Printed Circuit Design & Fab - March 2009 - (Page 29) test and reLIaBILIty the two test methods were developed. These methods were developed to understand and to quantify material degradation in response to thermal excursions. Cyclic Time to Degradation (cT260) is a thermal mechanical analysis method that ranks a material's robustness; Dielectric Estimation and Laminate Analysis Method (DELAM) is a capacitance-based method requiring specifically design test coupons (Interconnect Stress Test – IST). While cT260 establishes a material’s propensity to cause interconnect failures, DELAM confirms PCB construction and monitors for significant material degradation during reliability testing. Cyclic Time to Degradation at 260° C expands upon an established industry standard test method, developed and accepted by the IPC and contained in the IPC TM-650 Test Methods Manual, Time to Delamination (TMA Method) 2.4.24.1 testing with an isotherm of 260° C. For convenience, I will refer to this IPC method as T260, while the new cyclic method being proposed will be referred to as cT260. The IPC T260 method prescribes a constant ramp (scan) rate to an isotherm of 260° C or 288° C. The new protocol, cT260, expands on the T260 method by adding six thermal cycles before the isotherm. The equipment used for both the T260 and cT260 methods is a Thermal Mechanical Analysis (TMA) tester. TMA accurately measures small changes in sample size in response to temperature. By measuring changes in size through a tightly control temperature range, TMA allows one to determine the coefficient of thermal expansion (CTE) in parts per million and the glass transition temperature (Tg) based on changes in CTE. For the T260 method, TMA is measuring the time to a catastrophic material failure. Held at an isotherm, a sample may be monitored until there is catastrophic failure in the material or between laminated surfaces. The failure is usually expressed as a sharp irreversible increase in size. The test samples are cut to about 0.25 inches square and are usually taken from the edge of a test coupon or the PCB. The samples need to be laminated and have copper ground planes, with no holes or interconnect structures. The samples are placed on a quartz stage, and a quartz probe is lowered onto the top of the sample with a downward force of 0.005 Newtons (5g). A small electron furnace is lowered around the sample, probe and stage. Around the furnace is a refrigeration unit referred to as a “cold finger” which cools the furnace during the heating cycle. Cooling while heating a sample allows for precise control of thermal ramps and prevents over shooting the 260° C isotherm. Nitrogen gas floods the furnace, helping to conduct heat while providing an anhydrous and oxygen-free environment. Small changes in size are sensed through the probe with a LVDT (linear variable displacement transducer) and recorded in a file. Get your daily dose of PCB industry news and views. Subscribe to PCB UPdate The daily e-mail newsletter filled with news and resources for PCB engineers, designers, fabricators and assemblers. PCB UPdate does the work for you—finding and organizing the latest news and articles from Printed Circuit Design & Fab, Circuits Assembly and the World Wide Web into an easy-to-read, one-page e-newsletter. Experience the convenience of e-mail news delivery, combined with trusted brands and industry expertise. Subscribe now at www.pcbupdate.com. MARCH 2009 PRINTED CIRCUIT DESIGN & FAB 29 http://www.pcbupdate.com http://www.pcbupdate.com
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.