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). 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. This will tend to reduce compression of the balls in response to component weight on first reflow, potentially resulting in slightly higher collapsed height. 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. There's really no hard answer to this question. 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.
#Reflow bga free
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. You want fully melted, liquidus solder ball drop to ensure solder voiding is minimized, when you are soldering the BGA Ball system. If you have a mix in the PCB design, you will be challenged with heat thermally escaping your solder joint system through board artwork. Another consideration is land pattern design and via locations. 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. 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. It is a function of the Alloy and the liquidus phase you want for the solder to fully wet. Other service areas are supplier risk assessment, comprehensive outsource . Robert Fried helps leading electronics OEM's develop world-class sourcing strategies for PCBA, cables, precision metals, plastics, modules and complete end-products.
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