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28 The PCB Design Magazine • December 2014 feature When it comes to HDI reliability, what we must do is consider two parts: the copper inter- connects and the base material. What one can do is test the reliability with thermal cycling us- ing Interconnect Stress Test (IST) coupons. The IST coupon tests the copper interconnection and checks for material damage. The coupon is fabricated on the production panel with the PWBs and has all the attributes of the PWB. So the coupon has the same construction, copper weights, hole sizes, grid sizes, and copper plat- ing as is found in the corresponding board. The test thermal cycles the IST coupon, typically for 500 cycles, or until the coupon fails with a 10% increase in resistance due to cracks that develop in copper interconnections as a result of ther- mal cycling. By measuring capacitance change between ground planes, we can determine if there is any significant material damage in the coupon. One must measure the capacitance in picofarads be- tween adjacent ground planes before testing (to establish a base line), after preconditioning (a simulation of assembly and rework) and at the end of test. We then compare the measurement after preconditioning and at the end of test to An HDI PWB may be defined as a PWB with a higher wiring density per unit area than conventional PWB. They have smaller lines and spaces, smaller vias and capture pads and higher connection pad density than employed in conventional PWB technology. HDI boards utilize microvias, buried vias and sequential lamination with insulation materials and con- ductor wiring for higher routing density. HDI is an alternative to high layer-count and standard laminate or sequentially laminated boards. HDI boards are characterized by high-den- sity attributes including laser microvias, fine lines, smaller grid sizes and high performance thin materials. This increased density enables more functionality per unit area. Higher tech- nology HDI PWBs have multiple layers of cop- per-filled, stacked microvias, which create a structure that enables even more complex in- terconnections. These very complex structures provide the necessary routing solutions for to- day's large pin-count chips utilized in mobile devices and other high technology products. by Paul Reid HDI PWB Reliability

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