design rules for classic computer PCB projects

Hi! I've been doing some PCB design work for various classic computer projects and would like to discuss and hear some advice from experienced PCB designers regarding design rules. What design rules do you normally use for vintage and/or classic PCB projects? Clearly the design rules depend on the application and technology involved. Normally, my projects are in the style typically from the late 1970's and early 1980's style technologies such as PC/XT ISA bus, ECB, S-100, etc. The components are typically 74LSxxx TTL DIP parts meant for easy and reliable construction. There are no SMT components or very high frequency signals. My typical starting design rules are 17 mil traces for signal, 51 mil traces for power (VCC and GND), air gap (min clearance) of 11 mils. Grid size varies depending but usually is 25 mils although I prefer 50 mils. Component pads are typically 55 mils diameter with a 32 mil hole. Connector pads are 60 mils diameter with a 40 mil hole. These parameters allow for a single trace to be routed through the pins of a typical device. Vias are 45 mils in diameter with a 25 mil hole. There are no blind or micro vias allowed since all of my projects are use 2 layer PCBs. Normally the signals for my projects are in the range of 1-8 MHz. Occasionally there is signal as much as 16 MHz clock but those are few. The harmonic content would contain higher frequencies since this is almost entirely digital circuitry and square waves are prevalent. I aggressively review and optimize the PCB layout and trace routing to minimize vias and overall trace length. This phase can be very time consuming but I think it is worth it. Of primary concern to me is trace length due to legacy busses, board density, size, and mainly using 74LSxxx technology. I've found longer traces can result in greater impedance for both signal and power which can interfere with proper operation. As a result I am using the 17 mil signal traces and 51 mil power traces to minimize impedance and the resulting voltage drops. Long traces can also have other effects which can raise havoc on digital circuits. Solid power supply rails is important so when I think a design is going may use a lot of power I specify 2 oz copper weights although for most PCBs it is only 1 oz. The wide signal and power traces are where I think my designs might be overly conservative. The really old PCBs I have seen are as much as 20 mil signal traces but most recent PCBs are less and around 10 mils and sometimes smaller. Minimizing switching transients is important so I normally place a 0.1 uF bypass monolithic ceramic capacitor per IC. Normally there is a larger 22 uF electrolytic or tantalum bypass capacitor placed across the main VCC to GND supply rails at the entry point to the PCB. So far the PCBs have turned out to be fairly reliable and easy to build. There have been a couple of errors but those are cases of where signal or power traces were either connected wrong or left off entirely due to EDA library problems. Things seem to be working fine although I am curious as to what other designers are using and why. I would like to improve my projects to make them the best possible and if there are ways to reduce trace length and minimize the number of vias. There are some sources of information on the internet regarding PCB design but much of the recent material is focused on SMT, low power, and/or very high frequency components. Often times I see trace widths less than 8 mils which to me seem quite hazardous. Here is a good link for some discussion on how to design a PCB for reliable manufacturing. It is about the right age for the projects I like to work on and the design rules are fairly consistent with mine but not exactly. I tend to be a bit more conservative with regard to larger trace width than the author. http://www.pic101.com/using_tango.htm Thanks in advance for any advice and/or insight on proper PCB design. I appreciate your thoughts and comments. Please no flames, this is intended to be a serious and on topic question. If you are going to change the topic please make a new subject. Thanks and have a nice day! Andrew Lynch