Printed Circuit Design & Fab - May 2008 - (Page 33)

FIGURE 1. Types of material degradation in PCBs. FIGURE 2. Adhesive delamination between the B- and C-stage to copper interfaces. samples on equipment that simulates the assembly thermal profile. As long as the preconditioning is well controlled, both methods produce similar results. In reliability testing, thermal cycling is continued until there is a 10% increase in bulk resistance in any circuit, or until end of test at 500 cycles. Coupons tested “as received” are considered the control sample and the CTF is the sample’s entitlement. Entitlement establishes the relative robustness of the sample. It is the sum total of all reliability influences affecting the sample, including quality, thickness and uniformity of the electrolytic and electroless coppers, robustness of material, design influences and other fabrication and assembly process variables. Preconditioning reduces the entitlement of the sample, and the more aggressive the preconditioning, the fewer thermal cycles to failure. TABLE 1 demonstrates reductions in mean CTF (tested at 150˚C), based on an “as received” performance of 500 cycles, and illustrates the anticipated degradation in CTF commonly measured after tin-lead and lead-free preconditioning. This reduction in cycles to failure is observed when there is no significant material degradation expressed during testing. Frequently, samples that have delamination express increasing CTF in response to more aggressive preconditioning, giving artifact CTF results. Delamination that extends cycles to failure appears to be stress relieving and, as a result, reduces the damage accumulation in the metallic interconnect structures. On occasion, delamination reduces cycles to failure, but this is infrequent. Delamination that reduces cycles to failure appears to increase the stress in the interconnect structure. The delamination in the dielectric aligns with cracks in the barrel of the PTH. It appears that stress is MAY 2008 focused to the sidewall of the plated barrel, and may cause catastrophic failures in just a few thermal cycles. This somewhat rare occurrence is described as stress focusing delamination. In the majority of cases delamination stress relieves samples, stopping or slowing down the propagation of copper cracking in the plated through hole, and the internal innerlayer connections, which would have occurred in response to the higher z-axis expansion of the dielectric material. A typical reliability evaluation, where delamination is expressed in the data, is shown in TABLE 2. Note that with delamination present in the samples, the cycles to failure increases in response to more aggressive preconditioning. The samples tested “as received” and those exposed to six cycles of preconditioning to 260˚C both show a CTF of 500 cycles (no failures by end of test). This increase in CTF as a response to more aggressive preconditioning is counterintuitive. Recent studies have confirmed that not only delamination, but also other types of material degradation artificially extend CTF. Established IPC standards consider obvious delamination a condi- PRINTED CIRCUIT DESIGN & FAB 33 http://www.chemcut.net http://www.chemcut.net

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Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

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