Printed Circuit Design & Fab - March 2009 - (Page 31) test and reLIaBILIty tion in reliability testing are still under investigation. One (expressed in changes in Young’s and loss moduli) is not observable in microscopic evaluations, but it has an effect in changing how stress is distributed within the PCB during thermal excursions. The cT260 may be serendipitously reflecting the effect of chemical changes that are expressed in changes in physical attributes that affect thermal cycles to failure. For this reason, the new test method is called time to degradation rather than time to delamination. It is likely that the failures observed in cT260 are a reflection of a cohesive failure rather than a physical adhesive delamination. The term “delamination” in the original test T260 method may be a misnomer when testing today’s high reliability materials. taBLe 2. Capacitance Percent Change Layer 4/5. DELAM Testing A new method currently under evaluation determines the presence of material delamination in reliability test coupons. The method relies on specifically designed capacitance circuits that are sensitive to changes within the coupon’s dielectric construction. The test vehicle utilizes copper planes, at appropriate layers, across which capacitance is measured. There are a number of different measuring protocols using this technique, but the standard testing philosophy requires initial measurements that are compared to measurements after thermal excursions. In the most common application of this method, capacitance is measured before testing, after preconditioning and at the end of reliability testing. A 4% to 6% drop in capacitance suggests significant material degradation. Usually, adhesive delamination or cohesive failures are found to be the cause of the observed capacitance change. The second most common cause for capacitance change is moisture absorption or loss. Occasionally, changes associated with under curing of the b-stage are observed. This new test method has been given the acronym DELAM: Dielectric Estimation and Laminate Analysis Method. There are eight common factors that affect capacitance measurements in PCB test coupons. The first three establish the coupon’s initial capacitance and include the dielectric material, coupon design and PCB construction. Base materials present a dielectric constant (Dk), the circuit design determines the surface area of the copper planes and the coupon construction establishes the distance between the planes. There are five variables that may cause a change in capacitance readings, including the presence or lack of moisture in the dielectric, changes in the distance between copper planes due to plastic deformation of the dielectric, changes in the Dk due to curing or material degradation and propagation of cracks (delamination) in the dielectric between each pair of planes. This method provides a tool to find adhesive delamination, cohesive failure of the epoxy, crazing, material decomposition and the presence or lack of moisture in the dielectric (either absorbed epoxy or as a liquid in crevasses). MARCH 2009 There are two commonly used applications for this method: evaluating coupons for material changes like delamination, crazing or decomposition, and establishing a construction profile that allows an ability to identify and compare coupons with variations in construction. Construction Profile One useful application of the DELAM method is the ability to compare the construction for each production panel within a lot. This may be extended to compare production lots or panels fabricated by different vendors. Specializing in Quickturn Proto's Instant online Quotations & Ordering From Singlesided to 6 layers ML Leadtimes from 48 hrs Full DRC included on all orders Simply send your layout files and order online Internet pioneers with 15 years experience High Quality prototypes at LOW cost's TollFree USA: 1877 3908541 Email: sales@pcb-pool.com www.pcb-pool.com PRINTED CIRCUIT DESIGN & FAB 31 http://www.pcb-pool.com http://www.pcb-pool.com
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