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28 PCB007 MAGAZINE I SEPTEMBER 2018 Process Challenges It is clear that the mSAP processes share com- mon steps with the more traditional PCB pro- duction routes, so in principle it would ap- pear to be quite straightforward to update and stretch existing facilities in order to make the change from subtractive to mSAP production. However, due to many of the changes involved, it is typically seen as simpler, and as has been proven recently by those vying to enter the SLP supply chain, certainly better to make the re- quired investment into new equipment and processes which are tailored to the mSAP ap- proach. One obvious concern after such an in- vestment would be the expected lifetime of the mSAP processes. Fortunately, these can be fur- ther revised into an amSAP approach, which enables >20 mm L/S with limited further in- vestment being required. Table 2 shows a simple comparison of the technologies available, including their capabil- ities and major differences. For those looking to enter the SLP supply chain and begin to provide suitable boards, there are a number of challenges that need to be overcome, some of which can be resolved through investments, others which can only be achieved through working with experienced process suppliers. Improved Laser Drilling While the new generation of laser drill tools will surely arrive in due course, the current CO2 machines will remain the workhorse of micro- via processing for some time. Updated for com- patibility with the reduced foil thickness used in mSAP, chemical processes can pretreat Cop- per surfaces to maximize CO2 laser absorption and achieve improved hole shape, while ensur- ing the best possible conditions for subsequent plating operations Optimized PTH Reliable PTH coverage in microvias is criti- cal to not only mSAP production but the an- ticipated reduction in via dimensions, along with a broadening range of dielectric materi- als, means that the electroless copper process- es will also require review. Robust activation systems combined with optimized pretreat- ment systems will be required to work with high throwing power electroless copper baths to ensure via coverage, be that in vertical or, as is more common, horizontal tool sets. Pattern-Plated Copper Filled vias, be they full through-hole or stacked microvias, are essential to modern mo- bile PCB designs. However, for SLPs via fill must be achieved along with fine line pattern plating and excellent surface uniformity, all within acceptable plating times. Fine Line Formation <30 µm lines require fine line imaging pro- cesses not typical to subtractive processing. Table 2: Outline comparison of PCB production techniques.