The challenge
This board put every hard thing on one assembly: 0201 passives, high component density, BGAs, mixed metallurgy, and Class 3 through-hole fill on a thick 34-layer stackup.
Mixed metallurgy means lead-free SAC BGA balls on a tin-lead process. SAC melts well above eutectic SnPb, so the profile has to get the BGAs hot enough to fully collapse and mix the balls without overheating the 0201s and other small parts on the same board. A 34-layer board carries a lot of copper and thermal mass, so BGA sites and small passives heat at very different rates. Too little heat at the BGAs leaves balls incompletely collapsed and mixed; too much heat elsewhere stresses the 0201s and other small parts.
Class 3 also requires at least 75% vertical barrel fill on through-hole joints. On a thick, high-layer-count board, the inner planes pull heat out of the barrel, which is what causes short fills.
What BSU did
- Reflow profiling. BSU profiled the board with 5 thermocouples, placed to capture the temperature spread between the BGA sites and the small passives, and tuned the convection reflow profile to that spread.
- Paste and placement. Koh Young 3D SPI measured paste volume, area and height on every print before MyData placement, so insufficient or excess paste was caught before reflow.
- BGA inspection. Every BGA was X-rayed for ball collapse, bridging and voiding against Class 3 criteria.
- Through-hole. Through-hole joints were soldered on ACE selective solder, which heats each joint locally so the barrel gets enough heat to fill without stressing nearby SMT parts.
- Traceability. Lots and serials were tracked in E2 ERP.
Result
100% of the 93 boards passed final inspection and test. The customer approved anonymized use of this build.
