Even with the best production soldering process, you will inevitably come across the need to tackle electronic repairs ranging from simple to complex rework scenarios. Defective components need to be replaced safely after they are removed. Sometimes pads on the PCB need repair due to the identified defects causing local overheating. You may need to remove and replace pads or tracks as well.
Even though the fibreglass, copper and resin layers within the PCB’s construction are specified to resist heat, prolonged exposure to heat during rework might cause track damage or delamination, and also damage nearby components. Your rework strategy needs to take all this into account.
First, diagnose the fault in full. Find out what the question is, in full, before you try to answer it. Hold the PCB steady using tape or jigs and use the appropriate test equipment (oscilloscope, multimeter, PC etc.) to probe signals and waveforms across the various test points on the PCB in order to check continuity and to diagnose the damage. Sometimes a simple visual inspection will identify burned areas near a failed component, or a damaged pad or track. When you have identified the components or tracks on your PCB that aren’t functioning properly, follow these simple procedures below to remove and replace them.
You could use a hot air gun – but this method has a major disadvantage that heat dissipation by the hot gas stream does not stay localised to just the component you are working on. It reaches surrounding components as well, which subsequently can damage those components or their joints. Hot air or IR techniques are however the preferred method for BGA rework, and are applied in specialised workstations designed for the purpose.
For general use, instead of a hot air gun you should use a soldering iron, with a desoldering braid or solder suction tool to remove damaged components and solder residues. This is also a more stable means with which to fit replacement components for the same reasons of heat over-exposure.
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Here are the main criteria to observe for hot iron rework and repair:
- Keep Your Soldering Iron Clean & Tinned – we cannot over-emphasise the importance of keeping your soldering iron tip clean and well tinned. This will be second nature to the trained operator. Without a clean and well tinned soldering iron tip, the heat transfer rate from iron to workpiece is impaired.
- Reduce High Heat input for Board & Components – By ensuring your soldering iron tip is thoroughly cleaned and well tinned, heat is conducted far more efficiently and quickly to the workpiece. Therefore damage to other areas from prolonged heating times will be reduced, and the solder removal wicking action will complete much faster. By exposing your workpiece to high heat for a prolonged period, you risk damaging not only the board area and components in question, but also nearby solder joints and components.
- Match Your Desoldering Wick to Your Contact Pad – desoldering wick comes in a range of different widths. This is so you can match the braid to whatever it is you’re desoldering. If your wick is too thin, it won’t remove enough solder, but if the wick is too thick it will require more heat and this unnecessarily risks interfering with other components and joints on the board. This is a judgement issue : experience and training are valuable assets at this time.
- Match Your Tip to Your Desoldering Braid – Again as part of the judgement, you should also match your soldering iron tip size and the braid width. If the tip is too small, then you’ll need more dwell time and thereby spreading heat to nearby components. If the tip is too big, then you also run the risk of exposing other components to overheating. Optimising the soldering iron tip size and the braid width will eliminate problems and increase accuracy as a result, because the whole process is under control.
- Match Flux Type to Your Cleaning Method – when selecting a desoldering braid, it’s worth noting the various flux types available. The flux you choose will be determined by the application: the PCB & component type and by the end-use application of the product. If it is for example, a PCB for a low leakage piece of test equipment, the wrong flux can leave surface conductivity that will cause the product to fail its own test! Your cleaning process must therefore be matched to the flux, and also to the application. For example, Rosin-fluxed braid has a fast wicking action but does leave residues behind and requires more intense cleaning. Alternatively, no-clean flux braid is available for cases where this is suitable, and minimal cleaning is required.
- Protect Against Corrosion – ensure you use quality flux remover to clean the board. Without doing so, you could be putting your board at risk of corrosion through flux residues. Even with “no-clean” fluxes, a residue is possible because the flux specification assumes thorough heating (as-in a reflow tunnel) to deactivate the flux. Soldering by hand does not give this guarantee. Therefore removing even “no-clean” residues from boards is recommended in order to reduce the risk of corrosion in service (e.g. in damp environments).
It requires professional training to perform these procedures repeatably and to the highest standards, Advanced Rework Technology Ltd (ART) can help you with well designed training courses. We can teach you the skills required to perform to this level. We also offer bespoke courses for specific challenges you face in your production (please enquire), and industry certified training for inspecting the acceptability of all PCBs produced.
Whether you come to ART’s own facilities in Witham, or we bring the equipment to your facility, we can teach you all you require. Please call us today on 01245 237083.
