibit Bi and Pb „contamination“in HighTemperature applications!


Soldering of the Pb-free terminal components will typically require extensive changes to the board assembly reflow profile. SnAgCu based solders have a melt temperature that is approximately 40°C higher than eutectic SnPb based solders. It is preferable that Pb-free parts be soldered with solder pastes employing fluxes formulated for the associated higher process temperatures. Open solder joints and incomplete formation of the BGA-to-board solder interconnects can be a result of too low solder joint temperatures (Figures 4a-b). The thermal flow from the heated package through the solder ball towards the solder paste reservoir and vice versa must not be hindered by local separation or flux interlayer build-up. In this context it is advised to take care in the right choice of temperature stable fluxes that fit the thermal profile of the board soldering process. Possible incompatibilities of flux materials have to be excluded. This is an area where solder material suppliers need to share their experience with the user community.

Figure 4a: Open solder joints and incomplete formation of the interconnect at 203°C.

Figure 4b: Complete wetting and good formation and mix of the SnPb paste and SnAgCu solder from board and BGA.at solder joint temperatures of 225°C. Shown solder joint cross-sections are made after extended temperature cycling exercises far beyond the acceptance criteria. Visible crack formation is not of interest for this section where the focus is on the solder joint formation in the first place.


When solder joints are well formed, then there is equivalence between SnPbAg-to-SnPb and SnCuAg-to-SnPb and SnCuAg-to-SnAgCu (see later section on solderjoint reliability) with a tendency for superior performance of Pb-free joints. Careful inspection of solder joint cross-sections from interconnects that formed at different solder joint temperatures disclose the progress of homogenisation with increasing temperatures, Figure 5a-c.

203°C 210°C 225°C Figure 5a-c:: Increase of solder joint temperature leads to better mix and distribution of Pb from the SnPb paste into the originally Pb-free SnAgCu solder ball and the final homogenous solder interconnect. Package Warpage Large area packages are exposed to a thermo-mechanical phenomenon of package deformation known as “warpage”. It is a result of the more or less unbalanced construction and arrangement of material layers with not matching coefficients of thermal expansion (CTE). Directly after production the parts are not necessarily perfectly flat as built-in package stress finds relief in the described deformation. Thus the coplanarity of external leads can suffer from deformed packages. PBGAs substrates can lift the attached solder balls away from the solder plane on the board. This makes solder joint formation difficult or even prevents it totally, (Figure 6a-d).

6a

6b 6c 6d

6a

6b 6c 6d Figure 6a-d: Package warpage causes insufficient solder joint formation. Heat transfer from the package and board into the solderjoint is hindered. Package warpage is dynamic and changes with temperature, (Figure 7).


Figure 7: During board soldering the deformation of the package and substrate („warpage“)

changes with the temperature and reaches from a concave to a convex arrangement of the solder balls or vice versa.

It has to be pointed out that the printed circuit board also changes its curvature during soldering as same physics work there, too. Stabilization measures are recommended to support a continuous contact for ongoing heat transfer between the package solder balls and the printed paste on the boards. The effort will pay in good solder joint formation and better board yields. Reliability of typical solder joints Temperature cycling addresses performance and robustness of board-level solder joints. Various combinations of packages with different solder finishes are reflow soldered using conventional SnPb or the new Pb-free processing. During the thermal cycling stress the ohmic resistance of daisy-chained solder interconnects is watched. Example Weibull graphs show gathered results for the 64LQFP and the 132PQFP (Figure 8a-b). The conclusion is that the lifetime of matte Sn solder finish leadframe devices is at least as good as those with conventional SnPb plated leads no matter if soldered with SnPb or SnAgCu solder pastes on the printed circuit boards.

Package WarpageCriteria: 200µm or 8mils

Goal: 150µm or 6mils

0

65

130

195

T i m e

T e

m p

e r

a t

u r

e [

°C] Solderjoint Temperature

213°C - 235°C

Measured MoiréPeak Reflow Temperature

up to 255CView is dead bug with solder balls removed. Temperature Profile shown is a reference

curve for illustration purposes only.

