Fine pitch BGA solder joint split in SMT process

Fine pitch(?0.5 mm) BGAs were assembled on many kinds of products for many years, such as Smart Handheld Device, Mobile Phone, Network Device, Notebook main board, etc. Normally the BGA defects are solder joint bridge, solder joint open, and BGA solder joint crack during assembly process. Regarding the solder joint crack, most of cases are caused by external force, such as assembly operation, board testing operation, or unmatched mechanical parts assembly. In our case, we found the BGA solder joint crack is not the same root cause as mentioned above. When we got the two defect PDA (Personal Digital Assistant) devices, we did the FA (Failure Analysis) as usual. The first step, we did the functional test and X-ray analysis again to find the failure symptom. The device could work normally again in functional test by pressing the top of CPU (BGA package). We also performed CSA (Cross Section Analysis) and dye and pry test to realize the defect is BGA solder joint crack. Regarding the result, we suspected that the crack might not be caused by external force due to the crack did not locate at the corner or outer row of BGA. But we still can´t identify which process or operation to cause the defect. At the same time, we had sorted more same defect boards from our stock. We determined this is not the single case. So, we had three directions to analyze. For material, we had performed push test, CSA and EDS (Energy Dispersive Spectrometer) to verify the solder ball due to the solder crack was at BGA package side. For process, we had conducted two experiments to find out the suspected station and operation. From our experiment result, we had two conclusions. One is the defect could not be eliminated by changing the reflow parameters or the type of reflow oven, the other is the defect occurred in 2^nd side SMT process. So, to change the process sequence would be the short-term solution to prevent the defect happen again before we find out the root cause. We also foun- d the defects located at some specific points from our experiment result. Then we focused on the relationship between crack locations and PCB design and found that every crack solder joint has buried via beneath it. It means the buried via is the major cause of solder joint crack during SMT process. So, we had analyzed the structure and filling material of buried via of HDI (High Density Interconnect) PCB. We found the different CTE (Coefficient of Thermal Expansion) between PCB and buried via filling material would caused thermal stress remain around the BGA solder joint by CAE (Computer-Aided Engineering) simulation. When the fine pitch BGA solder joint was proceeded into 2^nd side SMT reflow, the thermal stress will split the BGA solder joint. So, how to eliminate the thermal stress will be the solution for BGA solder joint split. Referring to the article for more details discussed.