How to Rework PCB Assembly?

Reworking PCB assembly is often a possibility when manually creating a circuit board. Circuits can be rearranged to achieve the desired function or to troubleshoot.

When handling the circuit board, handle by the edges. This way, contaminates are reduced. It will also prevent the risk of snapping off any of the leads. Turn the circuit board over and check for proper soldering of the leads. Then use the volt/ohm meter to check for proper sequencing and placement of the diodes, capacitors, transistors and resistors. Start reworking only after properly assessing which of the components is in the wrong place, soldered backwards, component wrongly connected or placed backwards. All these can be remedied by reworking the PCB assembly.

Pcb assemblySolder can be removed using a solder wick or a braided copper tape. A solder wick is recommended for reworking single sided boards. This does not work well on two sided boards that have plated through-holes.

To rework a PCB with plated through-holes, a solder sucker can be used. This is a device with a plunger. Once the plunger is released, a vacuum is created, sucking the solder out through the hole.

If no solder sucker is available, the cheapest method to rework a PCB assembly with two sided boards with plated through holes is by cutting the errant component out. The component is removed but a considerable amount of solder may be left behind within the hole. The solder can be gently heated until it freely flows out for removal. Be careful when melting the solder off to avoid damaging the pad. Another way to rework the PCB is by inverting the board and melting the solder until the component is loose enough for the lead to be pushed off the hole.

Another way to de-solder is by applying liquid flux. Apply a small amount to the component’s solder joints. Align the tip of the de-solder to the lead of the component to be removed. Lightly make contact between the de-solder tip and the component lead. Make sure to keep the tip of the de-solder off the pad to avoid damage. Slide the de-solder tip around a film of solder. Avoid putting pressure on the lands and conductive patterns while moving the solder extractor tip. Wait until the solder starts to melt. Then rotate or move the de-solder tip in a circular motion.

Continue doing so until the solder in the solder joint has completely melted and the component is dislodged. Activate the vacuum immediately to extract the solder from the joint. While applying the vacuum, continue with the circular motion of the de-solder tip. This motion allows air to cool the lead on the component and the plated through hole. Cooling prevents the re-sweating of the component lead to the other side of the plated through hole. Once all the melted solder has been vacuumed from the solder joint, slowly remove the tip of the solder from the lead of the component being removed. All the while, continuously apply vacuum until the de-solder tip is cleared from any remaining melted solder. Then turn off the vacuum. Gently remove the component from the PCB assembly.

After removing the errant component, rearrange and re-solder the leads. Make sure to have good connection. No solders should overlap or leak through the opposite surface. If a connection is out of place, cut out a segment of the trace. To cut the trace, use an X-acto knife. Make two small slices on the trace, ¼ inch apart. Heat the cut trace segment with a soldering iron. Apply firm pressure on the segment and keep heating it up.

Once the segment loosens, gently lift off the cut trace segment off the board. Sand the edge of the cut trace segment if the trace needs to be rerouted. Sand just enough to expose the copper, that is, if the board has a solder mask. Make the necessary new connections with a wire wrap- a 30ga, insulated, single strand, tinned wire made of copper. Test the PCB assembly again to check for function.