The vast majority of caps and buttons are molded using ABS (acrylonitrile butadiene styrene). This plastic is chosen because of its excellent mold and surface finish qualities, and since the cap or button is a “first surface” which is seen by the user, quality of finish is very important. Also the ability to accept a wide range of color pigments, along with good general mechanical properties, including the molding of “snap-on features” typically required, makes ABS the preferred material for caps and buttons.
However, ABS has low softening and melting temperatures, typically around 100C and 120C respectively. Continuous service temperature of 70C is typically the maximum that ABS can withstand. What this means is that ABS is NOT suitable for automated solder processing, either wave or SMT. The typical preheat temperatures for wave solder are about 150C to 160C, and higher top-side board temperatures are seen when passing over the solder wave. This will distort or melt the caps, and therefore it is recommended to install the caps after any wave solder processing. SMT is even hotter, so ABS can never be considered for SMT.
Some ABS variants may withstand the wave solder process, but this will have to be evaluated on an individual basis by the customer using their specific equipment and processing parameters. Flame retardant additives can increase the softening and melting temps, but again these require evaluation by the customer with their specific processing equipment.
In summary, ABS is used for caps because it has excellent molding and cosmetic properties, but it is typically not suitable for automated manufacturing processing that exceeds 100C, or for end-use applications that exceed 70C.