Printed Circuit Design & Fab - March 2009 - (Page 33) test and reLIaBILIty fIGure 6. Types of Material Degradation. fIGure 7. Cohesive Delamination. is measure on a coupon “as received” (green line) and after preconditioning (blue line). Delamination between layers 3/4 causes a reduction in capacitance. The “as received” capacitance profile is plotted and compared to the coupon after preconditioning. The presence and location of delamination is obvious in “before” and “after” capacitance plots. taBLe 2 is the capacitances, in picofarads, of the layer 4/5 dielectric in a 10-layer coupon. Capacitance measurements were taken before testing (green), after preconditioning 4X260° C or 6X260° C (yellow) and at the end of test 150° C for 500 cycles (orange). The coupons were subjected to lead-free preconditioning with four or six thermal excursions to 260° C to simulate four assembly and two rework cycles. The coupons did not violate the 4% degradation as results of either level of preconditioning, although the coupons exposed to six thermal excursions had 2% or greater change. All coupons showed delamination by the end of the test. This data suggests that lead-free assembly degraded the material enough to fail during test to 150° C. fIGure 5 shows changes in capacitance above a threshold set to 5%. The sensitivity of the coupon design, coupled with the construction implemented by the fabricator, produced a coupon that was more sensitive to changes in capacitance than other design/ construction combinations. Using microsections as the referee, the capacitance threshold was increase from 4% to 5% for this application. The data (fIGure 6) shows that after six thermal excursions to 260° C to simulate lead-free assembly and rework layers 5/6, 9/10 and 13/14 were prone to delamination which was confirmed by microscopic evaluation (fIGure 7). The types of material degradation that have been identified with capacitance measurements include adhesive delamination, cohesive failure, crazing and material decomposition. It should be noted that in lead-free applications, lifted pads and cratering are also observed, MARCH 2009 but these material conditions are not found with DELAM methodology. The leading edge of capacitance testing is to define, or at least to establish, the effect of thermal excursions on capacitance during a thermal ramp. Initial studies suggest that CTE and Tg may be directly measured, or possibly inferred, by recording capacitance during the thermal cycle. It may be possible to observe changes in Tg, CTE and Dk by measuring capacitance during thermal excursions. This method may be able to demonstrate moisture loss, changes in Dk due to curing or material degradation, as well as define the onset of delamination. The data could then be compared to reliability measurements and failure modes to produce a comprehensive tool to analyze dielectric materials and determine their role in PCB reliability. Both of these methods, cT260 and DELAM, have been implemented in standard testing protocol by a number of companies. These methods offer two powerful tools in understanding how circuit boards fail, the role of material in PCB reliability and the consequences of process changes like lead-free assembly. These methods are now being requested as part of reliability testing and as stand-alone tests to investigate specific areas of concern in specific applications. PCD&F pauL reId the program coordinator at PWB Interconnect Solutions, in Ottawa, Ontario, Canada and can be reached at Paul.Reid@mail.pwbcorp.com. PRINTED CIRCUIT DESIGN & FAB 33
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