High-layer-count multilayer printed circuit boards (PCBs) present one of the most difficult cases for adaptation to the lead-free reflow assembly process. Often, these boards have through-hole and hand-soldered components, along with the requirement for two or more rework cycles. The slower wetting and higher reflow temperatures of lead-free solders place an enormous strain on the laminates and copper-plated hole barrels of the vias, with resulting loss of reliability.
Restrictions on Hazardous Substances
Printed circuits are coming under increasing requirements from environmental regulations. Waste Electrical and Electronic Equipment (WEE) directives and the European Union’s Restriction of Hazardous Substances (RoHS) are significantly affecting the requirements on the base materials used for manufacturing PCBs.
The most popular solder material so far consisted of the tin/lead (Sn/Pb) alloy. used for the assembly of PCBs for many years. The melting point of eutectic tin/lead alloy is 180°C and during assembly, reflow temperatures commonly reach peaks of 230°C. However, one of the major restrictions RoHS places is in the use of the element lead (Pb). This has resulted in development of alternatives to the tin/lead alloy, which are now replaced typically with the SAC alloy, whose primary ingredients are tin/silver/copper (Sn/Ag/Cu).
The SAC alloy has a melting point of 217°C, with reflow temperatures typically peaking around 255-260°C. This rise in the assembly temperature, coupled with the possible requirement of multiple exposures to these temperatures means the base material must possess improved thermal stability. Although there are several effects of lead-free assembly temperature on base materials, three effects deserve special attention for improving the thermal performance. These are:
* Glass transition temperature
* Coefficients of thermal expansion
* Decomposition temperature
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How Higher Temperature Affects Laminates
The traditional Sn/Pb assembly process exposed the PCB to peak temperatures of 210-245°C, with 230°C being a very common value. At these levels, most lamination materials do not exhibit significant levels of decomposition.
However, at temperature ranges of 255-260°C where the lead-free assembly process operates, traditional lamination materials exhibit a 2-3% weight loss. Furthermore, multiple exposures to these temperatures may result in severe levels of degradation. Thicker boards, many of which are 20+ layers, aggravate the situation, as many of the layers are power or ground planes.
Although one of the simplest ways of complying with the RoHS directive of lead-free assembly may be to change the base laminate and replace the tin-lead solder, this does not work out satisfactorily for thick, complex, high-layer-count PCBs.
Creating Custom Reflow Profiles
Creating custom profiles for high-layer-count PCBs works well for the lead-free reflow assembly process. If the new PCB has thermal requirements close to that of some other PCB already being assembled, tweaking the settings for the existing PCB may be adequate. However, for a new PCB whose thermal requirements do not match any existing types, there may be thermal challenges. In such cases, developing a new profile may be more cost-effective ultimately.
Conclusion
Redesigning to reduce the thickness and the number of layers of high-layer-count PCBs is the way out in achieving reliable lead-free reflow soldering. Moving over to HDI technology, together with the use of BGA connectors, offers a viable solution.
