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Plated Through-holes in Flexible Circuits

Plated through-holes can be tested/inspected in a number of ways.

• Conductivity: There are special electrical measurement devices that will precisely characterize the electrical conductivity of the through-hole. Any anomaly such as a plating void will change the electrical conductivity and be detected by the tester. This is a non-destructive test which is outlined in IPC-TM-650.
• Micro cross-sections: Cross-sections are created by encasing the through-hole in a hard epoxy and creating thin slices to inspect the through-hole sidewalls under high magnification.  Micro cross-section requirements are specified in MIL-PRF-31032/1 and IPC-6013.
• Visual Inspection: Through-holes can also be inspected by using special magnification with reflected lighting. This inspection is considered a “quick” method for spotting problems and can supplement more formal inspection methods.

Good process control techniques may require a combination of several different inspection methods. The most critical aspect is controlling the inputs such as chemistries. Some applications, markets and/or customers may have exact specifications on through-hole test methods, minimum thicknesses, and may require cross-section data and/or images to accompany individual manufacturing lots.

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.

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