Reworking SMD components

In all electronics projects I have done, there has always been few errors. Component value errors are easy to solve by just changing the component, but wrong components or wrong component pinouts are more difficult.

I think I've never managed to do any project where there wasn't any issues with component pinouts. Either some pins are swapped by accident, or I need to use other component (due to design error) which has different pinout.

In this post I will show how I work around wrong pinouts in SMD components.

Backgroud

Some time ago I designed a class D (subwoofer) amplifier . In the circuit I used a high and low side driver to drive the output H-bridge transistors. I selected ADP3120A from OnSemi for the driver, since I thought it would work fine with the 24 V power supply I was going to use.

However it turned out that the voltage rating of the component was not enough, and I needed to change the component. I could not find any pin-to-pin compatible replacement with higher voltage, so I decided to use a IR2104 from Infineon, which is logically compatible, and work my way around the pin differences.

The plan

The pinout for each chip is shown below. As can be seen, only 3 pins are directly compatible, marked with red squares.

Comparison of component pinouts

The plan how to fix the pin incompatibility is to solder the new chip in place with those 3 pins. The other ones will be lifted up with tweezers and then soldered to wherever the pins would go with jumper wires.

To ease the process, I checked the schematic and selected the components that I will solder the jumper wires to. In the following figure different colors indicate where the connection will be made. The blue numbers are the pinout of the new component, while the red numbers (originals) are from the old chip.

Planning of jumper connections in schematic

After this, I checked the layout drawing and marked the correct solder pads there also, with same colors.

Layout drawing with connections marked

After this step, it is quite straightforward to do the soldering.

Implementation

First, before soldering anything, I carefully bent up the pins that were not meant to be soldered down. Then using the 3 pins that were compatible between both chips, I soldered the chip to board.

After the chip was down, I put small pieces of polyimide tape under the lifted pins to prevent accidental solder bridges.

Pins lifted and small pieces of tape added under them

Then I measured pieces of thin single-strang wire-wrapping wire from each pin to the correct pads. I first soldered the pieces, one at a time, to the other pad (resistor pad, diode pad or similar larger pad) first, and then carefully routed the wire to the component pin. Adding a very tiny amount of solder, I then soldered the free end to the lifted component pin.

First jumper wire soldered to pad and pin

After the first jumper wire, I continued with other wires. Always measuring the length, soldering first to the other component, then carefully bending the wire into place and then soldering to component pin.

It is better to solder the wires first to a larger component because then the wire can be bent more safely without risk of breaking the component pin or the tiny solder connection between the pin and the wire.

Also with the wire I used, I found it easiest to not try to strip the wire with any pliers, but instead touch the end of the wire with hot soldering iron. This caused the insulator to shrink and expose just enough of the wire to allow soldering it.

It took some time to solder all the connections, but here is picture of the finished rework.

Finished rework

Maybe not the best soldering job I've done, but it does the trick. A good idea is to add some hot glue or tape to fix the wires into place then.

Happy soldering!