Replacing a 40 Pin COB With a 44 Pin QFP

Some years ago I found that my Mastech multimeter (of early 2000's vintage) was playing up, and so I made a reference to re-calibrate it, which I documented here.

However the meter kept deteriorating, and the best advice I could get was that it's internal reference was failing. I really like this meter and want it to work, so set out to fix it. At this point things got complicated.

After mapping out the circuit from the meter's PCB, I determined that it was using a 7106 voltmeter chip, which is widely used and commonly available. The meter's internal reference is inside that chip. It's normally available in 3 forms: a 40 pin DIP (dual in-line package), 44 pin QFP (quad flat package), or COB (chip on board). The COB package is very cheap for manufacturers to install and very reliable because the bare silicon is glued to the board, connected with very fine wires which are welded on, and covered with a blob of epoxy. Even if you could obtain a replacement, this makes it impossible to replace in a like for like manner.

I have read of someone removing one of these and finding the tracks underneath actually match the QFP version of the chip, which made for an easy replacement. I was not so lucky - the COB has only 40 pins, and Mastech provided the corresponding number of tracks. Not all the pins are in the same position, and the straight portion of the tracks is completely hidden by the new chip. Creative repair was necessary.

You need:

• Board with chip you need to replace
• Replacement chip
• Small files
• Glass fibre abrasive pen
• Thin magnet wire
• Soldering iron - temperature controlled is preferred
• Thin solder
• Wire cutters - precision type is preferred
• Fine tweezers
• Polyamide tape (Kapton is well known but "Koptan" is much cheaper)
• Acrylic paint. I used some from the £1 shop. It doesn't have much pigment.
• Small artist's paint brush (children's type is fine)
• Craft knife
• Desoldering pump
• Desoldering braid

Notes on supplies

The type of magnet wire you use is important. Don't use the Plain Old Enamel type. You need the sort where you can solder through the insulation (which is most common). Since magnet wire is expensive, it's good to save old transformers to take the wire from. You can dismantle ferrite core types by boiling them for a few minutes first. You need wire thinner than the width of the QFP pins, but not so fine that it's fragile. The 0.1mm wire I normally use for this type of thing is too fine, but the wire I actually used was too thick. 0.2 to 0.3 mm would have been about right. The problem with thicker wire is that it conducts heat too easily and melts the soldered joint you already made.

Don't use wire from motors, it's likely to be plain old enamel and unsuitable for this project.

My glass fibre abrasive pen came from Halfords many years ago. The original casing got crushed to bits, so it's made out of a biro casing now.

The paint isn't strictly necessary. It's a substitute for lost or missing solder resist, so you're just using it as laquer.

Depending on exactly what you are repairing, the best method for this may vary. I started by filing, but then found the abrasive pen to be much more effective (also causes less collateral damage). You could use a mini drill with suitable burr or abrasive attachment. I opted to not do this because of (a) dust, and (b) increased risk of damaging the board.

You'll see a cross-section view of the fine connecting wires as you go through. Remove the dust frequently by brushing it off and tapping the board over a bin. Don't blow it away because you don't want to breath it in.

At some point you'll encounter the actual chip. What I found happened was that it was left with a wall of resin on each side, which needed to be cut away. I could then lever the chip out with the tip of a craft knife.

Once you have cleaned away the chip and all the resin, paint over the exposed tracks and board with a few coats of acrylic paint. You can also touch up any damage caused to the solder resist elsewhere.

Make a pinout diagram of the original chip. This way you know where you are going with the new connecting wires. In my case I found that the COB pinout tracks the DIP pinout, so once I'd identified some key pins (V+, V- and Common) it was easy to just copy the correct pin identifiers to my drawing. The 44 pin QFP also tracks the DIP pinout, but it has 4 unconnected pins, to mess things up.

I decided to protect the tracks using acrylic paint. I used green for no other reason than the board is green. It's not essential, but I thought it would be better to keep dirt off the bare metal if possible. Apply two or three coats. If you do this, don't use black paint, as it's likely to be pigmented with conductive carbon.

Once the paint is dry, place the chip on the board and carefully compare the pinout of the new chip with the track positions that went to the old chip. You are looking for any opportunities to solder pins directly to the tracks. I was able to solder a couple down on each side. First though, you need to insulate the tracks from the pins as the position of the tracks (in my case at least) bears little relation to the position of the pins).

Stick some polyamide tape to the board where the chip will be going. I struggled to stick a single square of tape in the right place, so cut 4 separate strips to stick along the edges, which was much easier.

Put the chip back in place and again locate the pins you will be soldering directly. Using a craft knife, carefully cut through the tape on either side of each of these pins, and pull the pieces out with tweezers. Use the tip of the knife again to scrape the solder resist or paint from each of these spots.

Don't do what I did. I found 3 pins I knew I could solder down, and then later on discovered several more, after I had started soldering. Trying to cut and get out a piece of tape from under a pin is not something I want to ever do again!

Use the biggest soldering iron bit which can comfortably solder the QFP pins.

Solder down the pins you found matching tracks for. Check, check and check again for any other tracks that can be soldered in the same way. At this point it's still easy to unsolder what you've done and connect any extras.

Now the fun begins. I used wire that was a bit thicker than it ought to be. The problem with this is that the pieces are so short that when you solder one end it conducts the heat and melts the end you already soldered. Working with two different temperatures helped a lot. I managed with constantly adjusting my iron, but using a second iron would have been better.

The magnet wire needs quite a high temperature to melt the insulation. Set the iron too high however and it will also frazzle the insulation you don't want to melt.

So, tin the tip of your magnet wire, and pick a spot on the track to solder it to (I found it slightly easier doing the track first, rather than the pin), clean that spot, and tin it. Don't solder the wire on yet.

Find the spot along the wire which will touch the pin, and tin that spot. The ideal place on the pin to solder to is on top of the lower bend.

If adjustable, turn the iron temperature down to a low soldering temperature. Solder the tip of the wire to the track.

Cut the wire at the tinned spot so it will nestle down onto the pin, and push it down into place. Flush cutting wire cutters help here. Stick some polyamide tape over as much of the wire as possible, to stop it moving if the other end melts. Solder the freshly cut end to the pin. You may have to adjust - tweezers help, but pushing with wire with the tip of a craft knife lets you make tiny adjustments, and prize it out from between pins when it gets stuck.

Pre-form any wires which need to be bent so you can get the correct length.

Test each connection as soon as it is completed. Use a continuity tester to check from the IC pin where it enters the body of the chip, to whatever is at the other end of the track you soldered its wire to. This way you ensure the connection is device to device, and not just between two blobs of solder. Also check for shorts between pins, or other adjacent connections.

Finally, test your equipment! You can see from the photos that I had a bit of a connectivity problem with one of the display segments, however this proved to be due to the zebra strip not being quite aligned, rather than a soldering problem.