Issue link: https://iconnect007.uberflip.com/i/1497518
36 PCB007 MAGAZINE I APRIL 2023 costly and suitable for lead-free assembly. I will discuss this and several enabling technolo- gies, including laser-drilled microvias and new SMT connectors, in future columns. The Issue Tin/lead (Sn/Pb) alloys have been used for many years in the assembly of printed circuits. Eutectic Sn/Pb has a melting point of 183°C, and temperatures during assembly commonly reach 230°C. e primary alternatives to Sn/ Pb are tin/silver/copper alloys. ese alloys have melting points near 217°C, with typical peak assembly temperatures reaching 255°C to 260°C. is increase in assembly temper- ature, coupled with the possibility of multi- ple exposures to these temperatures, requires the base materials to have improved thermal stability. Several technical papers have illustrated impor- tant data on the effect of lead-free assembly on ba s e mater ial s 1,2 . W hi le there are many important properties to consider, t h e r e a r e a f e w t h a t deserve special atten- tion in light of current trends and the need f o r i m p r o v e d t h e r - mal performance. These include glass transition tem- perature ( Tg ) , c o e f f i c i e n t o f t h e r m a l e x p a n s i o n ( CT E s ) , and decomposition temperature (Td). As the temperatures which printed circuits are exposed to increase, as in lead-free assem- bly processes, the Td of the material becomes a much more critical property to understand 3 . e Td is a measure of actual chemical and physical degradation of the resin system. is test uses thermogravimetric analysis, which measures the mass of a sample versus temper- ature. e Td is reported as the temperature at which 5% of the mass of the sample is lost to decomposition. Experience shows that the Td is a critical property, and appears to be at least as important, if not more important, than the glass transition temperature when planning for lead-free assembly conversion. While the defi- nition of the Td uses a weight loss value of 5%, it is very important to understand the point at which 2-3% weight loss occurs, or where the onset of decomposition begins. In examining soldering reflow profiles, traditional Sn/Pb assembly processes can reach peak temper- atures of 210°C to 245°C, with 230°C a very common value. In this range, most FR-4s do not exhibit significant levels of decomposi- tion. However, if you examine the tempera- ture range where lead-free assembly processes are operating, you can see that the traditional FR-4 materials exhibit a 2-3% weight loss. Severe levels of degradation can result from mult ip le e x p o s ure s to the s e temperatures. This prob - lem increases when there are 20-plus layers, result- ing in thicker boards, and many are power or ground planes. ere are two primary fail- ure mechanisms for printed wir ing boards. Failures are mainly due to ther- mal or mechanical excur- sions. Plated through-hole (PTH) failures are the pre- dominant source of PCB failures in service and predicting them is the pri- mary goal of PCB testing at elevated temper- atures. PTH reliability testing should simu- late the thermal excursions of a PTH through- out its life. Generally, the most severe thermal cycles are experienced during assembly and rework. With that said, the materials that make up the board construction are thus critical to PTH reliability. As more boards are subjected to the higher temperatures required for lead- free assembly, layer counts, Tg, and Td must be considered. The Td is a measure of actual chemical and physical degradation of the resin system.