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June 12, 2018

BGA Solder Ball Collapse

Do lead free solder balls collapse the same amount as tin-lead solder balls?

N.Z.

Experts Comments

Need to be careful in answering this question.  Lots of variables to consider such as BGA pitch, solder mask openings, solder paste thickness, PCB plating thickness, etc., can cause different collapse amounts.
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Robert Freid
President and Founder
Contract Manufacturing Consultants, Inc.
Robert Fried helps leading electronics OEM's develop world-class sourcing strategies for PCBA, cables, precision metals, plastics, modules and complete end-products. Other service areas are supplier risk assessment, comprehensive outsource ...
It is a function of the Alloy and the liquidous phase you want for the solder to fully wet.  With some 3-4 metal phases and the fact that there is not a common Eutetic Alloy for metal mix to melt simultaneously, you may need to prolong the profile and/or temperatures to ensure balls become 100% liquidous.   

To help this process, its common to use TSF for both it's Flux cleaning properties, as much as a means to encourage heat transfer into the solder joint ball.   Another consideration is land pattern design and via locatiOns.  If you have a mix in the PCB design, you will be challenged with heat thermally escaping your solder joint system through board artwork. 

You want fully melted, liquidous solder ball drop to ensure solder voiding is minimized, when you are soldering the BGA Ball system. 
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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
Yes they do assuming the their weights per ball are the same and they both use appropriate peak temp and TAL.  If TAL and peak are less than optimum and or the weight of the components per ball are different, the collapse will be different.
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Ray Prasad
President
Ray Prasad Consultancy Group
Ray Prasad is the founder of Ray Prasad Consultancy Group which provides teaching, consulting and technical expert services in tin-lead and lead free technologies using SMT, BGA, BTC, fine pitch and through hole components. Mr. Prasad is a long time member of IPC, and is currently the chairman of BGA committee IPC-7095 "Design and Assembly Process Implementation for BGA" and Co-Chairman of recently created IPC-7093 "Design and Assembly Process Implementation for Bottom Terminations" surface mount Components (BTCs) such as QFN, DFN and MLF.
There's really no hard answer to this question. The one thing you can count on is that the Pb-Free alloys in common use all have higher Sn content and usually contain some Cu, and thus have higher surface energies. This will tend to reduce compression of the balls in response to component weight on first reflow, potentially resulting in slightly higher collapsed height.

In the scenario where the part experiences a second (inverted) reflow, the effect of the higher surface energy will be the opposite, limiting the stretching and resulting in less "sag" and therefore a lower height. That's theory. In practice there are a number of factors that play a large role in actual collapse height such as the oxide level on the sphere, the land and solder mask geometry on the PWB, the properties of the flux used, and the reflow atmosphere (N2 reduces surface energy). Because of the number of factors in play it can be devilishly hard to predict collapse heights, or to predict changes based on differences in alloy.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Actually, Dr. Prasad is correct, but with two caveats;
  1. The pad diameters of both the BGA pad and the PWB pad are of such a size that lead-free solder can easily flow out to the edges.
  2. Everything else is the same.
It is well-known that lead-free solder does not wet to any given surface finish as well as lead-bearing solder does at their respective liquidus temperatures.

Even small amounts of lead added to a tin alloy solder will improve the wetting, as well as the dissipation into the intermetallic formation. The laws of hydraulics stop both molten balls at the outer edges of the pads. But if the pads are sufficiently large enough, the lesser wettability of the lead-free ball will cause it to collapse less than the leaded ball, ie, the leaded solder ball will spread further than the lead free ball.

In the IPC-TM-650, there are methods for testing solder wettability to compare fluxes. If you perform a wetting test on bare copper, and you put two .035" solder balls on the same substrate cut into two exactly equal pieces, both covered with the same amount of the same flux, the leaded ball will wet out further than the lead-free ball. Try it.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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