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20 The PCB Magazine • November 2017 Process Controls for Fabricating Reliable Microvias The small blind vias we call microvias pres- ent a unique challenge to printed circuit fabri- cators. Unlike through-holes, the small micro- vias can be extremely difficult to drill, desmear, metallize and plate. Laser drilling can be quite challenging and is much more difficult that me- chanical through-hole drilling. There are seven fundamental quality issues, as seen in Figure 5. Unfortunately, all can only be known by mi- cro-sectioning. Thus, panel coupons dedicated to the fabrication of perfect microvias become a necessity. Bare Substrate Testing The testing of HDI PCB prototypes can be accomplished with flying probe electrical tes- ters, but volume production is still relegated to bed of nails type custom fixturing. Multiple- head flying probe testers do have more through- put, but even those models with twelve heads have only limited production capacity. It is the bed of nails fixture that will have to handle the production load. Unfortunately, the very high- density nature of HDI boards make custom fix- tures very expensive. Gridded fixtures work with modifications, but their 0.1-inch by 0.1- inch pin grid can handle only an average of 100 test points per square inch. HDI boards are usu- ally in the 75−200 average test points per inch with area array land pattern pitches down to 16 mils (0.4 mm). Non-contact and limited contact testing is the new area of product development. Capac- itance of nets to detect opens and shorts has been used for at least 15 years. I saw my first at Bull in France in 1982. They were using HP plot- ters outfitted with probes attached to HP high speed capacitance meters. By memorizing the capacitance of a known good board, the com- puter measured production boards very rapid- ly, looking for higher or lower capacitance, in- dicating open nets or shorted nets. Non-con- tact electrical testing machines have been de- veloped and used in Japan for the last couple of years. Foremost of the Japanese is Nidec-Read Corporation. Their automatic test systems for BGAs, PLGAs, PGAs, MCMs and HDI boards have belt conveyor handling, index tables and step & repeat test heads. Their newest is the TRL electrical tester. TRL stands for Transient Link technology, a signal stimulus injected by spring probe or non-contact transducer that is detect- ed by a non-contact capacitive sensor. It can test a small HDI substrate with 0.06mm pitch in ~4.5 seconds, 7.5 seconds if auto-alignment is required. Panel Coupons Special artwork designed into the panel edges or in unused space on the panel is essen- tial. For additional traceability, if the artwork shared common features on the assembly sub- panel, then there is continuity to whomever does the SMT assembly. Figure 6 shows a small coupon I designed that can be used on fabrica - tion panels as well as assembly sub-panel car- riers. The coupons are detachable and can be tested by production personnel during the fab- rication process. These coupons test for (and test method): • Desmear-metallization-plating quality (10 immersions of 10 seconds each in 288°C solder pot-inspection) • Layer-to-Layer registration and movement directions (continuity and cross-section) • Daisy chain electrical reliability (four immersions @ 288°C of 10 seconds each followed by 4-wire Kelvin measure) • First drilled hole of multiple diam.-last drilled hole of multiple diameter (inspection if present) • Layer-order windows (inspection if present) • CAF reliability (CAF test chamber) 4-Wire Kelvin Testing Electrical test companies need to do further research into the micro-resistance of small blind vias. Using the concept of multiple flying-head probes, they should be able to perfect either DC or AC 4-wire Kelvin measurements that tell us if there are any imperfections in the fabrication of small laser drilled vias. The CAD files make trace widths, thicknesses, vias and net lengths avail- able, to calculate these elements' contribution to a Kelvin resistance measurement. These cal- culations can be subtracted from the readings to measure the resistance of individual microvias. 35 YEARS OF HDI FABRICATION PROCESSES AND OBSTACLES FOR IMPLEMENTATION