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Lightning Speed Laminates: High-frequency Materials for Lead-free Soldering

Some OEMs’ qualification procedures dictate that PCBs be subjected to multiple passes through a lead-free solder reflow cycle. The qualification requirements differ from one OEM to another, but some will require five, six, or even 10 passes through a lead-free solder reflow process. There are a few different criteria for these tests, however the basic demand is that the PCB must remain mechanically intact and show no signs of delamination. The materials that make up the PCB can have a major impact on the ability of the PCB to survive the lead-free solder evaluations, and some materials perform better than others.

The material properties related to lead-free solder survival are typically coefficient of thermal expansion (CTE), glass transition temperature (Tg), decomposition temperature (Td) and moisture absorption.   

As a general rule, the material should have a CTE of 70 ppm/°C or less and closer to the CTE of copper, about 17 ppm/°C, which is best. Other general rules suggest a Tg value higher than 170°C, Td greater than 300°C and moisture absorption less than 0.5%.

The high-frequency circuit material industry offers many laminates and prepregs (or bonding material) to choose from. Some of the laminates and bonding materials are robust for lead-free soldering, and others are not. Additionally, there are some bonding materials which are not intended to be used at lead-free soldering temperatures.

A broad classification of bonding materials can be thought of in two categories. Some bonding materials are thermoplastic, while others are thermoset. A thermoplastic bonding material has the attribute of being able to reflow or melt when subjected to certain elevated temperatures. The thermoset materials will not melt or reflow when subjected to elevated temperatures.  However, at high temperatures, the concern can be related to material decomposition. Read the full column here.Editor's Note: This column originally appeared in the October 2013 issue of The PCB Design Magazine.