|Ask the Experts|
June 12, 2020
BGA Solder Ball Collapse
Do lead free solder balls collapse the same amount as tin-lead solder balls?
|Expert Panel Responses|
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.
President and Founder
Contract Manufacturing Consultants, Inc.
It is a function of the Alloy and the liquidus phase you want for the solder to fully wet. With some 3-4 metal phases and the fact that there is not a common Eutectic Alloy for metal mix to melt simultaneously, you may need to prolong the profile and/or temperatures to ensure balls become 100% liquidus.
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, liquidus solder ball drop to ensure solder voiding is minimized, when you are soldering the BGA Ball system.
Capital Equipment Operations Manager
Specialty Coating Systems
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.
Ray Prasad Consultancy Group
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.
Actually, Dr. Prasad is correct, but with two caveats;
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.
Even though tin-lead and lead-free solder differ in density & wetting ability, the flux ingredients & metal content of lead-free are designed for it to behave as much as tin-lead paste.
The amount of collapse should be around 5 mm. Other items you need to consider are paste volume, pad diameter & weight of package.
Senior Manufacturing Engineer
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