Reasons and analysis of PCB size expansion and shrinkage

- Sep 15, 2018-

During the processing of the transfer of the inner line graph from the base material to the outer line graph, it will cause the increase and contraction of the longitudinal and latitudinal direction of the plywood.

From the whole PCB flow-chart production, we can find out the reasons and processes that may cause the abnormal expansion and contraction of the plate and poor dimensional consistency:

1. The dimensional stability of the supplied material, especially the dimensional consistency between each lamination CYCLE of the supplier;Even if the dimensional stability of different cycles of the same specification base material is all within the specification requirements, the poor consistency between the plates can cause the first plate of the plate to be trial-manufactured to determine a reasonable inner layer compensation, and the subsequent batch production of the plate due to the difference between different batches of the plate size is out of tolerance.At the same time, there is also a material anomaly is the outer figure transfer to the shape process after the process of plate contraction found;In the process of production, some individual batches of board parts were found in the data measurement process before the shape processing, and their combination board width and shipping unit length relative to the outer figure transfer multiplier were found to have severe contraction, with the ratio reaching 3.6mil/10inch. The specific data is shown in the following table.After tracing, the X-ray measurement and the transfer rate of the outer layer pressure of the abnormal batch plate are all within the control range. At present, there is still no good method for monitoring in the process monitoring.

2. Plywood design: the plywood design of conventional slabs is all symmetrical design, which has no significant influence on the graphic size of finished PCB under normal graphics transfer rate;But part of the plate sheet to enhance utilization rate and reduce the cost in the process of using the asymmetry of the structure of the design, the different distribution of the finished PCB image size consistency will impact is very obvious, even we can be in the process of PCB processing in laser blind hole drilling and outer graphic transfer/curing exposure/character found such asymmetric design in the process of printing plate piece of alignment in each link relatively conventional panel is more difficult to control and improve;

3. Primary interior graphics transfer process: here it plays a crucial role in whether the size of finished PCB plate meets the customer's requirements;Such as inner graphics transferred film ratio compensation provide large deviation exist the not only can directly lead TO the finished PCB image size can meet customer's requirement, also can cause subsequent laser blind holes and connection plate at the bottom of the registration from abnormal LAYER TO LAYER between the insulation performance until the short circuit, and outer graphics in the process of transfer/blind hole matchup problems;

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

1. Monitoring of dimensional stability and dimensional consistency between batches of substrate materials: regularly test the dimensional stability of substrates provided by different suppliers, track the longitudinal and latitudinal differences between substrates and different batches of sheet materials of the same specifications, and properly use statistical techniques to analyze the test data of substrates;Thus, suppliers with relatively stable quality can be found, and more detailed supplier selection data can be provided for SQE and procurement department.As for the serious expansion and contraction of the plate after the transfer of the outer figure caused by poor dimensional stability of the base material of individual batches, it can only be found by the measurement of the shape of the first plate or the inspection of shipment.However, the latter of the higher requirements on batch management, in a number of mass production is easy to be mixed board;

2. In the aspect of plywood design, symmetrical structure design scheme should be adopted to keep the expansion and contraction of each shipment unit in the plywood relatively consistent;If possible, it shall communicate with the customer to suggest that they allow the specific identification of the location of each shipment unit in the plywood by means of etching/character, etc. on the process edge of the plate.This method in the way of asymmetric design effect is more obvious in the panel, even if every makeup internal asymmetric graphics cause individual unit size out-of-tolerance, even can cause the partial blind hole bottom connection exception can be very convenient to determine abnormal units and handle to pick it before shipment, not outflow caused by abnormal encapsulation incur complaint;

3. Make the multiplier head plate, and scientifically determine the multiplier of the first inner layer graph transfer of the production plate through the bow plate;This is especially important when changing other supplier base material or P sheet in order to reduce production cost.If the plate is found to be out of control, it should be based on whether the pipe hole of the plate is secondary drilling.If the plate is a conventional processing process, it can be released to the outer figure transfer according to the actual situation through the film multiplier for appropriate adjustment;In the case of secondary bore-plate parts, special care should be taken to deal with the abnormal plate parts to ensure the graphic size and distance between the target and the pipe location hole (secondary hole).List of head ratio of the attached secondary laminate;

4. Process monitoring: use the target data of the inner layer of the plate part measured when the outer or secondary outer plate part produces the bore pipe location hole with X-ray after lamination, analyze whether it is within the control range and compare it with the corresponding data collected by qualified first plate to determine whether the size of the plate part is abnormal with expansion and contraction, and the following table can be referred to;After theoretical calculation, the multiplier here should be controlled within +/-0.025% to meet the size requirement of conventional plate.

By analyzing the reasons for PCB size increase and decrease, the available monitoring and improvement methods are found out. It is hoped that the vast majority of PCB practitioners can get inspiration from this and find the improvement solutions suitable for their company based on their actual situation.