Printed Circuit Design & Fab - March 2009 - (Page 30) test and reLIaBILIty taBLe 1. cT260 Isotherm Profiles. fIGure 1. TMA – cT260. The T260 protocol requires a laminated sample to be heated with a well-controlled, constant scan (ramp) rate to an isotherm at 260° C. Time to delamination is defined as the time from reaching the isotherm to an irreversible increase in size. The abrupt change in size is expressed as a spike at the time of failure. The specified scan rate is 10 degree sign/minute, although the 100° C scan rate option has proven to be more effective. The goal of the cT260 method is to emulate assembly and rework cycles prior to the isotherm established in the T250 method, referred to as preconditioning. In the current cT260 method, an IST 260° C preconditioning profile was emulated. Any preconditioning profile may be programmed, but by using the exact preconditioning used in IST reliability testing, a direct relationship is established between reliability cycles to failure and cT260 results. The cT260 method uses six preconditioning cycles that are defined in five increments in taBLe 1. It can be described as heating to a lead-free precondition temperature of 260° C in exactly three minutes. There is no hold time at temperature; instead, the sample is cooled as fast a possible to below Tg before the next thermal cycle. On the seventh thermal cycle, the sample is held at the 260° C isotherm to failure. A failure is considered any spike in size during the preconditioning cycles or at isotherm. The isotherm is held for a maximum of 90 minutes, which is considered the end of test. The cT260 thermal profile may be broken into two zones: the preconditioning zone (Zone 1) and the isotherm zone (Zone 2). One would expect material decomposition to occur in Zone 1, but experience demonstrates that most materials fail after achieving the isotherm. On occasion, a sample will fail during the preconditioning phase. It is thought that materials that fail in the preconditioning phase may have a mechanically-induced failure mode, such as a breakdown of oxide coating on copper planes or out gassing of trapped moisture. Since the TMA samples are small and there is a high ratio of edge to 30 volume, it appears out gassing of water and other volatiles is accommodated and has less of an impact in these relatively small samples as compared to PCBs or larger reliability test coupons. Most often, the failure occurs during the isotherm. It has been observed that samples tend to shrink before failing. This observation would be consistant with an epoxy that was continuing to cross link, building internal stresses until the material fails. The results of these tests demonstrate that preconditioning some materials can significantly reduce time to delamination while other materials are unaffected. The most surprising finding is that cT260 protocol identifies material degradation but does not necessarily predict delamination in reliability test coupons tested by thermal cycling. Frequently, material samples that failed in less than 10 minutes would produce reduced reliability cycles to failure, but the failure mode was barrel cracks and interconnect failures – not delamination! This counter intuitive relation between time to delamination results and reliability failures has been observed repeatedly over the past five years. Usually, reliability test coupons will fail and microscopic examination will reveal an understandable cause for the failure. Most often, thin electrolytic or poorly applied electroless copper, poor copper distribution or columnar copper crystals in the PTH will account for the failure. Interconnect problems may relate to drilling or hole preparation that is easily observable in microscopic evaluation. On occasion, microscopic evaluation presents a perfectly fabricated PTH with adequate copper and hole preparation and no observable defects. Frequently, when objective reliability data demonstrates a compromised PCB with no discernable fabrication problems, a cT260 evaluation (fIGure 1) of a sample taken before testing will degrade in less than 10 minutes. It appears materials that degrade in less then 10 minutes produce reliability coupons that fail in only a few thermal cycles. The implications of this test method and its applicaMARCH 2009 PRINTED CIRCUIT DESIGN & FAB
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.