PCBs
Prevention: It is vital that conditions within a manufacturing environment are controlled through regular maintenance and management, which should all be part and parcel of adhering to quality compliance and standards.
4. Problem: Copper too thin The copper thickness on a PCB is important, especially for high current applications. High current tracks require more copper thickness and vice versa. The etched and finished copper width is also important and mandatory to maintain. Reason: If it is too thin when used for high current applications, it may act like a fuse. Prevention: Any professional PCB assembler worth its salt should ensure the finished or base copper thickness is designed to the exact requirement, therefore avoiding any issues to emerge later down the line. It should be checked after finished PCB finalisation, either by destructive testing or microscopic testing of the copper tracks.
5. Problem: Insufficient wetting Good solder wetting is crucial to ensure it can adhere properly to the component for an optimal solder joint. Ideally, the solder should achieve 100 per cent wetting with the pad and pin, leaving no gaps or spaces exposed. PCBs with insufficient wetting problems have low strength, and the circuit is not connected or on and off. Reason: Poor wetting can be categorised in two ways, de-wetting or non-wetting. Non-wetting happens when the solder only partially covers a surface, leaving some exposed copper behind. De-wetting occurs when the molten solder covers a lead or pad, but then retreats and leaves behind a mound of solder.
The surface of the welding area could
be contaminated or is stained with solder resist. It can also be caused by failure to apply heat to both the pad and pin, therefore not giving the solder enough time to flow, which can lead to insufficient wetting and lack of bonding. Another reason could be that a metal oxide layer is formed on the surface of the bonded object. Prevention: Using quality components that are still within their usable shelf lives will minimise this issue during the assembly stage. Alternatively, cleaning the board thoroughly is a quick and effective way to remove grease or any dirt that could impede solder wetting.
6. Problem: Environmental factors PCB quality can be affected by environmental factors within a manufacturing facility where the assembly takes place, potentially leading to failure. Boards can become corroded, contaminated, clogged or warped. Reason: Ongoing exposure to elements such as moisture, dust, heat and cold can severely compromise the printed circuit board’s function. For instance, temperature changes can make elements in the PCB contract or expand, leading to the soldering joints and boards becoming damaged or warped. Excess moisture can cause corrosion, rust and oxidation, harming the PCB. Extra dust around the PCB can also gradually build up, eventually clogging the board, which can cause the PCB board to overheat and require electronic repairs. Hair and even bugs can get into the PCB. Another common failure is from not keeping PCBs clean during and after manufacture.
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7. Problem: Starved thermals Thermals are small traces surrounding pads and are utilised to interface pads to the plane. These thermals enable the pads to more effectively disband heat, and are important components during the soldering process. Connectivity issues, soldering problems and PCB overheating can all occur because of starved thermals. Reason: If a thermal is applied inconsistently, the PCB could ultimately have connectivity issues. Insufficient thermals stall the process of heat transference between the pads and planes. As a result, it can make it difficult to solder a board correctly. When constructed under these conditions, the PCB is liable to overheat once it is utilised. Prevention: Identification and replacement of such faulty thermals can be carried out by an experienced PCB manufacturing expert before they have a chance to cause problems in the circuit board.
8. Problem: Electromagnetic issues Two problem areas for circuit board designs are electromagnetic compatibility (EMC) and electromagnetic interference (EMI). EMC is the ability of an electronic system to function satisfactorily in an electromagnetic environment without generating electromagnetic interference in nearby devices or systems. EMI refers to the unwanted and damaging effects of EMC.
Reason: Too much EMI can cause a product to become defective. Prevention: Reducing EMI can help by increasing the circuit board’s ground area and compartmentalising the board specifically for EMI. Increased EMI interference can be caused by 90-degree angles, so should be avoided for most components. Using shielded cables during cable assembly and metallic packaging will absorb EMC, thereby reducing EMI.
A PCB manufacturing expert can help spot and diagnose these potential design defects and if EMC and EMI are identified as issues during the prototyping stage, a redesign may be required.
www.ukcircuits.co.uk Components in Electronics March 2023 43
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