Issue link: https://iconnect007.uberflip.com/i/1453746
FEBRUARY 2022 I PCB007 MAGAZINE 77 drilled blind vias, finer lines, and stud-bumped flip-chip for the IC. All this could be done in four-layers and at one-quarter the size. ese three alternatives are shown in Fig- ure 4. e four-layer option was too radical for them, but the blind-via option turned out to work very well and was adopted. Later, the Jap- anese followed the miniaturized-HDI option. We built samples of the two options, with two versions of the HDI option: one in rigid FR-4, the other in polyimide flex. e remark- able feedback was, "Can we keep these for test- ing? Our R&D says the HDI boards are not possible to build." IPC-2315 HDI Design Guideline e design report on the HDI alternative proved to be so successful that an IPC subcom- mittee was formed to create a new document, the IPC-2315 HDI Design Guideline. is was first published in 2000 and the committee was comprised of interested OEMs, PCB fabrica- tors, and interested suppliers. e guideline is still available today, with updates and the coop- eration of the JPCA. Density Modeling If the goal in using ultra-HDI is to take advan- tage of its density reduction capabilities, then the use of density models in the design report or new design guideline is appropriate. ese models can help calculate the density improve- ment and size or layer reduction with using finer traces and spaces from ultra-HDI design rules. If the board has already been designed, then the CAD system can report on the "Manhat- tan distances" of the traces on each layer. Mea- suring the total routed area on each layer will give you the centimeter per square centimeter metric to calculate what the size would be for ultra-HDI routings. If the boards have not been designed yet, then there are seven wiring mod- els that I have used and studied 2 and the one I favor is the statistical wiring model by Coors, Anderson, and Seward of the Colorado School of Mines (Figure 5). Where Do You Start? Between 2000 to 2010, I recommended to OEMs and fabricators that they should not jump into using HDI until some preliminary Figure 4: The three different constructions and two size options for the new cellular phone in 1993.