CIRCUIT BOARD REPAIR AND ENGINEERING CHANGE FOR BGA
Author: John G. Davis Company: IBM Microelectronics Date Published: 9/10/1996
Surface Mount International
Abstract: In the past, it had been absolutely essential to be able to repair an internal circuit line. Circuit board layer counts were large (>20) and the probability of successfully connecting all layers correctly was difficult, particularly in the early stages of development. The result was an expensive board rendered useless. The choice was to either fix or start new and allow the law of probabilities to eventually yield a completely good board. Those were the days of the Large Board Product (700mm x 600mm), with low melt solder and, pin-in-hole technology that helped enable the Mainframe Product Line. The ability to repair over 1,000 internal circuit lines was one of the technology items IBM had to master. In the early 1990’s, a change from Bi-Polar Chip Technology to CMOS Chip Technology created three welcomed “windfalls”. First, fewer boards were required to create a high end system. Secondly, the layer count was greatly reduced. Thirdly, the circuit line spacings increased. The boards became simpler to build and therefore at higher yields. Circuit board repair had lost some of its business. At the same time, the BGA module was introduced to the Large Board Product. The Large Board Product became a relatively simple mix of Pin-in-Hole and SMT components with grid pitches of 0.100” (2.5mm) staggered and 0.050’’(l.25mm) straight. This simplified board design, however, was short lived. More function on the module level and the board level was created by increasing BGA Module pin outs (> 225), using more BGA modules per board, and packing those modules closely. The higher number of pin outs per board unit area and the restrictive 0.050” wiring channels necessitated an increase in layer count (>20) and with the most aggressive circuit line widths/spacings design to date.