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November 2017 • The PCB Design Magazine 41 pressure, which ensures that the microvias are properly plated. Positioning the solder mask correctly against the pattern poses a significant challenge, since extreme components, for example 01005 and μBGA circuits with 400 μm or finer pitches, have to provide registration down to 37 μm, or in extreme cases, 25 μm. To achieve this, CCD exposure units are required. PCB makers now have the possibility to use special LDI units to expose the soldermask, as the soldermask manufacturers have developed special soldermask inks, to support HDI designs that require lower energy to polymerize. Looking "Under the Hood" NCAB Group has to carry out a thorough examination of all aspects of a factory's produc- tion processes and equipment when assessing whether it meets the demands for HDI manu- facturing. It's like looking under the hood and servicing the car before buying it. If a factory claims that it has laser drills, and is therefore capable of producing reliable HDI PCBs, it's rather like saying that all you need to do to become a new Michelangelo is to get yourself a hammer and chisel. We know that la- ser drilling equipment is not the start and finish when it comes to HDI production; it's equally as important to have the right kind of plating equipment and the right chemistry, as well as knowing how to handle, control and verify the full plating process. We also look at what kind of chemicals and methods they're using, image transfer equipment and procedures and this is in conjunction with understanding the num- bers behind the factories' real experience in this field and their performance, both of which are crucial factors. NCAB's strategy is to ensure that we maintain and develop a best-in-class and secure factory base. We emphasize the importance of the facto- ry being able to not only manufacture HDI PCBs, but also in keeping the number of production er- rors to a minimum. Producing this type of board according to the 3-4b-3 method involves lami- nating, drilling and plating it four times. If they return a 10% rate of failure during each round in the factor y, the number of boards they would end up scrapping would exceed the volumes they deliver. In such a case you need to question the quality of the items that make it through to the delivery stage. When you consider that the components on the board can cost more than 100 times the board itself, it's imperative you can rely on the quality of the board. Otherwise, it can be incredibly expensive, if you're forced to scrap the product at a later stage. Get the Design Right from the Start Yet another aspect you need to prioritize with advanced boards is the design itself. The margins are tiny with regard to such factors as conductor widths, isolation distances between copper features, impedance requirements, hole sizes and their relation to capture and target lands. All this poses a considerable challenge at the layout stage. The design rules should be re- alistic and adapted to volume production right from the start. There are several pitfalls when only considering prototype factory design rules: One example may be making the inner layer cores too thin to produce a good capaci- tive coupling. It might work in a prototype fac- tory, where great care is taken to basically hand- process these thin inner layer cores. However, it could lead to major problems when the product is in volume production, since they may have different capabilities and in this instance the thinner cores may easily get stuck during the processing through long, volume-oriented etch- ing lines since they are basically too flimsy. We thus recommend working with a dielectric spac- ing for a microvia layer of 60-80μm as below this may be problematic and our experience tells us that this design guideline works well across our higher-technology factory base. HDI PCBS: MAKE THE RIGHT CHOICE FROM DESIGN TO VOLUME " When you consider that the components on the board can cost more than 100 times the board itself, it's imperative you can rely on the quality of the board. "