What is printed circuit board assembly (PCBA) rework?
Rework is the process of repairing or refinishing an electronic printed circuit board assembly (PCBA) by de-soldering and re-soldering surface-mount components (SMD), through-hole technology components (THT), or the PCB itself.
Why Rework for printed circuit board assemblies (PCBA)?
Rework is performed manually by hand or at a rework station by specialized personnel trained to identify and replace damaged or faulty electronic components, and to repair soldering mistakes. Some other reasons for rework include incorrect component placement, bad solder joints, electrical defects due to electrostatic discharge (ESD), and exposure to high voltage current.
Rework may also be necessary if counterfeit or refurbished parts have been supplied from brokers or free distribution, which lead to non-functional or semi-functional electronic boards.
Another reason for rework is an assembly mistake made at the EMS company. In other cases, the electronic design of the circuit is the cause of a non-functional board.
Reworking boards can save expensive PCB assemblies. The process can also be used to help uncover (and repair) production errors – and the information gleaned from reworking can be used to help prevent these mistakes from occurring in the future. Rework of printed circuit board assemblies is standard across all industries.
What Makes Rework Challenging?
Printed circuit board assemblies typically have both surface-mounted (SMD) components, as well as thru-whole technology (THT) components that are assembled by hand or with a wave soldering process. Since completed boards have both types of components, the boards can no longer be put in the Reflow oven – this would damage the THT components. For this reason, a special rework station may be needed. This is especially true for finepitch or BGA components.
Why not just de-solder and re-solder by hand?
Some rework is done by hand, however there are many reasons why a full manual process for de-soldering and re-soldering may not be suitable for reworking printed circuit boards assemblies: • Electronic components or PCBs can be damaged during the de-soldering process.
- Hand soldering can put more stress on electronic components and PCBs due to inconsistent or excessive application of heat.
- Hand soldering is very expensive and time-consuming.
- Hand soldering is not a defined process that can be systematically reproduced. Reproducibility of results is an important aspect of quality assurance, particularly for problematic printed circuit board assemblies.
- Some electronic components (BGAs, FPGAs, etc.) cannot be placed manually by hand soldering – a precision camera and precision pick-and-place is needed to solder these parts.
Our Rework Station: ERSA HR550
We have an ERSA HR550 Rework Station at ESO Electronic. This is a guided rework station that uses an infrared heating system to apply heat to the boards from both the top and bottom. The embedded camera system provides computer-aided component placement. An integrated vacuum pipette and force sensor allow for accurate component removal and placement. A process camera captures the entire soldering process and documents the steps performed for each rework job.
What Must Be Considered Before Starting Rework?
The following preparatory steps must be taken before beginning rework of printed circuit board assemblies:
- Electronic components and PCBs have to be baked to ensure that they do not have any humidity, which could lead to damaged circuits after expansion of water vapor.
- THT components have to be protected from heat by applying reflective material or tape, or they may have to be removed and re-soldered after the rework process.
- The rework technician must identify which component(s) have to be reworked, including their position and package/case.
- The rework technician must check the manufacturer datasheet in order to ensure that the maximum temperature for the component is not exceeded.
- The rework technician must identify the thickness of the PCB, the layer count, the final copper thickness and the density of the assembly in order to determine the correct program for appropriate heat application.
- The suitable de-soldering and soldering program has to be selected on the rework station. The rework station offers standard programs, but in special cases, the rework station operator can also select their own custom program.
What is the Rework Process for PCBA?
The following steps are performed to rework printed circuit board assemblies on a rework station:
- The PCB is placed in the rework station and secured in the rework frame.
- The electronic component to be reworked is positioned with a laser beam.
- Temperature sensor is placed next to the board. This monitors the temperature during the de-soldering and soldering process.
- The machine then goes through every step of the rework program. The steps are displayed on the machine’s monitor for the worker to follow. Each step is also recorded with a camera.
- During the de-soldering phase, the machine heats the PCB until the solder becomes liquid.
- In the next step, the machine removes the component from the board and places it on a tray.
- Now the worker can inspect the soldering pads and clean them if necessary.
- The worker then places the replacement component on the tray. The rework station assembly head can now pick up the component.
- Using its camera system, the assembly head precisely aligns the part for positioning on the PCB. The worker ensures the correct positioning and polarity.
- In the next step, the assembly head places the component on the board and solders it to the PCB.
- The process is recorded. Documentation of the rework job can be archived and/or provided to the customer.
How Much Solder Do You Need for Rework?
For BGAs, the solder pads on the PCBs have to be very carefully cleaned. BGAs are made of many little soldering balls, and this means that they usually do not need extra solder for assembly since the solder is built into their design. If additional solder is applied, the BGA will not sit flat on the PCB or may cause a short-circuit. However, if too little solder is applied, the ball might not solder, resulting in an open circuit.
For other electronic components, it is usually the case that the leftover solder from the previous soldering attempt will be enough to solder the new component. Additional solder paste, flu or flux-gel can be applied as needed.
How Do You Know if PCBA Rework was Successful?
After reworking a printed circuit board assembly, there is still some testing that needs to be carried out. For BGAs and other electronic components with leads on the bottom, the parts should be X-rayed after performing rework. This is because you cannot see whether all leads are soldered correctly with visual inspection, as the leads are underneath the component. Another way to inspect BGA is to use a side-microscope, which allows you to look between the small gap between the PCB and the BGA. The first two rows are inspected by focus-shift and the paths between the solder balls are inspected by through light (if the light goes through, there is no short-circuit).
Other electronic components can be inspected with microscope control. The boards may also be tested for functionality using the standard protocol. Solder joints may be cleaned as needed. The rework process should also be documented and provided to the customer as required.
In most cases, properly reworked boards cannot be distinguished from non-reworked boards and have the same life expectancy as non-reworked boards.