Impedance testing for sensitivity to delamination in printed circuit boards.

Using embedded structures in printed circuit boards, changes in moisture content have been monitored using route-impulse-energy measurement, capacitance measurement and weight gain. All three methods showed good correlation for different prepregs with similar structures. Two designs of capacitor plate, a solid plate and a multi slotted plate, showed significantly larger percentage increases in capacitance values (10%) than the corresponding mass increases (0.35%) from moisture ingression effects. A good correlation was also obtained between the RIE measurement results and the board mass changes The RIE values showed significantly larger percentage increases (20%) than the corresponding mass increases (0.35%) over a similar timeframe. Although some results indicated a slightly greater moisture take-up for the 2116 prepreg samples compared to the 7628 prepreg samples, the majority of measurements did not show significant performance differences between the two prepreg types. Both the RIE measurement and capacitance methods show promise in evaluating the moisture content of printed circuit boards. Capacitance measurements using capacitors of a solid plate design, have the disadvantage of inhibiting the take up of moisture if the plates are near the surface of the PCB. RIE coupons have an aspect ratio which enables them to be relatively easily incorporated into the break-off panel that commonly occurs around the outside of many PCB designs during manufacture. Provided that similar coupons had been characterised in the dry condition previously, and that the PCB has the same build characteristics, both types of coupon could be interrogated immediately prior to assembly, to determine the current level of moisture content.