Vapor Phase Versus Convection Reflow

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Volume 9 Number 4 April 2009

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vapor pHase vs. convection reflow in roHs-compliant assembly

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20 – Global SMT & Packaging – March 2009 www.globalsmt.net

Vapor phase vs. convection reflow in RoHS-compliant assembly

introductionVapor phase (VP) reflow technology has been in existence since the early 1970s as a reflow process for surface mount technol-ogy (SMT) assemblies. While used for some defense applications and in smaller volume production settings, the disadvantages associated with the initial processing tech-nology limited its widespread acceptance. These disadvantages included environmen-tal concerns about the fluids being used, throughput limitations, applicability only to single-sided printed circuit board as-semblies (PCBAs) and an inherent problem with tombstoning.

Advances in VP technology have addressed many of these shortcomings through continued development of im-proved machines, chemical selections and process controls. Doublesided PCBAs are easily processed in current equipment. As a result, VP is becoming a viable alternative

to consider in volume manufacturing.Today’s VP reflow process makes use of

the heat produced by a boiling fluorinated polymer or fluid. This boiling fluid pro-duces a uniform temperature zone (vapor blanket) in which the PCBA is exposed for solder purposes. Heat is transferred to the PCBA as it is immersed into the vapor area until the PCBA reaches temperature equi-librium with the boiling point of the fluid. The primary soldering benefits of VP in comparison to infrared (IR) or convection include an oxygen free (inert) environment without the need for nitrogen, fixed upper temperature exposure and superior heat transfer on thermally challenged PCBAs.

VP also offers distinct advantages in the realm of lead-free soldering. Key benefits include a lower peak reflow temperature, an inert environment without nitrogen, improved solder wetting and flow and a reduction in profiling time

Keywords: Vapor Phase, Convection Reflow, IR Reflow, Lead-Free

The contract manufacturing in-dustry is changing rapidly from lead-based soldering to lead-free soldering. There is no stopping the transition or the reality that lead-free components are going to be introduced in lead-based processes. This challenge to engineering and quality is a huge concern and one that needs scrutiny and a watchful eye. EMS providers rely on compo-nent suppliers to ensure that the lead-free transition on the com-ponent terminations is seamless to their soldering processes, but that rarely happens. Termina-tion changes require additional modification to solder profiles and flux chemistries in order to ensure proper wetting of the sol-der to the lead-free termination. The need for nitrogen to be used in convection reflow is becoming a requirement more than an op-tion, and nitrogen is costly.

This paper will look at the advantages and disadvantages of vapor phase (VP) and convec-tion reflow in RoHS-compliant processing and discuss associ-ated design for manufacturing (DFM) issues.

by Dan Coada, EPIC Technologies, Norwalk, OH USA


This paper was originally published in the proceedings of the SMTA International Conference, Orlando, Florida, August 2008. Figure 1. Vapor phase soldered, lead free, ENIG surface fnish (U26 partially removed in shear/tensile test).

Global SMT & Packaging – March 2009 – 21www.globalsmt.net


lower peak reflow processingVP reflow requires a lower processing temperature. In convection or IR process, temperaturescanreach245˚Cto265˚Cat the component level. VP temperatures stay at the boiling point of the fluid, typicallyestablishedbetween230˚Cand240˚C.Thelowertemperaturesmakeitpossible to reduce the cost of the PCB by using lower Tg

/Td material selections for

SMT assemblies. Savings of 10-15% and even more could be seen on laminate costs alone. VP also offers processing advantages with large mass components, such as con-nectors, because the thermal equilibrium is better. In convection reflow, particularly in higher temperature lead-free processes, correctly soldering large mass connectors may overheat the rest of the PCBA.

inert environmentThe inert environment and consistency of heat transfer allow VP to be more forgiving with lead-free component terminations. Less active no clean flux chemistries have proven to be adequate in soldering lead-free terminations that demand high activ-ity fluxes in the convection reflow process.

