Analysis of reasons and countermeasures for PCB size expansion and shrinkage

During the processing process from the inner line pattern transfer of substrate to the outer line pattern transfer for several times, it will cause the different growth and contraction of the warp and weft of the slab.

From the FLOW-CHART of the entire PCB production, we can find out the reasons and processes that may cause abnormal expansion and shrinkage of board parts and poor size consistency:

1. Dimensional stability of incoming substrate materials, especially the dimensional consistency between each laminating CYCLE of the supplier; Even if the dimensional stability of different cycles of the substrate of the same specification is within the specification requirements, the poor consistency between them may lead to the out-of-tolerance of the graphic size of the subsequent batch production of plates due to the difference between different batches of plates after the first plate trial production has determined a reasonable inner compensation. At the same time, there is a material anomaly is found in the outer pattern transfer to the shape of the process of the plate shrinkage; In the process of production, it was found that the width of the slab and the length of the shipping unit had a serious contraction in the ratio of the outer pattern transfer ratio to 3.6mil/10inch during the data measurement before contour processing for some batches of sheet parts, and the specific data was shown in the table below. After tracing, the X-RAY measurement and the outer pattern transfer ratio of the abnormal batch of plates after the outer layer compression are both within the control range. At present, no good method has been found for monitoring in the process monitoring.

2. Panel design: The conventional panel design is symmetrical, and has no obvious influence on the size of finished PCB under the condition of normal graphic transfer ratio; However, in order to improve the utilization rate of board materials and reduce the cost, a part of the board uses the design of asymmetric structure, which will bring a very obvious impact on the consistency of the graphic size of finished PCB in different distribution areas. Even in the process of PCB processing, we can find that the alignment of the plate with this kind of asymmetric design is more difficult to control and improve than the conventional plate in the process of laser blind hole drilling and outer pattern transfer exposure/flux resistance exposure/character printing.

3. An inner graphic transfer process: It plays a crucial role in whether the size of finished PCB board meets customer requirements. For example, if there is a large deviation in the film ratio compensation provided by a transfer of inner LAYER graphics, it will not only directly lead TO the failure of the finished PCB graphic size to meet customer requirements, but also cause the subsequent abnormal alignment between the laser blind hole and the bottom connecting disk, resulting in decreased insulation performance between LAYER TO LAYER and even short circuit. And the pass/blind hole alignment problem in the process of outer pattern transfer;

Based on the above analysis, we can take appropriate measures to monitor and improve anomalies.

1. Monitoring the dimensional stability of incoming substrate materials and the dimensional consistency between batches: conduct dimensional stability tests on substrates provided by different suppliers regularly, track the difference of longitude and latitude data between different batches of sheet materials of the same specification, and analyze the test data of substrate materials with appropriate statistical techniques; In this way, suppliers with relatively stable quality can be found out, and more detailed supplier selection data can be provided for SQE and purchasing department. The severe shrinkage of the sheet after the transfer of the outer pattern caused by the poor dimensional stability of the substrate in individual batches can only be detected by the measurement of the contour production first plate or during the shipment review. However, the latter has higher requirements for batch management, which is easy to appear mixed plate in mass production of a certain number.

2. The design scheme of symmetrical structure should be adopted in the aspect of panel design, so that the growth and contraction of each shipping unit in the panel remain relatively consistent; If possible, communicate with the customer to suggest that they allow the process side of the board with etchings/characters and other ways to specify the location of each shipping unit in the board; The effect of this method will be more obvious in the plate with asymmetric design. Even if the size of each unit in each plate is out of tolerance due to the graphic asymmetry, or even the partial blind hole bottom connection anomaly caused by it, it can be very convenient to determine the abnormal unit and pick it out before shipment, so as not to flow out and cause complaints from customers.

3, make the multiplier first plate, through the first plate to determine the production of the first inner pattern transfer multiplier; This is particularly important when changing substrates or P-pieces from other suppliers in order to reduce production costs; When it is found that the plate is out of the control range, it should be processed according to whether the unit pipe position hole is secondary drilling. If the plate is a conventional processing process, it can be released to the outer graphic transfer according to the actual situation through appropriate adjustment of film multiplier; In the case of secondary drilling plates, special care should be taken to handle abnormal plates to ensure that the graphic size of finished plates is the distance between the target and the pipe hole (secondary drilling); A list of first plate multipliers of secondary laminates is attached;

4. Process monitoring: Using the inner target data of the outer or sub-outer plate measured in the X-RAY production of drilling pipe position holes after its lamination, analyze whether it is within the control range and compare with the corresponding data collected by the qualified first plate to judge whether the size of the plate has abnormal expansion or contraction. The attached table is for reference. According to theoretical calculation, the ratio here should be controlled within +/-0.025% to meet the size requirements of conventional plate parts.

By analyzing the causes of PCB size increase and contraction, we can find out the available monitoring and improvement methods. We hope that the majority of PCB practitioners can get inspiration from this and find the improvement plan suitable for their own company based on their own actual situation.