Sn36Pb2Ag Solder Paste is liquid at 179°C.Sn4.0Ag0.5Cu Solder paste is liquid at approx. 225C

Concave on customer board


Convex on customer board


260 Example shown:388 PBGA Nitto HC100X2EMC Package Flatness Baseline Performance to Pass Solderability

dz = -130um (5.12 mils)

dz = 80um (3.14 mils)

dz = 100um (3.93 mils)dz = 100um (3.93 mils)


Package WarpageCriteria: 200µm or 8mils

Goal: 150µm or 6mils

0

65

130

195

T i m e

T e

m p

e r

a t

u r

e [

°C] Solderjoint Temperature

213°C - 235°C

Measured MoiréPeak Reflow Temperature

up to 255CView is dead bug with solder balls removed. Temperature Profile shown is a reference

curve for illustration purposes only.

Sn36Pb2Ag Solder Paste is liquid at 179°C.Sn4.0Ag0.5Cu Solder paste is liquid at approx. 225C



Convex on customer board


260 Example shown:388 PBGA Nitto HC100X2EMC Package Flatness Baseline Performance to Pass Solderability

Example shown:388 PBGA Nitto HC100X2EMC Package Flatness Baseline Performance to Pass Solderability

dz = -130um (5.12 mils)

dz = 80um (3.14 mils)

dz = 100um (3.93 mils)dz = 100um (3.93 mils)



Figure 8a: Board-Level Solder Joint Reliability of 64LQFP

Figure 8b: Board-Level Solder Joint Reliability of 132PQFP

Number of Temperature Cycles -40°C/125 °C

%

Packages

Failed


%

Packages

Failed

Peripheral Device: Board Level Solder Joint Reliability, 64LPQFP

Sn – SnAgCu 4has no fails

as of 13407 cycles,dashed line represents worst case estimation.

Finish - Solder

2) Backwardcompatible220°C reflow

3) Forwardcompatible240°C reflow

4) Pb-free, 240°C reflow

1) Current220°C reflow

Sn - SnAgCu 4SnPb - SnAgCu 3Sn - SnPb 2SnPb - SnPb 1

Both combinations,Sn finish in SnPb solder

andSn finish in SnAgCu solderperform equal to or better

than SnPb finish in SnPb solder.

Board assembly with SnPbsolder paste was performed at

the standard SnPb profile.


%

Packages

Failed


%

Packages

Failed

Peripheral Device: Board Level Solder Joint Reliability, 64LPQFP


as of 13407 cycles,dashed line represents worst case estimation.

Finish - Solder





Sn - SnAgCu 4SnPb - SnAgCu 3Sn - SnPb 2SnPb - SnPb 1

Finish - Solder





Sn - SnAgCu 4SnPb - SnAgCu 3Sn - SnPb 2Sn - SnPb 2SnPb - SnPb 1SnPb - SnPb 1


andSn finish in SnAgCu solderperform equal to or better

than SnPb finish in SnPb solder.



Peripheral Device: Board Level Solder Joint Reliability, 132PQFP


andSn finish in SnAgCu solder

perform equal to or better than

SnPb finish in SnPb solder.

Board assembly with SnPbsolder paste was performedat the standard SnPb profile.

Finish - Solder


3) Forwardcompatiblehigh-temp reflow

4) Pb-free, high-temp reflow


Sn - SnAgCu 4

SnPb - SnAgCu 3

Sn - SnPb 1

SnPb - SnPb 2


as of 6961 cycles,dashed line representsworst case estimation.

Peripheral Device: Board Level Solder Joint Reliability, 132PQFP


andSn finish in SnAgCu solder

perform equal to or better than

SnPb finish in SnPb solder.

Board assembly with SnPbsolder paste was performedat the standard SnPb profile.

Finish - Solder





Sn - SnAgCu 4

SnPb - SnAgCu 3

Sn - SnPb 1

SnPb - SnPb 2

Finish - Solder





Sn - SnAgCu 4

SnPb - SnAgCu 3SnPb - SnAgCu 3

Sn - SnPb 1Sn - SnPb 1

SnPb - SnPb 2SnPb - SnPb 2


as of 6961 cycles,dashed line representsworst case estimation.


