Printed Circuit Design & Fab - March 2009 - (Page 32) test and reLIaBILIty fIGure 2. Capacitance Construction Profile. fIGure 4. Capacitance Change Delamination. fIGure 3. Construction Profile. fIGure 5. Graph of Capacitance Change. Errors in PCB lay-up are easily observed by measuring the capacitance between layers in the coupons and plotting the coupons construction profiles. If the construction has a different dielectric build or if the coupon is laid up with mixed materials or has reversed layers, a different construction profile will be observed. This comparison test, based on capacitance measurements, has great utility in demonstrating variations between coupons. fIGure 2 demonstrates two different 6-layer constructions. The capacitances in picofarads (pF) are measured for each layer in both coupons and then plotted, demonstrating how the two boards will present different construction profiles. If many construction profiles are plotted together, coupons with different constructions are readily identified. fIGure 3 identifies the possibility of incorrect dielectric thickness at layer 6/9 in coupon A1. Since the capacitance is higher, one might expect a missing sheet of “B-stage” material. If the layers were laid up out of 32 order, one would also expect a change in capacitance profile. Whatever the cause, changes in the capacitance profile identifies a problem in construction that requires investigation. If all the coupons were laid up incorrectly, but in the same manor, there would be no practicle method of determining the error. It is conceivable that dielectric spacing, surface area of the plane and Dk of each material in the stack up could be used to calculate the expected capacitance and to compare those results to measured values. The practical approach is to establish a capacitance profile on a “golden board” and use that to compare to subsequent lot profiles. DELAM Testing The most useful application of this method is testing to identify the presence of delamination and other types of material degradation. Coupons subjected to reliability and DELAM evaluation will reflect damage to conductors and base materials. In fIGure 4 capacitance MARCH 2009 PRINTED CIRCUIT DESIGN & FAB
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