There is also cost savings due to lower energy consumption. In addition to the elimination of nitrogen, electricity usage with VP is much lower.

improved solder wetting and flowVisual inspection of micro sections indicates that VP creates good solder joint performance. When larger thermal load components or clusters of components are present, time above liquidus (TAL) should be increased beyond the 60-90 seconds rec-ommended by solder paste manufacturers to accommodate thorough heat transfer. Comparatively, in convection processing it can be more difficult to ensure good joints on components with high thermal mass be-cause achieving TAL in larger components may result in smaller components overheat-ing. There is no chance of overheating smaller or isolated components with VP because VP cannot heat a component higher than vapor temperature.

reduction in profiling timeAnother advantage of VP’s heat transfer characteristics and the uniformity at which it accomplishes heat transfer is that it makes it easier to understand the profiling relationship between PCBAs. In traditional profile development for a new PCBA, sample assemblies with thermocouples are run through the reflow process numerous times in order to get the right profile, with manual inspection of the solder joints and flux residue used to determine if the target profile is correct. Engineers develop matrix charts on board size, layer count

and complexity to get the profile close so that process development time is kept to a minimum.

Conversely, VP reflow profiling can be classified by process type to the point where there are fewer profiles to develop. For instance, standard multi-layer 0.062” thick PCBAs can follow the same lead-based VP profile regardless of component complexity. With advances in VP process systems, monitoring of heat load during the soldering process allows the systems to profile almost automatically. Ramp rates and soak times at peak temperature can be defined by the engineer and controlled by the systems regardless of the product mix during the process. In a true one-piece flow on a prototype, it is much easier to get it right the first time using VP processing. The days of inadequate reflow tempera-ture or over temperature on the first piece are virtually eliminated by use of the VP systems.

comparative data on lead-free and tin-lead solder joint creationStandard test boards (Figure 1) populated with common components, including BGAs and QFPs, were built with vapor phase and convection soldering technology and tested at an EPIC facility. Reliability testing demonstrated that lead-free and tin-lead joints produced by vapor phase to be equally robust as those from convection reflow. The controllable, lower peak solder temperature makes vapor phase ideal for soldering complex assemblies having sensi-tive lead-free SMT components.

Figure 2. Convection lead-free profile.

Figure 3. Vapor phase lead-free profile.

Figure 4. Vapor phase tin-lead profile.



Test DescriptionTests were conducted using a VP reflow process. Vehicle boards were used with tin-lead HASL or lead free immersion silver, immersion tin or ENIG surface finishes, as appropriate. Boards were populated with tin-lead or lead-free components, printed, assembled and soldered using standard reflow or VP production equipment. The solder pastes selected for testing included tin-lead and lead-free no-clean and water soluble formulations. Assembled test boards were thermal shocked between -45˚Cand+125˚Cwith20minutesdura-tion at each limit for 500, 1000 and 2000 cycles in EPIC’s Failure Analysis Labora-tory. Other test boards were subjected to acceleratedagingat85˚Cand85%relativehumidity for 1,000 hrs. The JOCY test vehicle is populated on only one side, al-though it is equipped with plated through-holes (PTH) for mixed technology tests.

Test boards were populated with dum-my 402, 603, 805, 68 pin PLCC, TSOP32, SOIC TQFP QFP208 and daisy-chained BGA169 and BGA352 components.

Standard lead-free convection reflow

profiles (Figure 2) provided a peak tem-peratureof245˚CandaTALinthe60-90second range recommended by paste sup-pliers. Vapor phase soldered boards were soldered in an EPM-IBL SLC500 vapor phase soldering chamber using Galden LS/230 Perfluorinated heat transfer fluid. The vapor phase profiles developed provided a TAL of about 90 seconds and amaximumtemperatureof230˚C,atemperature that is governed by the vapor temperature. After a vapor phase profile is established, TAL can be modified to achieve any time required without exceed-ingthe230˚Cmaximumtemperature.

The vapor phase equipment first preheats the board using infrared. Next, the work is lowered into the vapors at a programmedratetoregulateΔTandTAL.After the work reaches the maximum va-por temperature, the duration of its expo-sure is preprogrammed. Several soldering programs can be developed by the engineer and stored in memory to suit the needs of different lead-free or tin-lead board types. ΔTandTALarecontrolledbytheprogramdeveloped by the engineer.

Visual inspection for solder balls, tombstones, bridging, voids and dewetting indicated no apparent difference between the two methods of solder joint creation. No tombstones were experienced on the JOCY test vehicle boards in either case.

