Printed Circuit Design & Fab - May 2008 - (Page 34)

LEAD-FREE RELIABILITY FIGURE 5. Material decomposition. FIGURE 3. Cohesive failure aggravated by mechanical stresses. FIGURE 4. Materials crazing, dark field microscopy. tion to be rejected. Delamination is unacceptable in electronic products for a number of reasons, including the degradation of electrical attributes by reducing the dielectric properties of the material. One of the concerns is that delamination provides a path for conductive anodic filament (CAF) growth that can occur over time in the end use environment. Material degradation is not easily observed by inspection, even with magnification. Many times delamination is present in PCBs or test vehicles that show no signs of discoloration or material deformation. Internal delamination that is not apparent externally may be found by microscopic examination. The microsection method of delamination inspection suffers from two disadvantages, first, microscopic evaluation is expensive and time consuming, forcing the sample sizes to be small, and the second even greater limitation to this method is the random nature of delamination. Delamination may be present in discreet areas and not continuous throughout the PCB or coupon. This sporadic nature of delamination can confound microscopic evaluations and lead to false positive conclusions. An electrical, non-destructive method for detecting material degradation has been developed in response to this problem. In order to determine if material degradation had occurred within the construction, circuits were included in reliability test cou34 pons (enhanced IST test coupons), which could detect material changes across the entire sample. Changes in capacitance were measured between internal planes before preconditioning or testing (to establish a baseline), after preconditioning, and at end of test. Relative changes in capacitance occur when materials are degrading. Not only can this method discern material degradation, it can also measure the physical changes in materials in response to thermal cycling. Armed with an objective method for determining which test samples had material changes, microsections were processed and material conditions were observed. Although microsections are random by nature and limited to viewing only a small area of a sample, a strong correlation was found between electrical changes and the presence of material degradation. Studies conducted over the past five years have established that a 4% change in capacitance (on specifically design coupons) demonstrates that material degradation has occurred. This method confirmed that, when present, delamination was extending thermal cycles to failure in most test groups. In a tin-lead application high reliability materials generally do not exhibit degradation. In a lead-free application, even high reliability and lead-free capable materials can exhibit significant degradation. It is significant in that the degradation is observable electrically and can be found upon microsection evaluation. Since there was a new method for identifying the presence, location (layer) and degree of material degradation, knowledge of failure modes by means of microsection analysis was advanced. Thermal cycles to failure analysis, including damage onset and acceleration, were now being evaluated against the presence and onset of delamination. It became possible to identify delamination and other material conditions that influenced reliability. One of the challenges in discussing material failures was the lack of a common vocabulary. Material degradation may be categorized in five often overlapping conditions; adhesive delamination, cohesive degradation, crazing, material decomposition and cratering as shown in FIGURE 1. These definitions may be unique to PCB fabricators and have different connotations in other disciplines. For the purposes of this article the following definitions apply. MAY 2008 PRINTED CIRCUIT DESIGN & FAB

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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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