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New plastic materials have been developed
over the past few years to specifically address the needs of surface mount
reflow processing conditions. Liquid Crystal Polymers (LCP), Polyphenylene
Sulfides (PPS) and Polycyclohexylene Terephthalates (PCT) have excellent
high temperature properties due to their high Heat Deflection Temperature
(HDT) and excellent moisture resistance. Additionally, they have excellent
dimensional stability, low warpage and can be molded in thin sections
to reduce the amount of mass to be heated.
Typical plastic materials used in connectors are shown in the accompanying
chart. Insulator materials such as glass filled polyesters, which are
well established for wave soldered through-hole connectors, are generally
not desirable for reflow soldering due to their low heat deflection
temperature. Additionally, other high HDT plastics have proven unsuccessful
in many surface mount applications. Heat deflection temperature can help
predict the short term effect of high temperatures, but does not necessarily
indicate when permanent distortion or thermal relaxation of molded in
stresses might occur in an actual connector. Because of subtle differences
in design or molding processes, two connectors molded with the same material
could perform differently in the same application. Likewise, with two
identical connectors except for the plastic, it is possible for the one
with the lower HDT to outperform the one with the higher HDT in a particular
application.
When specifying surface mount connectors, it is important that the actual
processing times and temperatures be noted. Quoting heat deflection temperatures
is not sufficient.
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