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40 The PCB Design Magazine • November 2017 More Production Stages Widespread miniaturization is putting far greater demands on the production equip- ment at the PCB factories. Many of the stages in the production of HDI boards are similar to those used for the manufacture of convention- al boards. However, HDI production calls for considerably more sophisticated equipment to achieve the tiny geometries that are required. Not only does incorporating several layers of buried vias and/or microvias into the boards require several additional steps, but these also need to be repeated several times and all that increases the degree of complexity and the risk of error. All the geometries are much smaller on HDI boards, which calls for better dedicated equipment designed for high-tech manufac- turing. Many factories do have laser drills, but there aren't, unfortunately, as many who also possess the appropriate plating equipment and processing experience to enable them to make good-quality, reliable HDI boards. NCAB puts a great deal of time and effort into qualifying and verifying a factory before giving it our seal of approval to manufacture HDI boards for our customers. The first consideration in generating micro- vias is that of advanced laser drills that can gen- erate blind microvia holes with finished sized of 75 μm and below, although most microvias normally have a diameter of around 100 μm. The latest generations of these machines are ca- pable of drilling hundreds of holes per second. Following on from that, the transferring of the circuit pattern onto an HDI board is an equal- ly critical operation that calls for the highest precision, which the traditional photography- based techniques can't achieve. Instead, HDI board makers either use CCD camera aligned imaging machines with parallel lighting, or la- ser direct imaging (LDI) systems, which images the pattern directly onto the bonded photo-im- ageable material. This makes for improved qual- ity as no phototool film is used, thus enabling a much greater accuracy of transfer of pattern features down to 50μm. Prerequisites: Correct Equipment and Clean Rooms To ensure the best possible result in the imag- ing transfer process, it's vital that it's performed in special clean rooms with carefully controlled temperature and humidity levels. The clean rooms that have been used for these processes meet the US federal standard 209E Class 10000. This class has constituted the industry standard for many years now and stipulates that the con- centration of airborne particles ≥0,5 μm (a hu- man hair is typically 20–50 μm thick) in size should not exceed 10,000 particles per cubic foot. Today the best factories have clean rooms that meet the Class 1000 requirements. To give another idea of what this means; the air in our normal everyday environment contains 1 mil- lion particles, of the same size, per cubic feet. However, good quality clean rooms are expen- sive, both to buy and to properly maintain. Producing HDI boards also requires a differ- ent type of plating line. For non-HDI boards, one can usually make do with ordinary plating lines, with vertically held panels that use me- chanical and air agitation, allowing you to get the plating chemicals to facilitate good copper plating onto the surfaces and into the holes (the through hole part of the plating requires good solution flow within the holes or you will not obtain a reliable or uniform plating thickness). However, this method isn't really suitable for HDI boards with blind holes that can measure 100 μm or less in diameter. That's why most factories use both horizontal plating lines as well as vertical continuous plating (VCP) lines. These methods involve spraying the plating chemicals onto the pads under high HDI PCBS: MAKE THE RIGHT CHOICE FROM DESIGN TO VOLUME " NCAB puts a great deal of time and effort into qualifying and verifying a factory before giving it our seal of approval to manufacture HDI boards for our customers. "