PCB Assembly Test:
Given the same test cost, circuit board testing will compromise the defect rate and the yield indicator. For example, if the defect rate is quite high, most of them need to be diagnosed. It is more economical to use an automatic tester to obtain a circuit board image for diagnosis, but to use an automatic tester, it is necessary to provide test contact points for all nodes on the circuit board. This indicates that at least one node of each wiring network needs to be connected to an automatic tester. If the item defect rate is low, manual diagnosis of each defect is required, which takes more time than using an automatic tester, which reduces the impact on the yield, but the node test rate is less than 100%. In theory, if the defect rate is quite low, a 0% node test rate is allowed (no needle bed test is required). As long as a simple pass and fail test is adopted, it is impossible in today’s reality. Here, people only need to use the etched dedicated test connector, and all they have to do is throw away the unqualified assembly instead of performing a diagnosis.
Factors to consider when determining the node test rate include:
1. Defect rate
2. Judgment ability
3. Yield rate
4. Circuit board area
5. Circuit board layers
When determining the node test rate in the circuit board wiring design, it is necessary to balance all the many factors corresponding to manual fault-finding, and also consider the impact on the yield of the made circuit board. Unless there are no defects in the assembly, full node testing is still the best option.
It should be realized that once the design of the circuit board is completed (the node test rate is fixed) and its test plan has been designed, the defect rate becomes the primary factor in reducing costs. Therefore, defect report analysis and defect correction and prevention are necessary. This can include establishing close relationships with sub-suppliers to reduce problems with supplied components and circuit boards, as well as reducing processes and operations in the production workshop that may cause problems.
Another problem to be solved before assembling wiring is to study the rework strategy. Due to the limitation of testing inner layer conductors, the rework of multilayer printed circuit boards is quite difficult or even difficult to achieve. A typical rework process includes cutting the inner conductor with a special tool, aligning the working position on the circuit board with a programmable drill bit, and then drilling a hole with a controllable depth to break the connection with the inner conductor or raising the feet of the surface package component. This is why the circuit board manufacturer needs to fully test the circuit board products before assembly, so as to avoid the complicated testing necessary for the rework process of the installed component circuit board. For surface-mounted circuit boards with densely populated components, sometimes a component has to be removed to enter the rework area (where the inner conductor has been cut). SMT component application rules stipulate: Unless absolutely necessary, components cannot be removed from the circuit board. This will reduce the adhesion strength between the pad and the circuit board and cause the pad to warp.
The rework strategy includes: dismantling the components and reinstalling new components. In this way, test through-holes, land shapes and any details related to assembly need to be considered, so that any details related to rework, and repair need to be considered, so that the tools necessary for rework, and repair have the ability to complete their special workability.
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