www.globalsmt.net

The Global Assembly Journal for SMT and Advanced Packaging Professionals

Volume 9 Number 9 September 2009

ISSN 1474 - 0893

Mike NelsonInterview Inside

NEW PRODUCTS

INDUSTRY NEWS

INTERNATIONAL DIARY

PoP (Package on Package) and vaPor Phase technology

a baseline study of stencil and screen Print Processes for wafer backside coating

fatigue & creeP wearout in electronics: a historical retrosPective

sPecial Packaging feature:

imbedded component/die technology (ic/dt®): is it ready for mainstream design applications?

Plus: ic packaging and interconnection technologies’ 4th dimension challenge

10 – Global SMT & Packaging – September 2009 www.globalsmt.net

PoP (package on package) and vapor phase technology

Increased use of PoP technology is driven by continued product miniaturization. Smaller products require increased printed circuit board assembly (PCBA) density. Stacking memory is one way to achieve both goals for enhanced functionality and greater packaging density.

PoP technology is not without manufac-turing challenges. While the ball grid array (BGA) packages used are well understood, a poor quality process carries a far higher cost in terms of rework or scrap, since two or more BGAs are affected.

Placement requirements and process

technology are not significantly different from individual BGAs. Following screen printing, the first component is placed. Then the second component is flux dipped and placed. In some cases, up to two or more BGAs may be stacked. The only significant line modification is purchasing a flux dip module for placement machines. Thermal profiling during reflow is the same as boards of similar thermal mass.

Quality needs to be carefully moni-tored. Any distortion in the bottom com-ponent will be reflected in the components above it.

Keywords: Package on Package, PoP, Vapor Phase Reflow, Solder Joint Formation

The trend toward smaller, more densely populated electronic assemblies is driving increased use of package-on-package (PoP) technology ,and electronics manufacturing services (EMS) providers are developing viable manufacturing solutions.

by Dan Coada, EPIC Technologies LLC, Norwalk, Ohio, USA

PoP (package on package) and vapor phase technology

Figure 1. A 2-D x-ray image of primary and secondary stacked BGAs pre-reflow indicated excellent placement accuracy.

12 – Global SMT & Packaging – September 2009 www.globalsmt.net

PoP (package on package) and vapor phase technology

Excessive heat in convection reflow processes when lead-free components are used can be a concern because the top component is flux dipped; there is no addi-tional solder volume. The use of VP reflow is the only one that actually helps PoP technology by controlled peak temperature, no ΔT, and SVP (soft vapor phase) technol-ogy that gives an accurate control on ramp rates and reflow time.

VP reflow technology is not a new process to the EMS industry. It is simply an alternative process for SMT reflow that has been in existence since the early 1970’s. Today’s VP reflow process makes use of the heat produced by an environ-mentally friendly boiling fluorinated fluid. The vapor blanket is a uniform tempera-ture zone in which the PCBA solder is reflowed. Heat is transferred to the PCBA through condensation as it is immersed into the vapor area until the PCBA reaches temperature equilibrium with the boiling point of the fluid.

The primary soldering benefits of VP, in comparison to infrared (IR) or convec-tion, include an oxygen-free (inert) environ-ment without the need for nitrogen, fixed highest temperature exposure, and superior heat transfer on thermally challenging PCBAs.

Vapor phase reflow offers two main advantages for PoP assembly:

Better thermal transfer, which • reduces the potential of “potato chipping” or cracking of the components that can occur if the proper thermal mass temperature is not achieved. Reduced cost by avoiding a ni-• trogen requirement for reflow.

In our Reliability Laboratory at EPIC, we performed analysis on PoP assemblies reflowed using VP technology. The aim was to verify that solder joint formation between PoP BGAs and the PoP assembly and board pads conformed to IPC stan-dards. The test vehicle used a BGA305 and BGA128 assembly on a PCB200-12mmcad Practical Components testing board. Kester Tacky Flux RF743 was used for the flux dip operation and Henkel no-clean lead-free Multicore LF318 DAP88.5 for the solder paste. Five PoP BGA sets were placed and reflowed at 240˚C. Electrical testing, x-ray and cross sectioning were used to assess solder joint quality.

In vapor phase reflow, heat is trans-ferred when the hot, saturated vapor condenses on the PCBA surface and gives up its latent heat of vaporization. The fluid boiling point is the governing factor in peak temperature. The vapor encapsulates the entire surface of the board, resulting in

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Figure 2. A micro-section image showed excellent ball collapse on primary and secondary packages after reflow.

Figure 3. A darkened image of micro-section indicated excellent solderability, wetting, and ball collapse on primary and secondary BGAs. Shown under a stereo (top) and metallurgical (bottom) microscope.

Global SMT & Packaging – September 2009 – 13www.globalsmt.net

PoP (package on package) and vapor phase technology

the smallest ΔT at short dwell times of the board in the condensing vapor. Thermal transfer is independent of form, color, mass and mass distribution of the PCBA.

Detailed x-ray inspections were performed on each BGA group, looking for evidence of solder bridges, voids and open connections. Voids were measured using RINCON measuring software and found compliant with IPC-7095 where the accepted voiding area is less than 25% (the percentage of joint cross-sectional area occupied by the void). Voiding was mainly caused by flux out-gassing within molten solder joints. (These bubbles form and pop open when they either grow too large or migrate to the edge of a joint; upon solidi-fication, the bubbles become voids.) While cross-sectioning was not perfectly centered on the BGA balls, the solder joints exhib-ited in the analysis met all requirements of IPC-610 Revision D for component alignment, voiding, solder ball spacing and connection (Figure 1).

Cross sections of the components that used Multicore LF318M DAP solder paste together with VP reflow indicate adequate solder volume and uniformity (Figures 2 and 3). The calculated solder volume used is within 5% of the solder volume that should normally occur on a production surface mount line.

As a surface finish, ENIG offered good solder-ability, increasingly significant strength and a considerably improved board finish.

Visual inspec-tion under 40, 150, 600 and 1200X magnifications re-sulted in 100% first pass yield (FPY) for all BGAs that used Multicore LF318M DAP solder paste under VP reflow.

All five BGAs (U1, U5, U8, U11, and U15) were suc-cessfully tested for continuity.

All criteria for this process were satisfied with excel-lent test results.

references:Munroe, C., “Beating 1.

the RoHS Heat,” Circuits Assembly, March 2008.

Wooten, R., “Vapor 2. Phase vs. Convection Re-flow Technology” SMT, July 2009

Dan Coada is a process/manufacturing engineer

with EPIC Technologies. He can be reached at coaddc@

epictech.com.

Figure 4. Vapor Phase profile that has been used to reflow the boards.

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compact systems.

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