How to build high reliability prototype circuit boards
by Sandro Sfregola [21010294] <chroma.tech.one@gmail.com>If you have to build a prototype or a single piece of a circuit for your own use (like those you can find in my projects), you don't need a traditional printed circuit board; you can build a high reliability substitute that, if well done, will last for life; here is a description of materials and tools required: with a little practice you should be able to build very compact boards, with a component density comparable with normal (not SMT!) double sided PCB.
Wire-wrapping vs. soldering
Wire-wrap is widely used for prototyping in pre-production environments as well as an alternative to traditional soldering in industrial production; I've seen a NASA paper that recommend wire-wrap as a high reliability substitute to soldering in aerospace applications (high G force); nevertheless, I never use wire-wrap for the following reasons:
- high overall thickness of the finished board ( not suitable for permanent installation )
- a lot more work if you have to do modifications of the circuit
So, unless you are planning to put your circuit in a new satellite, I suggest you to consider point to point soldering; to do this, you need near the same materials and tools as for wire-wrapping.
Required tools and materials
- a pre-drilled, 1/10 inch grid, fiberglass prototype board; you could find it in several sizes in a good electronic store; it is easy to cut fiberglass with a hack-saw
- teflon insulated, solid strand AWG 30 wire; this is relatively expensive but teflon is easy to strip and stable at soldering temperature
- soldering wire; use the best you can find, 60/40 alloy with an organic fluxant
- a precision wire cutter
- a special AWG 30 wire stripper (expensive)
- a good soldering tool: if you are serious about soldering you need a good, temperature controlled tool and some different tips; the suitable working temperature should be about 300 - 330 Celsius degrees
- a spray de-flux solvent and hard brush for solder-flux cleaning
- the usual electronic workshop tools
Ready to go ...
As a general rule, I recommend the following steps in board construction:
- first, a good planning in components position; if you have a PCB design software, use it and try different solutions; an efficient positioning mean a better layout; place decoupling capacitors near the respective ic; remember that you need fixing holes on the board!
- solder all the components and ic sockets in place (I recommend good gold-plated ic sockets) and clean all the solder flux with solvent and brush; dry with compressed air
- start the connection work with the shortest wires; try to not overlap wires if possible; keep separate analog and digital supply/grounds; remember: if you run a long wire parallel to another, you make a transformer! in digital/only circuit this should not be a problem but in mixed/signal situations it can be; as a general rule, crossing wires minimizes both inductive and capacitive coupling
- it is better to work with a copy of the schematics and mark the connections you are doing with a pencil
- perform a final control with a multimeter; check all the connections and for unwanted adjacent pins shorts
- clean the residual solder flux as above
- insert IC in sockets
An example
Here is an example of a useful tool that I designed and built about 13 years ago; it is an in-circuit, EPROM emulator for firmware development and debugging; as you can see it is very compact although the circuit complexity.
