Excepted from Section 1.9, “Existing Manufacturing Conditions,” in the recent book “Implementing Lead-Free Electronics,” by Jennie S. Hwang, Ph.D., Copyright 2005, The McGraw-Hill Companies, Inc., New York, NY.
In this section, Dr. Hwang shows how the manufacturing parameters commonly used for Pb/Sn eutectic soldering are also suitable for some “drop-in replacement” lead-free solders.
Manufacturing conditions allowing “drop-in” lead free solder.
For the last 23 years of SMT establishment and evolution, 63Sn/37Pb (or its equivalents, 60Sn/40Pb and 62Sn/36Pb/2Ag) has been uniformly used in the assembly industry. Its melting temperature of 183° C serves as a reference point. With this melting temperature, if everything was performed at realistically close to optimal conditions, the reflow process could be carried out in a range of approximately 203 to 210° C.
However, based on numerous on-site examinations of production lines around the world over the years, the actual reflow temperature that has been used for the process across the industry and over various geographic regions has largely fallen within the range of 220 to 230° C, and the actual wave soldering was from 245 to 250° C, for various reasons.
In other words, the use of an “excess” temperature in relation to the melting temperature of 63Sn/37Pb has been common practice with Sn/Pb solder. What this means is that the process temperatures up to 230° C for reflow and up to 245° C for wave soldering have provided sound manufacturability. It also means that the manufacturing system, including materials, PCB bare boards, and all components, is compatible with these reflow and wave soldering temperature settings.
Thus, it has been proven that the peak temperatures of 230° C for reflow and 245° C for wave soldering are robust process parameters for PCB assembly within the existing SMT infrastructure.
IC components have demonstrated a temperature tolerance up to this process temperature. According to J-STD-20B, IC component temperature tolerances are as follows.
for packages larger than 350mm: temperatures less than 245° C
for packages smaller than 350 mm: temperatures less than 250° C
The temperature tolerance of PCB material varies with the specific type. Table 1.3 lists the glass transition temperature of various PCB materials.
Table 1.3. Glass transition temperature of PCB substrate polymers.
The majority of PCBs used in SMT are of the FR-4 type or polymer substrate materials that have even a lower glass transition temperature than that of FR-4. The cost of the polymers with higher glass transition temperature is one primary issue. FR-4, at glass transition temperatures higher than 125° C, can withstand a process temperature up to 230° C, which has been the proven real-world performance.
Finally, the following summarizes the established temperature tolerance levels in the existing SMT manufacturing environment.
- IC Component
- PCB (FR4 or lower) withstand process temperature
- Reflow peak temperature
- Wave soldering temperature
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This material was excerpted by permission from Section 1.9, “Existing Manufacturing Conditions” in “Implementing Lead-Free Electronics” by Jennie S. Hwang, Ph.D. Copyright 2005, The McGraw-Hill Companies, Inc., New York, NY