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April 2015 • The PCB Magazine 13 NExT-GENERATION ULTRA-THIN HDI PCB MANUFACTURING CHALLENGES continues FeAture The first UV lasers introduced in 1997 were Nd:YAG lasers @355 nm. These lasers were well- focused with small spot diameter, so trepanning and spiraling methods were used. They were ca- pable to cut copper and resin very nicely but had problems with FR-4 due to the poor ab- sorption by glass. The product need to use resin coated copper as a build-up material instead of FR-4. Productivity was low and machines had issues with power stability. While stability has improved and rated power increased dramati- cally, glass ablation remains a problem and pro- ductivity is not comparable with today's CO2 lasers, so UV remains a niche process for special applications. Some companies started to use UV lasers in combination with CO2 lasers. This can be an- swer for prototyping and small volume produc- tion but is not cost effective answer for the high volume production. 1998 marked a dramatic increase in demand of the microvia boards. In response, the main- stream PCB makers standardized etching + CO2 laser and new lasers with no missing pulses and higher rep rates were introduced to market. Pro- ductivity improvements at last made this cost effective for high-volume production. Process was also very robust and stabile. By the mid- 2000s the leading edge PCB makers started to develop direct drilling trough copper. The cop- per is etched down in thickness to 5–12 µm and the surface of the copper is treated to be rough and dark prior drilling. The merit of this process is to reduce cost and eliminate via ap- erture etching. This is the leading method to- day for producing any layer microvias, how- ever, the weakness is that the process window is very narrow and rework impossible. On the quality point of view it is a great challenge to have stabile production for microvia less than 100 µm with optimized microvia shape without excess copper hang, glass bundles, resin smear etc. as these will cause downstream problems in desmear and plating. The future development of CO2 laser drill- ing is going to be dominating for a while. How- ever new lasers with picosecond and femtosec- ond pluses are coming to market that show very promising results in speed, quality and productivity. This may be the direction where industry is going when facing the challenge of small laser via size. Thermal damage for the Figure 3: The top picture is 150 µm microvia with 100 µm dielectric, the typical dimensions in the late 90s. In the middle 100 µm microvia with 60 µm dielectric the typical design from early 2000. The bottom picture is today's 60 µm microvia with 40 µm dielectric.