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26 The PCB Magazine • April 2014 An HDI PWB may be defined as a PWB with a higher wiring density per unit area than con- ventional PWB. They have smaller lines and spaces, smaller vias and capture pads and high- er connection pad density than employed in conventional PWB technology. HDI PWBs uti- lize microvias, buried vias and sequential lami- nation with insulation materials and conductor wiring for higher density of routing. HDI PWB is an alternative to high layer-count and stan- dard laminate or sequentially laminated boards. HDI PWBs are characterized by high-density attributes including laser microvias, fine lines, smaller grid sizes and high performance thin materials. This increased density enables more functionality per unit area. Higher technology HDI PWBs have multiple layers of copper-filled, stacked microvias, which create a structure that enables even more complex interconnec- tions. These very complex structures provide the necessary routing solutions for today's large pin-count chips utilized in mobile devices and other high technology products. When it comes to HDI reliability of PWBs, what we must do is consider two parts: the cop- per interconnects and the base material. What one can do is test the reliability with thermal cycling using Interconnect Stress Test (IST) cou- pons. The IST coupon tests the copper intercon- nection 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 construc- tion, copper weights, hole sizes, grid sizes, and copper plating as is found in the corresponding board. The test thermal cycles the IST coupon, for typically 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 thermal cycling. By measuring capacitance change between ground planes we can determine if there is any significant material damage in the coupon. What one does is to measure the capacitance in picofarads between adjacent ground planes be- by Paul Reid HDI PWB Reliability F e a t u r e