Issue link: https://iconnect007.uberflip.com/i/745476
November 2016 • SMT Magazine 63 TO BAKE OR NOT TO BAKE Delamination Due to Moisture Currently, pre-baking is avoided at many PCB assemblers due to the time and effort re- quired for the process. Many of our customers consider the indicator of a successful lead-free PCB assembly process to be the lack of delami- nation. However, it is important to first look at what causes delamination. As the moisture turns into water vapor, it exits the PCB. The most similar failure mode from the SnPb halcyon days are blowholes. Blowholes occur when moisture exits the PCB innerlayers through the copper plated hole walls—typically the weakest point on a PCB. In lead-free PCBs, however, it appears that the laminate itself is the weakest point for vapor evacuation. In this case, the vapor seems to be exiting in a vertical (Z-axis), which is responsible for the ubiquitous cases of delam- ination. That brings up a question, though. Just be- cause a board hasn't delaminated, does that mean everything is OK? Or is there a negative impact on the vias from the mechanical stresses of water vapor escaping from the PCB? Via Integrity During the early days of the lead-free tran- sition, I attended an Isola seminar focusing on laminate selection for Pb-free products. One of the salient points was that you can no lon- ger rely solely on Tg for reliability. For exam- ple, their 170° Tg product, FR-4-06, only had a 290° Td vs. the 340° Td of their lead-free capable product. Life cycle testing revealed that vias had only 50% life (as measured by thermal cycling tests) as opposed to vias passed through a stan- dard SnPb reflow process. Part of the reason for this was that the FR-4-06 had higher coefficient of thermal expansion (CTE) values than their Table 3. Figure 3: Example of surface delamination. Figure 4: Example of board delamination.