In the process of PCB processing, from the PCB substrate to the inner circuit pattern transfer through several times of pressing until the outer circuit pattern transfer, the warp and weft of the board will expand and shrink in different directions. From the whole PCB production flow-chart, we can find out the reasons and procedures that may cause the abnormal expansion and shrinkage of the board parts and the poor size consistency:

The dimension stability of PCB substrate, especially the dimension consistency between each laminating CYCLE of the supplier. Even though the dimensional stability of PCB substrate in different cycles of the same specification is all within the specification requirements, the poor consistency between them may lead to the out-of-tolerance of graphic size of subsequent batch production of PCB due to the difference between different batches of board after the reasonable inner layer compensation is determined in the trial production of the first board. At the same time, there is also a material anomaly in the process of outer graphics transfer to the shape of the plate shrinkage. In the process of production, it was found that the width of the panel and the length of the delivery unit had a serious contraction in the transfer ratio of the outer graphics, reaching 3.6mil/10inch. After investigation, X-ray measurement and outer graphic transfer ratio of the abnormal batch of plates after layer upon layer of external pressure are both within the control range. At present, a better method for monitoring has not been found in the process monitoring.

The conventional panel design is symmetrical and has no obvious influence on the graphic size of finished PCB under the condition of normal graphics transfer ratio. However, in order to improve the utilization rate of the board and reduce the cost, part of the board uses the design of asymmetric structure, which will bring a very obvious influence on the consistency of the graphics and size of the finished PCB in different distribution areas. Even in the process of PCB processing, we can find that the alignment of such asymmetric design of the board is more difficult to control and improve than the conventional board in each link in the process of laser blind hole drilling and outer graphic transfer exposure/solder resist exposure/character printing.

Factors of an inner layer graphic transfer process An inner layer graphic transfer process plays a very key role in whether the size of finished PCB board meets customer requirements. If there is a large deviation in the film ratio compensation provided for the transfer of inner layer graphics, it can not only directly lead To the size of finished PCB graphics unable To meet the customer’s requirements, but also cause the abnormal alignment between the laser blind hole and the bottom connecting plate To cause the insulation performance of layer-to-layer To decline and even short circuit. And the through/blind hole alignment problem in the process of outer graphics transfer.

According to the above analysis, we can take appropriate measures to monitor and improve the abnormal;

Monitoring of dimension stability of PCB substrate incoming material and dimension consistency between batches The dimensional stability of PCB substrate supplied by different suppliers is tested regularly, from which the difference of longitudeal-latitude data between different batches of the same specification board is tracked, and the test data of PCB substrate can be analyzed by appropriate statistical techniques. In this way, suppliers with relatively stable quality can be found, and more detailed supplier selection data can be provided for SQE and purchasing department. As for the severe expansion and contraction of board parts after the transfer of outer graphics caused by the poor dimensional stability of PCB substrate of individual batches, it can only be found through the measurement of the first board in the shape production or the inspection of shipment. However, the latter has higher requirements for batch management, and it is easy to appear mixed plate when a certain number is produced in large quantities.

The design scheme of symmetrical structure should be adopted as far as possible to make the expansion and contraction of each shipment unit in the jigsaw board keep relatively consistent. If possible, the customer should be advised to allow specific identification of the location of each shipment unit in the board by etching/character identification on the process edge of the board. 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 first plate, scientifically determine the multiplier of an inner layer graphics transfer first plate, scientifically determine the multiplier of an inner layer graphics transfer of the production plate through the first plate; This is especially important when changing PCB substrates or P sheets from other suppliers to reduce production costs. When it is found that the plate is beyond the control range, it should be processed according to whether the unit pipe hole is secondary drilling. If it is a conventional processing process, the plate can be released to the outer layer according to the actual situation and transferred to the film ratio for appropriate adjustment; In the case of secondary drilled plates, special care should be taken in the treatment of abnormal plates to ensure the graphic size of finished plates and the distance from target to pipe hole (secondary drilled holes); Attached is the first plate ratio collection list of secondary laminated plates. 4. Monitor the PCB board making process by using the inner target data of outer or sub-outer plates measured during the X-ray production of drilling pipe position holes after lamination to analyze whether it is within the control range and compare it with the corresponding data collected by qualified first plates to judge whether the size of the plates is abnormal in terms of expansion and contraction; According to the theoretical calculation, the multiplier here should be controlled within +/-0.025% to meet the size requirements of conventional plates.

By analyzing the causes of PCB size expansion and contraction, we can find out the available monitoring and improvement methods, hoping that the majority of PCB practitioners can get inspiration from this, and find the improvement plan suitable for their own companies according to their own actual situation.