Same equivalence as identified for peripheral packages and all solder material and process combinations is found also for the soldering of area array packages. Acceptance citeria of >2000 temperature cycles are fulfilled for such ball grid arrays (Figure 9a-b) as well.

Figure 9a: Board-Level Solderjoint Reliability of PBGA388

Figure 9b: Board-Level Solderjoint Reliability of TBGA480

Area Array Device: Board Level Solder Joint Reliability, PBGA388

Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4

Both combinations,SnAgCu ball in SnPb solder

andSnAgCu ball in SnAgCu solder

perform equal to or betterthan SnPbAg finish in SnPb solder.



Area Array Device: Board Level Solder Joint Reliability, PBGA388

Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4

Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4






Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4

Area Array Device: Board Level solder Joint Reliability, TBGA480






Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4

Ball - Solder





SnPbAg - SnPb 1

SnAgCu - SnPb 2

SnPbAg - SnAgCu 3

SnAgCu - SnAgCu 4

Area Array Device: Board Level solder Joint Reliability, TBGA480







Whisker Growth Spontaneous Sn whisker growth (11 - 13) from plated Sn based solder finishes (Figures 10a-b) is considered a risk by the aircraft and space industry (14, 15) but needs carefull review and investigations also for other critical applications. There is a strong indication that mechanical stress is the main reason for the extrusion of these crystalline needles or other formations on Sn surfaces. Copper/tin intermetallics along Sn grain boundaries and originating from the diffused Cu of the leadframe base material are suspect to be responsible for the stress that drives whiskers (9, 13) from surface grains. The situation is different for Alloy42 leadframe material where the thermal mismatch between the steel and the Sn solder plating is identified to promote whisker growth (16, 17) from stressed grains in the solder finish.

Figure 10a: Figure 10b: Whisker on a Sn surface. Whisker grown from SnPb surface. The mechanisms for Sn whisker growth are not fully understood, yet. Figure 11 shows a fishbone diagram detailing key influencing factors for the formation of Sn whiskers. A combination of environment, materials, methods and equipment interact. Our investigations found whiskers on all Sn-based solder finishes and sooner or later all surfaces of Sn, SnBi, SnCu and also SnPb plated finishes exhibited more or less whiskers (10).

Figure 11: Influencing factors for whisker growth

red = promotes whisker growthgreen = prevents whisker growth

1 - 8 µm

> 10 µm

Organic Brightening

(>2%) AlloyingPlating thickness

Plating grain size

Ni Underplating

Tin WhiskerPrevention


Environment

EquipmentMethods

Materials

Base metal

Plating Process

Temperature

High current density plating baths

HumidityIntermetallic formation (Cu6Sn5)

Surface oxidationExternal mechanical stress

Physical imperfections caused in assemblyAnnealing

> 150ºC50º - 70ºC

< 1 µm< 0.5 µm

CuBrass, Zn

Hot DipElectroplating

red = promotes whisker growthgreen = prevents whisker growth

1 - 8 µm

> 10 µm

Organic Brightening


Plating grain size

Ni Underplating



Environment

EquipmentMethods

Materials

Base metal

Plating Process

Temperature





> 150ºC50º - 70ºC

< 1 µm< 0.5 µm

CuBrass, Zn


1 - 8 µm

> 10 µm

Organic Brightening


Plating grain size

Ni Underplating



Environment

EquipmentMethods

Materials

Base metal

Plating Process

Temperature





> 150ºC50º - 70ºC

< 1 µm< 0.5 µm

CuBrass, Zn



Storage in 60°C/95%RH created whiskers on SnCu after only 3 weeks, it took about 7 weeks for SnPb and 13 weeks for Sn and SnBi. The shortest whiskers were found on SnPb, followed by Sn, SnBi. SnCu grew the longest whiskers. SnPb shows the least amount of whiskers, there are more on SnBi, SnCu and most whiskers are on plated Sn. Figure 12 summarizes the results of a comprehensive 60°C/95RH whisker study using QFP components.