Visual inspection indicates that while vapor phase created solder joint perfor-mance and micro-section appearance on the board is very good, it might be a good idea to explore increasing the lead-free TAL above the 60 to 90 seconds recom-mended by solder paste manufacturers to accommodate thorough heat transfer to larger components or clusters of large components. Larger thermal load compo-nents, especially in clusters, tend to retard the complete melting of lead-free paste. It is more difficult to ensure good joints on components with high thermal mass in convection processing because while trying to achieve a sufficient TAL on larger components, smaller components in less populous areas may tend to overheat.

Much discussion in trade magazines and forums such as the IPC TechNet has focused on the question of soldering tin-copper and SAC-alloy-terminated BGAs and other components with standard tin-leadsolders.Usinga230˚Cvaporphasesystem, even liquification of these termina-tions ceases to be a problem while posing little chance of overheating heat sensitive components. Similarly, risks associated with lower T

g substrates and temperature

sensitive components is reduced relative to lead-free convection processing.

Since cleanliness had been studied using ion chromatography for a previously published report2, a cleanliness compari-son was made for this report using ROSE techniques. An Omegameter operating above100˚Fwasemployed.Nodifferenceswere detected in ionic cleanliness between boards soldered using convection reflow and those soldered in vapor phase. Lead-free no-clean samples tended to have 50% higher contamination levels than standard tin-lead boards due to the type and level of flux used in lead-free pastes. All results were well below IPC limits.

Resistance across soldered BGA daisy chain arrays of 40 and 80 joints were the same for convection and vapor phase reflowed test boards within the limits of experimental measurement. (Daisy chained dummy 169 and 352 termination BGAs containing four daisy chains each were used.) Solder joint conductivity did not ap-pear to deteriorate measurably after either 2000 thermal shock cycles or 1000 hours ofacceleratedagingat85˚C/85RH.Dur-ing the 2000 thermal shock cycles and

Initial 2000 CyclesThermal Shock

1000 hrs.Accel. Age

SnPb No Clean Convection 19.1 23.1 23.6

Vapor Phase 22.8 21.6 19.0

Lead-free No Clean Convection 27.6 25.6 26.6

Vapor Phase 27.3 27.6 26.2

SnPb Water Soluble Convection 23.7 20.3

Vapor Phase 23.5 20.5

Lead-free Water Soluble Convection 26.3 27.6 25.0

Vapor Phase 26.9 28.0 25.7

Table 2. Shear/tensile force required to remove SOIC16 (pounds of force).

2000Cycles

1000 hrs.Accel. Age

SnPb No Clean Convection 19.5 * 9.34 *


Lead-free No Clean Convection 2.11 9.16


SnPb Water Soluble Convection 14.5 No data


Lead-free Water Soluble Convection 1.31 23.7


Table 1. Average percent resistance change (absolute value).

* Tin-Lead No-Clean resistance measurements tended to decrease slightly. Others in-creased or were mixed hence, absolute values to were used to assess changes.

Continued on page 34


Conquering SMT stencil printing challenges with today’s miniature components

mounted into a stencil frame, tension is applied to electroformed foil by the sten-cil frame’s polyester mesh. This tension pulls on the foil causing slight shifts in the locations of the stencil apertures. In most cases, the electroformed stencil aperture locations will be long, or further away from their expected locations. If the PCB has SMT pad locations that are short of expected locations and the electroformed stencil has aperture locations further away than expected, there can be a significant shift, or misalignment, between the stencil apertures and PCB pads.

A shift between the stencil aperture and PCB pad reduces the amount of solder paste in contact with the surface of the PCB pad. This lowers the adhesive force between the solder paste and PCB pad, effectively reducing the ability of the board to pull the paste from the stencil. Minia-ture components already have very low surface area ratios. The lower the surface area ratio, the more critical the alignment between the stencil aperture and PCB pad. The Fine Grain stencil in this DOE was cut in the frame on the new LPKF high-power short-pulse fiber laser. The intent was to minimize stencil aperture registration errors, thereby increasing the alignment accuracy between the stencil and PCB. The results (27 position errors for the Fine Grain stencil and 2,307 position errors for the electroformed stencil) below show a marked improvement in aperture registration when compared to an electro-formed stencil.

conclusionAs advancements continue in component and PCB technologies, will the stencil tech-nology of today provide current and future solutions to the challenging assembly issues faced by OEMs and CMs? Is electroformed technology the right solution or have new developments in stencil laser and material technologies caught up with and surpassed the electroformed technology of today?

