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20 PCB007 MAGAZINE I MAY 2024 resist + 35-µm foil divided by 75-µm space or 0.87. e aspect ratio of a 50-µm line etched in 17-µm foil with the same resist is 0.94. From this, we can assume the minimum aspect ratio for a successful subtractive etching process is around 1 (the closer to 1, the more difficult it is to etch but it's still possible for most manufac- turers). It is possible to achieve successful sub- tractive etching with aspect ratios higher than this, but it requires utmost attention to detail and optimization of every process step. For the purposes of this discussion, let's use 1 as the minimum aspect ratio. If you are doing 75-µm lines and spaces on 35-µm copper with a 30-µm resist now, then you should be able to do finer lines and spaces with a reduced aspect ratio without any more problems than you have now (hopefully). Reducing the aspect ratio by using thinner foil might not be acceptable due to increased heat genera- tion, but reducing the thick- ness of the etch resist is cer- tainly a viable option. Dry films come in various thick- nesses depending on the appli- cation. e thinnest I have seen for general application is 15 µm. Going to this thickness will make it possible to do 50 µm spaces in 17 µm foil (aspect ratio 0.64) and just possible to do 25-µm spaces in 9-µm foil (aspect ratio 0.94), which is a help but may still be short of what is needed. And, of course, a 15-µm resist is going to cost signifi- cantly more than a 30-µm resist. e best alternative for reducing the aspect ratio is to use a liquid resist. ere are liq- uid resists available that are basically liquid dry films that can be applied to circuit foil in thicknesses of 5 µm or a little less. ese have an additional advantage in that whatever type of exposure process you use, it doesn't have to deal with a cover sheet like that on hot roll laminated dry films. With a 5-µm resist, the with some difficulty) 25-µm (0.001") lines and spaces in 5-µm (1/8-ounce) foil. e ability to etch a 25-µm line and space in 5-µm copper sounds good with the demand for this type of density becoming more com- mon. Unfortunately, there is something else that must be taken into consideration, in addi- tion to the fact that the thinner foil is more expensive. A line in thinner copper has a much lower cross-sectional area and is more resis- tant to current flow than the same width line in thicker copper. is means that a lot more heat is generated for the same current flow, which may be unacceptable when considering how tightly packed the circuit lines are. e specification for the high-density lines may also include requirements that a 25-µm line has to be 17- or even 35-µm thick. ere are some things t h a t c a n b e d o n e t o improve upon these line and space limits. e key is to reduce the aspect ratio (the combined thickness of the etch resist and the foil divided by the space to be etched). e higher the aspect ratio the more difficult it is to get fresh etchant down into the space and remove spent etchant. In practical terms, this means that, for equal thicknesses of resist and foil, the etch rate in the spaces gets slower as the spaces narrow. Tests have shown that the etch rate in a 25-µm space is only about 60% that of a 125-µm space for the same thick- ness of resist and foil. As a result, there is more time for sideways etch and the ratio of down- ward etch to sideways etch can go from 3 to 1, 3 to 2, or even 1 to 1. e limits above were achieved using a 30 µm (0.0018") thick dry film resist, which is probably the most common thickness in use today. at means that the aspect ratio of a 75-µm space etched in 35-µm foil is 30-µm The best alternative for reducing the aspect ratio is to use a liquid resist.