Figure 12: Incubation time, maximum length and number of whiskers found on a variety of plated solder finishes during a 60°C/95RH storage. NEMI (National Electronics Manufacturing Institute Inc, USA), SOLDERTEC (Tin Technology Ltd., Europa) und JEITA (Japan Electronics and Information Technology Association, Japan) formed a team to run a program (18) to study the nature of Sn whiskers, their growth mechanisms and to develop and propose possible prevention and test measures. Ni or Ag barrier layers seem to be effective diffusion barriers and buffer between the plated Sn and the leadframe base materials. Infineon, STMicoelectronics, Philips and Freescale Semiconductor form E4 (Environmental 4) and share their expertise and results for joint progress in the field of EPP (19). An evenly grown SnCu intermetallic during a one hour anneal @ 150°C after plating acts as diffusion barrier and mitigates the Sn whisker growth especially during storage of Cu leadframe components. Reflow soldered parts exhibit a very low level of whiskering. NEMI proposed whisker tests of -55°C/85°C air-to-air temperature cycling or extended storage at ambient or in 60°C/93%RH did not show whiskers lengths above 25µm (20). Among others the above methods and tests strategies are being further investigated. Manufacturers are invited to join the mentioned working groups to shed more light into the possible impact of whisker growth for the electronic industry and in support of successful use of Matte Sn as a viable and reliable Pb-free plated solder finish.


Pb-free or Green Product Availability The customer demand for volume shipment of Pb-free components or Green Product is not yet widespread. While Pb-free parts don’t have Pb in the solder terminations of leads or solder balls and are processable at Pb-free temperatures, Green Products’ additional feature is the use of halogen- and Sb-free moldcompounds. Besides SnPb-solderable NiPdAu parts, Freescale Semiconductor repectively MOTOROLA SPS have been shipping Pb-free products since 2001, based on specific customer requests. In a proactive manner, we will consider any new product a Pb-free candidate after Q1, 2003. Our policy is to evaluate each Pb-free opportunity by component type. For manufacturing readiness by packaging type, see Table 3. For sample or product availability please contact a Freescale sales office or the Freescale Technical Information Center via http://www.freescale.com . Daisy-chain PBGAs are well suited to evaluate the process capability of board assembly lines also with respect to minimum solder joint temperature and maximum package peak temperature of the components on the boards.

Table 3: The demand triggers the production of “Pb-free” components.


Pb-free Conversion Plan At this time, conversions are taking place only on a customer request basis. Conversion of existing SnPb finish products to Pb free products will be handled by our standard Product Change Notification system. All customers are requested to report and list their needs for Pb-free components in order to allow for proper production planning. Execution of the Environmental Packaging Technology Roadmap (Figure 13) pends market needs. The graphic below contains information about features and timing that are projections and subject to change per market and business development.

Figure 13: Freescale Semiconductor Environmental Packaging Technology Roadmap After completion of Pb-free termination development and selection for production start the key effort has now turned towards finalizing the evaluation and the best choice of future mold compounds in order to fulfill the obvious need for improved Resistance against Soldering Heat (RaSH) and upcoming requirements of halogen- and Sb-free packages.


