PCB assembly houses encounter problems despite having precautionary measures in place. Each day, the engineering department responsible for product development and enhancement addresses these issues to ensure meeting deadlines and adheres to standards to maintain quality. Below are issues an assembly house experiences and the simple solutions the assembly team may do to prevent them from recurring.
Faulty PCBA due to parts substitution
Many factors cause faulty electrical connections in a printed circuit board. One factor is due to parts substitution. When a part or some of the components are unavailable, sometimes assembly team uses substitute to replace incomplete parts. In some cases, substitution is okay and the final output is not affected.
However, substitution during PCB assembly is only applicable if the bill of materials (BOM) indicates possibility of substitution. Most of the times, the substitute are the same in all aspects such as dimension and functionality except for brand. However, when substitution is different by only a fraction of a millimeter in dimension or one function is missing in an integrated circuit, the final batch output becomes faulty.
Never use substitute when it is not in the BOM. A fraction is already a big difference to the final output. If in case the part is not available and substitution is not viable, the assembly department should postpone PCB assembly until the part is available. Alternatively, the team should contact the customer for any possible substitute.
Paste Stencil Problems
Solder paste is placed into the board before reflow soldering. The process is paste stenciling. Correct stencil is crucial in this process because it tells the machine how much of the solder paste should be “squeezed” into the board solder pads. Problems encountered with stencils are incorrect design, insufficient thickness of the stencil and incompatibility issues.
These problems affect the paste printing process. Incorrect paste printing is tantamount to defective printed circuit board. To avoid such situation, inspection is the best remedy. Before loading the stencils, make sure that the stencil forwarded to the assembly department is the correct one. Otherwise, request for the right stencil before proceeding.
Incomplete components delay operation. When the situation is recurring, the assembly house should evaluate what causes it. One cause might be supplier problems. If this is the cause of delay in operation, perhaps it is time to look for a new supplier who can deliver the goods in time.
Another cause might be lead inventory time and quantity, wherein the buffer inventory until the next delivery of the supply is not sufficient. Factors that affect lead inventory are the influx of customer orders, defective components, and environmental factors that affect delivery time. Re-evaluate these factors and adjust order quantity to make up for the insufficient quantity of components for assembly.
Accumulation of Moisture
Improper practices cause moisture to accumulate inside the packaging of parts and board. When moisture accumulates, “baking” is necessary to remove the moisture. Baking is the process of heating the assembled board, which may add stress to the soldered components. Besides, the process entails additional expenses to the total overhead costs.
To avoid moisture accumulation, the PCB assembly team should exercise proper handling and storing of the components and board. Use cabinet with moisture sealants to stack and store work-in-process. Always inspect packaging for any tear and immediately replace it. Avoid delays in loading components to the placement machine. Keep the work place free of moisture.
In any PCB assembly department, problems arise from time to time. To avoid any issues, inspection before proceeding is the best remedy. Another way of preventing issues on faulty boards is to create a prototype before mass production.