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|A New Method for Non-Destructive Characterization of Through-Holes|
|Keywords: Through-hole Reliability, Through-hole Failure Analysis, Silicone Casting|
|Printed circuit boards (PCBs) require robust plated-through-holes (PTHs) that can withstand the thermal stresses of solder reflow processing during electronic card assembly. However, the laminate materials used in PCBs have a coefficient of thermal expansion (CTE) much greater than that of copper plating, especially above the laminate glass transition temperature (Tg), resulting in significant stress on the PTHs during thermal cycling. Excessive stress can cause circumferential barrel cracks in the PTHs, resulting in open circuits and electrical failures. High aspect ratio PTHs are especially susceptible to this failure mechanism. PTH cracking is accelerated by defects or irregularities in the through-hole resulting from drilling, desmear, or plating processes that create local stress concentrations [1-3]. Currently, cross-sectioning is most commonly conducted to evaluate plating quality. However, detection of plating abnormalities with cross-sectioning is a time consuming and destructive process. Furthermore, half of the PTH is lost to grinding and only one vertical plane can be analyzed at a time. Computer tomography (CT) x-ray techniques have also been used to evaluate PTH defects. However, this requires expensive equipment and the images produced can be difficult to interpret . To overcome the limitations of cross-sectioning and CT x-ray, a novel, non-destructive method of evaluating through-hole quality and reliability was developed. Briefly, the samples of PCBs with through-holes of interest were submerged in a two-part silicone elastomer mixture under vacuum to promote complete infiltration into high aspect ratio PTHs. After curing, the elastomer was removed from the PCB samples, resulting in a full replica of the inside of each PTH and intact PTHs that could be used in further testing or applications. Qualitative optical and scanning electron microscopy showed that the castings captured the micro-topography and roughness of the plating surface. A quantitative assessment of the plating surface was achieved by measuring the casting roughness using a laser scanning microscope. To validate the method, current induced thermal cycling (CITC) coupons selected from lots before and after a drilling process change were cast and the surface roughness was measured. The drilling process change was known to have increased the number of CITC cycles to failure and was theorized to have done so by creating smoother hole walls, thereby minimizing stress concentration. Roughness measurements of the castings confirmed that the CITC coupons with a higher number of cycles at failure had smoother plating than coupons with a lower number of cycles at failure. These results indicate that casting roughness measurements can be used to help predict PTH reliability. Therefore, this method can be used as a quick pre-screening tool before entering a battery of reliability testing to assess likelihood of success or as a health of the line check to ensure that the process is stable and that PTH reliability is maintained. Since the casting method is non-destructive, the analysis can be done on functional PCBs or PCB coupons. Furthermore, this technique can be used as a process indicator or failure analysis tool. With PTH castings, evaluation of the effects of a drill, desmear, or plating process change on plating surface topography can be attained. Also, if a PTH failure were to occur, the castings can provide information about the plating quality that was previously difficult to obtain with cross-sectioning alone which could assist in determination of a root cause. In addition to plating surface characterization, this technique can be used to evaluate other PTH features. For example, castings can be used to evaluate back drill quality of many PTHs in an array at once, including determination of depth, concentricity, and the presence of stubs. Diameter at any point along the PTH length can also be non-destructively measured using the casting method. Lastly, castings from through-holes after drilling or after desmear, prior to plating, can be created to evaluate the effects of those processes as well. Thus, this new PTH characterization technique can be utilized in a range of applications to assess through-hole quality and ultimately PCB reliability. Usage of this technique could also be expanded to include assessment of laser or plasma processed holes on laminate, ceramic, silicon, and glass chip carriers and as such, be valuable in reliability analysis and prediction in first level packaging applications.|
|Sarah Czaplewski, Advisory Engineer