The answer to these important ques-tions is in our view an unequivocal ‘yes.’ Stencil laser and material technologies have advanced to the point where laser-cut stencil performance is beyond that of current electroformed technology. Using the new LPKF high-power short-pulse fiber laser technology and the new Fine Grain material, stencil performance is significant-ly improved over electroformed, especially when printing miniature components. Improvements in stencil laser and mate-rial technologies have lead to significant improvements in solder paste release down to a surface area ratio of 0.45 as well as improved aperture registration accuracy. These improvements are critical to meet-ing future requirements when printing miniature components like 01005s. The technology summary is as follows:

At a cost savings of 30-50 percent compared to electroformed, the ability to produce multi-thickness (step) stencils, and the option of same day turn times, Fine Grain stencils, cut with the new fiber lasers, are a marked improvement compared to the high-performance stencil solutions available today. OEMs and CMs can get the performance they need while reducing costs and meeting critical delivery schedules. The new stencil laser and mate-rial technologies available today give stencil manufacturers the tools and materials needed to supply an ever-changing industry for many years to come.

AcknowledgementThe authors would like to thank Stephan Schmidt and Sebastian Gerberding of LPKF Laser Electronics (www.lpkfusa.com) for their contribution to this article.

Robert F. Dervaes is V.P. technology and engi-neering for Fine Line Stencil, Inc. Jeff Poulos,

is V.P. of manufacturing and sales, Alternative Solutions, Inc. Scott Williams is product/ac-

count manager with Ed Fagan, Inc.

Table 1: Stencil technology summary.

Technology

Minimum surface

area ratio Cost Material

Aperature registration accuracy

Chemical etch 0.66 Low SS, Alloy Moderate

Traditional laser-cut 0.66 Low SS, Alloy Very High

Traditional laser-cut 0.55 Low Slic™ Very High

Electroformed 0.5 High Electroformed Nickel

High

Advanced laser-cut 0.45 Medium Fine Grain Very High

accelerated aging, the average absolute change in resistance on measured daisy chains is summarized in Table 1. The differ-ence between the performance of VP sol-dered and convection soldered test boards is insignificant considering the limited data set. The resistance change for each sample is reported as the average of absolute values of the changes in resistance for a set of sam-ples. No special preparation or seasoning of samples was performed. Resistance values were recorded ‘blind.’ A small amount of ohmmeter drift was experienced at the low resistances measured.

The shear force required to cause SOIC joint failure was measured and found to be the same for convection and vapor phase reflowed test boards. Shear force was measured on an SOIC16 (U25 and U26) exerting a combination shear and tensile force that pushed the component parallel to the plane while lifting the component bymeansofa30˚wedge.Thesemeasure-ments did not deteriorate after shock and accelerated age. While thermal shock re-sults were measured at 500 and 1000 shock cycles, only those from the 2000 cycle test are reported here. Results are summarized in Table 2.

conclusionVP key benefits include:

Lower peak reflow temperature •Inert environment without •nitrogen Improved solder wetting and flow •Reduction in profiling time. •

As the data above indicates, thermal profiles using vapor phase soldering equip-ment are controllable with the maximum temperature dictated by the specific ther-mal transfer fluid employed. The tin-lead and lead-free solder joints created using vapor phase technology have equivalent performance to those created using convec-tion equipment, while offering a uniform fixed maximum temperature of controlled duration. Vapor phase solder joint creation offers a viable alternative to convection reflow. Convection reflow has less uniform maximum temperatures over complex circuit board surfaces.

referencesMunroe, C., “Beating the RoHS Heat,” 1. Circuits Assembly Magazine,” March 2005, pp. 38-47.Fraser, S. and Munroe, C, “Lead-Free 2. Using Vapor Phase Reflow in Lead-free Processing,” SMT Magazine, April 2005, pp. 48-49.

Vapor phase vs. convection reflow in RoHS-compliant assembly, continued from page 22

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Vapor Phase Versus Convection Reflow