Grafting 2 Damaged Circuit Boards

In the picture above we have two circuit boards from two Dental Table Top Sterilizers. The board on the left has had a solder joint crystallize, arc out, and set fire to the back of the board. The board's logic section is still working properly but the burned section of the board is too badly damaged to repair. The board on the right is in good cosmetic condition but has had a failure in its logic circuits. I attempted to replace the eprom chip but I was not successful. When these boards are working properly the logic section of the board provides output to a view screen, processes user input from a key pad, senses temperature and pressure input, as well as door position and controls AC line power output to 5 circuits that heat and control the flow of water and steam. This brand and style board has chronic specific board failures and the sterilizer itself is a very popular brand. These specific design problems with the board coupled with how expensive it is to buy a replacement from the manufacturer has created a niche business opportunity. I have reverse engineered most of the board and since the board is quite expensive a market has developed for repaired second hand boards.

My plan for these two boards is to separate the logic section and the AC driver sections in such a way that I can make one good board out of the two.

I begin the process by removing the components that will be in the path of my separation cut through the board. I have also removed addition components that I typically replace when I overhaul a board. The route of the cut will be planned to lie along the designed high to low voltage isolation area of the board. For this reason the cut will pass between the coil connections and the switched connections of relays, between primary and secondary transformer windings, and between the input and output terminals of opticouplers.

Above is the front and back of the board that was burned due to the arcing of a solder joint and subsequent fire. The lower part of the board will be discarded. The upper part of the board has been cleaned but the discoloration of the epoxy coating remains. It is a cosmetic problem and will not affect the operation of the board. The logic on the board is separated from the AC power section by relays and optocouplers so it was possible to cut the bad part of the board from the good and only sever 4 traces. Notice that there are mounting holes in the four corners of the board and also a hole midway down each side. We will be using these midway holes in one of the next steps of the process.

Here is the front and back of the board with the bad logic section. Notice I have cut this board high enough so that the top edge extends above the cut on the other board. I will use the center holes as alignment and bolt the two boards together. I will then use a jigsaw and follow the contour of the planned cut on the first board to make a matching cut on the second board.

Here is the good logic board, with its strategic cut, bolted, using black screws, over the good AC power control board. I will now carefully jigsaw the lower board following the contour of the existing cut to get a mating edge at the proper location on the lower board.

The lower board has been cut and you can see how well it mates with the upper good logic section. The next step is to clamp the boards into a proper relative position and repair the cut traces and glue the cut. To glue the cut I use a high temperature 500 degree epoxy from Permetex. The cut ends of the traces are cleaned of masking and bridged with a wire and solder. For one very fine trace I ran an additional redundant wire from the trace beginning to it's end so that I could be confident that I would have good future connection integrity.

Here are the two board sections in the positioning vises with the first run of epoxy. I have used black electrical tape on the back side of the boards to prevent the epoxy from dripping through. The detail picture on the right shows two traces that have been repaired and epoxied over. Note that the optocoupler has also been reinstalled across the cut. As soon as the epoxy gets hard we will install the relays and transformer across the cut as well. Once all the components are reinstalled on this side of the board we will flip the board over and epoxy the crack from the back side.

Here is the backside of the board once the epoxy has been applied. I have also reinstalled the fuse clips and the relays. Since this board operates in a moist environment green colored sealer has been applied to newly soldered joints. If you look along the left side of the board you can see the white redundant trace wire that I have patched in. The original trace was less than a millimetre wide and seemed to be a risk for future failure. There is also a horizontal trace in the middle of the board that had to be cut twice and that has now been repaired and epoxied.

The first picture is the completed circuit board. Though this board is now in great functional shape it will not be used in a clinic sterilizer. Most dental equipment repair facilities have sterilizers that are used as loaners while a clinic sterilizer is being repaired. This board will be used to repair a loaner sterilizer and will sell at a reduced price due to its history. The second picture shows the left over bad parts from the process. We have a test jig in the shop that allows us to run a battery of tests on the boards that have been repaired and overhauled. This has given us a zero field failure rate over the last 100 boards that we have processed. In addition to supplying these reconditioned boards to dental service companies and dentists, who want to do their own repair, we also offer a consulting service where we answer questions and assist with trouble shooting disabled sterilizers of this particular brand.

John