Summary The conversion to Pb-free demands special effort on the manufacturer and user side of integrated circuits. Technical solutions are made available where matte Sn was chosen as the mainstream Pb-free solder finish for leadframe devices besides the already introduced NiPd(Au). Ball Grid Array packages receive SnAgCu solder balls. Sufficient to good solderability is given for all Pb-free terminations in conventional SnPb and SnAgCu board solders. MSL/PPT processability rules have to be followed per J-STD-020B. Introduction of new moldcompounds is often necessary to comply with requirements for halogen- and Sb-free materials and to achieve the necessary package robustness for the elevated process temperatures. Biphenyle epoxy resins with metal-hydrates and -oxides or multi-aromates as flame retardents have a good chance to replace present plastic materials. Board-Level reliability with Pb-free terminations proves fully acceptable or even better when compared to traditional solder joints. Application specific investigations are encouraged to collect further supporting data. Finally it has to be noted, however, that all along with the proactive and technically oriented efforts by the manufacturers there is still reluctance for Pb-free conversion on the business side. The customers have to let us know what they need, when which product is required “Pb-free” and in what quantities. During the actual economic situation, some still not fully elaborated legislative wording and also no finally defined material weight% limits, it is not justifiable to proceed with the changes and to make further investments when the political and economic situation don’t trigger clear market signs. Customers might prefer or need a different solution than so far prepared for. All along the value chain, customers and suppliers have to keep each other informed about their Pb-free roadmaps and expectations from early on in the development phase of new products, materials and processes. Joint effort and shared learnings will result in successful product and business for all parties envolved together. Literature 1) Directive 2002/96/EC of the European Parliament and of the Council of 27 January 2003 on waste electrical and electronic equipment (WEEE) 2) Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS) 3) Commission Decision of 27 June 2002 amending Annex II of Directive 2000/53/EC of the European Parliament and of the Council on end-of-life vehicles (ELV) 4) IPC/JEDEC J-STD-020B: “Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices”, July 2002 and later versions 5) Klein, Ch. et al (2003): “Qualification Temperature Profile of Electronic Devices for Lead-free Reflow Soldering”, ZVEI Workshop “Lead-free Production in Automotive Business”, Frankfurt, Oct.23, 2003.


6) Wulfert, F.W. et al (2002): “Assessment of Pb-free Finishes for Leadframe Packaging”, Electronics Circuits World Convention, ECWC 9, Cologne Trade Fair, Germany, October 2002 7) Wulfert, F.W. et al (2002): “Development of Pb (lead) and Halogen Free Plastic Ball Grid Array, PBGA, Components”, Electronics Circuits World Convention, ECWC 9, Cologne Trade Fair, Germany, October 2002 8) IEC 60068-2-58 Environmental testing – Part 2-58: “Test methods for solderability, resistance for dissolution of metallization and to soldering heat of surface mounting devices (SMD)”, 2nd. Edition 1999-2001 9) Dittes, M. (2002): “Lead-free Post-Mold Plating for Semiconductor Devices”, Electronics Circuits World Convention, ECWC 9, Cologne Trade Fair, Germany, October 2002 10) Vo, N. et al (2001): “Pb-free Plating for Peripheral Leadframe Packages”, Second International Symposium on Environmentally Conscious Design and Inverse Manufacturing Proceedings, Tokyo, Japan, Dec. 11-15, 2001 11) Lee, B. et al (1998): “Spontaneous Growth Mechanism of Tin Whiskers”, Acta Materiologica, Vol. 46, No. 10, 1998, pp. 3701-3714 12) http://www.nemi.org/projects/ese/tin_whisker.html 13) Zhang, Y. et al : "Understanding Whisker Phenomenon — Driving Force for the Whisker Formation", APEX 2002, San Diego, CA. 14) http://nepp.nasa.gov./whisker 15) http://www.space.com/businesstechnology/mexican_satellite_000830.html 16) Dittes, M. et al (2003): “Tin Whisker Formation – Results, Test Methods and Countermeasures”, ECTC 2003, New Orleans, May 29, 2003 17) Dittes, M. et al (2003): “The Effect of Temperature Cycling on Tin Whisker Formation”, IPC/JEDEC 4th International Conference on Leadfree Electronic Assemblies and Components Proceedings, Frankfurt/Germany, October 21-22, 2003 18) "NEMI Sn Whisker Project," Tin Whisker Joint Meeting: NEMI, JEITA & ITRI, May 2003, Tokyo, Japan, ftp://nemi.org/webdownload/newsroom/Presentations/JEITA_paper.pdf or ftp://nemi.org/webdownload/newsroom/Presentations/JEITA_presentation.pdf or Galyon, G.T.: “Annotated Tin Whisker Bibliography and Anthology, NEMI, March 2004, ftp://nemi.org/webdownload/newsroom/TW_biblio-July03.pdf 19) http://biz.yahoo.com/prnews/040603/nyth132_1.html 20) Su, P. (2004): “Effects of 260°C Reflow on Sn Whisker Growth”, NEMI Tin Whisker Workshop @ ECTC 2004, Las Vegas, June 2004

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