Issue link: https://iconnect007.uberflip.com/i/291105
14 The PCB Magazine • April 2014 Experience is showing that the decomposition temperature is a critical property, and appears to be at least as important, if not more important than the glass transition temperature when planning for Pb-free assembly conversion. " " LEAD-FREE REFLOW FOR HIGH-LAYER-COUNT PCBS continues the number of through-holes, contribute to increase routing density that allows the lower layer usage. Finally, by replacing through-hole connectors with surface mount connectors, higher connector density and improved electri- cal performance can be realized. The resulting new multilayers are not only thinner, cheaper, and easier to design, but are less costly and suitable for lead- free assembly. Challenges of Lead Free Environmental regulations are placing increased require- ments upon printed circuits. The European Union's Restric- tion of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) directives will signifi- cantly affect the requirements placed upon base materials. Among other elements, RoHS restricts the use of lead (Pb). Tin/lead (Sn/Pb) alloys have been used for many years in the assembly of printed circuits. Eu- tectic Sn/Pb has a melting point of 183°C and temperatures during assembly com- monly reach 230°C. The primary alternatives to Sn/Pb are tin/silver/copper (Sn/Ag/Cu or "SAC") alloys. These alloys have melting points near 217°C with typical peak assembly temper- atures reaching 255–260°C. This increase in as- sembly temperature coupled with the possibil- ity of multiple exposures to these temperatures requires the base materials to have improved thermal stability. Recent technical papers have illustrated important data on the effect of Pb- free assembly on base materials [1,2] . While there are many important properties to consider, there are a few that deserve special attention in light of current trends and the resulting need for improved thermal performance. These in- clude: • The glass transition temperature (T g ), • Coefficients of thermal expansion (CTEs), • Decomposition temperature (T d ) Effect on Laminates As the temperatures to which printed cir- cuits are exposed increases, as in Pb-free assem- bly processes, the decomposition temperature of the material becomes a much more critical property to understand [4] . The decompo- sition temperature is a measure of actual chemical and physical degradation of the resin system. This test uses thermogravimet- ric analysis (TGA), which mea- sures the mass of a sample ver- sus temperature. The decom- position temperature is report- ed as the temperature at which 5% of the mass of the sample is lost to decomposition. Expe- rience is showing that the de- composition temperature is a critical property, and appears to be at least as important, if not more important than the glass transition temperature when planning for Pb-free as- sembly conversion. While the definition of the decomposi- tion temperature uses a weight loss value of 5%, it is very impor- tant to understand the point at which 2–3% weight loss occurs, or where the onset of decomposition begins. In examining solder- ing reflow profiles, traditional Sn/Pb assem- bly processes can reach peak temperatures of 210–245°C, with 230°C a very common value. In this range, most FR-4s do not exhibits sig- nificant levels of decomposition. However, if you examine the temperature range where Pb- 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 multiple exposures to these temperatures. This problem is increased when there are 20+ layers, resulting in thicker boards, and many are power or ground planes. Consequences for Multilayers While the simplest steps to comply with Pb-free assembly may be changing the base laminate and replacing the tin-lead finish, this may not be suffiecient for thick, complex, high-