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December 23, 2024 | Todd Kolmodin, Gardien ServicesEstimated reading time: 1 minute
As technology advances, our substrates become increasingly smaller, and chip technology has progressed to where HDI and UHDI are household terms. Ten years ago, this new design architecture would strike fear in the hearts of any plating engineer. Unfortunately, North American and European manufacturing has lagged due to excessive offshoring of commercial and some high-reliability product lines. Fortunately, most of the military and aerospace product has remained controlled. But in doing so, the designs utilized by the mil/aero and medical sectors’ reliability lines were corralled into the manufacturing capabilities of the North American and European providers. This is changing with some of the more advanced PCB manufacturers now revamping lines and increasing capabilities to embrace HDI and UHDI substrates.
One of the leading defect pareto bars has always been related to plating, specifically drilled holes and microvias. Plating anomalies in the via barrel or bonding in the microvias will, many times, not be detected during a standard continuity test. Thresholds in the standard electrical test adhere to manufacturing specifications such as IPC-9252, Requirements for Electrical Testing of Unpopulated Printed Boards. Continuity test requirements here are 10 ohms continuity for Class 3/C boards. The problem here is that minute changes in the copper thickness in a plated barrel or microvia bond will not change to the extent that the 10-ohm continuity threshold will be violated. These types of anomalies will go undetected and ultimately result in a field failure, as it may take some time for the anomaly to deteriorate under heat or stress.
The Kelvin Bridge
Although 4-Wire Kelvin testing is nothing new, it has not been used significantly in validating printed circuits until more recent times. Four-wire Kelvin or high-resolution resistance testing came about through the innovation of William Thompson (1824—1907) also known as Lord Kelvin. He was instrumental in the formation of the first and second laws of thermodynamics. The absolute temperature scale is expressed in Kelvin units (Kelvin scale) in his honor due to his work determining the exact temperature in Celsius and Fahrenheit of the value of Absolute Zero.
To read this entire article, which appeared in the December 2024 issue of PCB007 Magazine, click here.
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