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32 The PCB Magazine • April 2015 Testing of higher-density product has be- come ever more challenging with the advance- ment of chip technology. BGAs, CCDs and other active components have decreased in size so much that historical industry accepted test methodology can no longer effectively test these newer substrates incorporating this high- er technology. Universal grid test machines in North America, for the most part, are limited to sin- gle and double-density grids. Dedicated (wired) fixtures can combat some of the density issues faced in today's test arena, but to successfully succeed in the high-volume demand market a test solution must be found to efficiently test the product and also provide automation to reduce cost. Universal multi-plate fixtures routinely use test pins down to .3 mm. As pin size decreases, the need for extra plates in the fixture increas- es, so as to stabilize the pin and provide accu- rate loading of the pin. However, critical mass is soon reached as the fixture can no longer be solved due to inadequate grid contact availabil- ity. Further, as the density is maximized the fix- ture loses internal support and finally becomes unstable producing false defects, excessive trou- bleshooting and lost time. Additionally, there is an ever-increasing de- mand for 4WK measurement especially on HDI boards, requiring two probes contacting the same pad simultaneously, which can only be by Todd Kolmodin GarDIen SerVICeS Splitting Hairs: The Manufacture of HDI and Substrate Test Fixtures FeAture