Question: What are common methods of preventing blisters from lead free assemblies?
Answer: Control humidity within the manufacturing environment. Bake samples to drive out moisture, and select high reliability materials that can withstand elevated soldering temperatures. The cause and prevention of blisters in lead-free assemblies is discussed in the following article.
A certain company began a lead-free pilot run. Upon removal from the reflow oven, the first set of soldered printed circuit boards (PCBs) were full of blisters.
The customer’s board design had six layers, full ground, and power planes. The board was approximately 6" x 7" in size. It was not a high density board. The most difficult item to attach was a transformer. The reflow process temperature profile for the lead-free (Pb-free) solder assembly peaked at 250°C on the board itself. The substrate was made of FR-4 with a glass transition temperature of 170°C.
Initially, the customer considered that humidity was the cause of the blistering. To explore this possibility, one board was baked for 8 hours at 125°C. This was followed by reflow. Typically, this would bake out any moisture. Again, almost the same amount of blistering appeared (Figure 3-1). Next, after 3 months of storage, a PCB was reflowed at 250°C and a tiny blister appeared. The customer requested an analysis of the phenomenon and advice on correcting their process.
Lead-free solders have a higher melting point than SnPb solders. Due to an increase in vapor pressure with increase in temperature, components are more sensitive to moisture which may have accumulated during fabrication. If left in the device, this moisture will vaporize at soldering temperature, leading to delamination, soldering voids, and device cracking. The process specifications used by the customer for peak temperatures (240 - 250°C) and the board bake out for 8 hours at 125°C are well within the limits established in IPC-HDBK-001 with amendment 1; Handbook Guide to Supplement J-STD-001. The bake out should remove water accumulated during the fabrication process and absorbed during storage. Extended bake out can degrade the PCB and component solderability.
A potential source of failure is the use of conventional FR-4, which may not withstand multiple lead-free reflow soldering temperatures of 250°C. Alternatively, high reliability FR-4 can tolerate the elevated soldering temperatures. The choice of laminate is determined by two critical temperature values. Traditionally, the glass transition temperature is the sole consideration in laminate selection. An equally important metric in determining the choice of laminates is the material decomposition temperature. This is defined as the temperature at which a weight change of 5% occurs. Degradation can occur with as little as a 2 to 3% weight change during thermal cycling; therefore, a high decomposition point is critical in providing reliability.
Another factor in selection is the resin system. Phenolic cure materials known as high reliability FR-4 have higher temperature resistance to support the 250°C peak soldering temperatures. Appropriate examples of other PCB laminate materials that are specifically developed for the higher lead-free soldering temperatures and also satisfy the material requirements are Polyclad Getek, Isola IS 415, and Nelco N4000-12.
The EMPF agreed with the customer’s moisture testing procedures and results. Based on the EMPF’s lead-free processing expertise, the EMPF concluded that the problem was not due to the customer’s soldering process but due to improper bare board manufacture. This was likely due to the manufacturer’s use of expired prepreg. Fabricating the board in an uncontrolled humid environment is also a potential source of blistering. This absorbed moisture and/or expired prepreg could prevent full resin curing during multi-layer PCB lamination steps.
Conclusions
Board manufacture-induced errors can give rise to blistering independent of processing. From a processing perspective, prior to initiating a lead-free trial run, first confirm that the board material can tolerate lead-free soldering temperatures. Some materials have been suggested to better support the higher temperatures required for lead-free assembly. High reliability FR-4 can be substituted for conventional FR-4, but beware that these substitutes may be more costly and reduce performance of the finished product.
