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In theory, all solder pastes are pin probe testable. However, very few pastes possess the broad process window that allows them to perform successfully under a wide variety of pin testing methods. In addition to paste chemistry, the pin probe testability of a solder paste directly depends on several process factors. The most important factors are length of time between reflow and testing and the type and number of reflow profiles to which the paste is exposed. Other factors include the type of probes utilized, the spring forces and the type of parts being probed.
In addition to process considerations, reliability also must be considered when attempting to use a pin probe testable, no-clean solder paste. Several no-clean solder pastes that are marketed as pin probe testable suffer from poor surface insulation resistance (SIR) and electromigration values. In fact, many of these pastes actually fail these reliability tests when tested to industry specifications such as IPC and Bellcore tests. Often, this failure is the result of solder manufacturers trading the insulating properties of paste residues for those that will remain soft over long time periods. Long-term reliability issues could result, resulting in premature field failures in products assembled with these pastes.
To simplify the pin probe testing process, a no-clean solder paste has been developed that is adaptable to the many variations of pin probe testing. In addition, the new solder paste has demonstrated very good reliability, passing both SIR and electromigration testing.
As a result of an increase in intrusive reflow production and, to a lesser degree, a decrease in spare board real estate for test pads, in-circuit testing via pin probes has become increasingly prevalent. However, due to the relatively large amount of flux residues left by no-clean solder paste reflow, especially during intrusive reflow, ensuring the reliability of pin probe testing has become a complex task.
Variables of Pin Probe Testing
As with any process, several variables can affect the pin probe testing operation, including the quantity and quality of the paste residue and the time between paste reflow and probe testing. Other factors such as probe point style, barrel and socket material, spring force, and design and length of component leads will also affect the ease and results of in-circuit testing.