Guidelines for Populating and Initial Testing of PCBs

Although it is tempting to immediately solder on all of your parts to your newly arrived PCB, a more organized approach will often save you a tremendous amount of debugging work. The following guidelines will help you to increase your chances of getting your PCB working the first time.

WARNING - Both soldering and powering up a board for the first time are significant eye hazards. Always wear eye protection when performing either of these tasks.

Initial Inspection and Testing

  1. Visually inspect your board - make sure the traces look clean and the holes are aligned properly. Confirm there aren't any blotches or bad spots on your board.
  2. Check for shorts between your power lines/planes and ground. Hopefully you won't have any - if you do, these are identified and fixed much more easily without parts on the board.
  3. Check for shorts between your signal lines and power/ground. Once again, identifying these early on allows the easiest fixes.

Fixing Boards

Most problems with PCBs can be fixed relatively easily if the error is on an outside layer and is a low-speed line. Problems with high-speed lines and lines on inner layers will require advanced solutions.

bulletShorts. Shorts can be fixed by cutting the offending portion of wire. Draw a sharp knife across the trace to cut it. Confirm that it is disconnected with an Ohmmeter.
bulletUnconnected lines. To add a connection (jumper), follow the traces you wish to connect to a good connection point such as a hole or pin. Holes make the best connections, but you can carefully connect to the legs of most SMT parts as well. Solder in a piece of fine-gauge wirewrap wire. Do not try to solder a wire directly to a trace unless you are truly desperate. This is unlikely to work and almost certain to fail over time.
bulletChanges. Combine cuts and jumpers to correct your circuit. Think before cutting! Careful planning can make the difference between an easy job and an impossible task.
bulletAdding ICs. To add an IC, you'll need a place to put it. If you didn't put in an array of holes for this purpose, your best bet is to glue the chip upside down to the PCB and carefully solder wirewrap wires directly to it. This can be tricky and is easily broken, so avoid this unless necessary.

Recommended Population and Testing Procedure

The procedure listed here phases the board population procedure over time. A single system such as a power supply or ADC is placed on the board, powered on, and checked out. By following this method, placement, PCB, and soldering errors can be isolated to the system most recently placed, which will save a great deal of debugging effort in the long run.

  1. Have a couple of team members start with the power supplies. The other members should proceed to step 2. Populate a power supply that is powered by your primary power source, turn it on and see if it produces an appropriate voltage. If it's close, great! Move on to the next one. If not, check your work against the schematic and get it working before moving on. (Remember that switching power supplies often require a nominal load in order to operate.)
  2. Most systems have a primary control chip (MCU or other) with an in-circuit emulator (ICE). While some team members are working on the power supplies, others should familiarize themselves with the ICE and development system. Most ICE systems have evaluation boards you can practice with - make sure that you can connect to and download programs into the evaluation system before trying it on your own board.
  3. Once (1) and (2) are complete, place your primary control chip (MCU or other) and any necessary accessories such as crystals and bypass capacitors. Hook up your in-circuit emulator, power it up and see if it connects. (Remember that you often have to specify to the ICE whether the chip is powered by the board or from the ICE - do so appropriately.) If you can connect, you probably have the chip hooked up reasonably well. If not, there may be a soldering error, a design error, a failed ICE, or a lack of understanding of how the ICE works. Check for soldering problems first, then make sure that the ICE still talks with the evaluation system, then check over your design.
  4. Hook up your analog systems that don't require MCU control (i.e. amplifiers and filters). Establish that these work at a basic level.
  5. Most external digital systems (i.e. ADCs) require control from the MCU, so you'll need to write basic device drivers to check if these work. Solder on the components for the system and write basic device drivers concurrently. Fire them up and see if they work! If not, your problems could be soldering, hardware design, software bugs, timing issues, MCU problems, etc. This is the part where you'll be glad you're only working on one small system at a time.

Kevin Bolding January 02, 2007