Precautions against Substrate Warpage
Starting from the resin formula
Using relatively long molecular chains, better flexibility of resin and curing agent, which is an important way to overcome the substrate warpage. Paper-based CCL solved problems of CCL warping effectively since the use of tung oil modified phenolic resin, which provided reference for other types of laminate and CCL.

Choose a good substrate carefully
In the paper-based CCL production practice, we found that the same glue, the same production conditions, using paper CCL of different production plant to produce CCL, single-sided CCL will from warping into anti-warping when using the paper from some factory. According to this phenomenon, in the production of paper-based CCL, we intentionally mixed with the different warping of the paper were mixed on the plastic mixture and stacked material, found that we can get good flatness copper clad laminate. Its PCB process is also very stable, this approach can learn from other CCL plant. We have not yet found that changing the substrate produced by different manufacturers will change from anti-Alice CCL is Alice situation, but different manufacturers of glass fiber cloth made of warp CCL significantly different, the material Purchasing should be directed.

Control the production process parameters strictly
In the CCL production process, control the production process parameters strictly, to ensure the consistency of the resin content, fluidity, gel time for prepreg, it is the necessary measures to improve the smoothness of CCL. Which fluidity and gelatinization time to determine the technical indicators, is a highly technical problem, subject to a large number of production practice to accumulate data in order to find a better control index and production process conditions (there are many manufacturers to use the current pressure Instrument to measure the technical parameters of prepreg, the control accuracy is high, and can be used for dynamic simulation of the laminating process test, has many advantages).

Tension control
When coating glue on the substrate, the glue machine’s tension should not be large. The warp and weft of stacked material may not cross, the fabric of different factory can not mix, the fabric of different specifications should not be mixed, the prepreg that different gluing machine produced, the prepreg that different manufacturers produced do not mix (because of their different tension).

Temperature Control
When the product was hot-pressing, it will be best if heating them with oil, throughout temperature difference of hot plate is smaller than steam heating. Heating rate should be moderate, not too slow, it should not be too fast. Note that the degree of liquidity of prepreg , gel time of prepreg, and making a appropriate regulation depending on the laminated product variety and thickness, to minimize the amount of glue flow. If the layout of the hot plate and the arrangement of the import and export of heat source is unreasonable, can also cause uneven temperature distribution of hot plate, will increase the product warping, this time should be modify hot plate.

Layered menu is reasonably designed
The preheating temperature and the rate of temperature rise and pressure in process of reasonable design and laminating is the key point to press good plate and reduce the warping. The quantity of pad paper and softness influence is also very large, must be used flexiblely. Thickness and hardness of stainless steel plate has a certain influence to the substrate warpage , if having conditions ,as far as possible using a little thicker and harder point of stainless steel plate.

Cooling rate slowly
When the CCL was hot-pressing, it will find that outer layer warping of each BOOK is greater than the inner part of the board after careful observation, this is related to the cooling rate of the outer layer is greater than the inner plate. After the end of the heat curing stage, some CCL plants adopt sub-cooling cooling production process, that is, with warm water or warm oil (for the oil heating system) cooling, so that the first section cooling of products are more moderate, and then cold water or cold oil cooling, the effect of this approach is very good, it is worth reference for other factories.

reduce the molding pressure
As far as possible to use of vacuum pressing machine to hot press forming, the vacuum is higher, it can make the product to achieve higher density with a lower pressure due to low molecular weight is easier to discharge, the pressure is lower, the product stress is also smaller, so using low pressure molding can help to reduce product warpage.

Reducing flow glue, low pressure molding is one of the important way for improving the flatness of CCL , Reducing the white edge. People have studied to use vacuum chamber to suppress CCL or PCB products, because the product is all-round force, and is uniform, consistent, and can be made without white edges, high flatness products. However, at present there is no CCL plant to use it in industrial production.

Packaging and storage

It must be sealed packaging when placing substrate, at present,many CCL factories has taken moisture-proof and seal packaging, which has benefit for reducing the base material storage process warping. Substrate should be placed flat, not vertical, also should not be pressed up the weight. If the stack storage, the package should be separated with hardwood between package , the practice has proved that not separated from hardwood, the following products will be deformed.

design balance of PCB circuit graphic

PCB line graphic design can not be very balanced, such as the face of a large area of conductive patterns, should be make its grid as far as possible in order to reduce stress.

drying board firstly before PCB processing

it is best to dry the plate before the substrate is put into use (bake for several hours at the temperature near the substrate TG), to relax the stress of the substrate and reduce the warping of the substrate in the PCB process.

It can take lower processing temperature when PCB processing, to reduce the strong thermal shock.

Processing direction consistency

The direction of the trademark character on the CCL substrate indicates the force direction of the product process, commonly known as portrait. In the PCB manufacturing process should be as far as possible in the direction of line graphics and substrate longitudinal line in order to reduce stress and reduce product warpage. To do multilayer, we should pay attention to the longitudinal and longitudinal curing of each layer of PCB consistent.

Warping CCL leveling measures:

Roller – type leveling machine application:

Roll leveling machine leveling method was taken: In the manufacturing process of PCB, will pick out the plate of relatively large warpage firstly, then leveling by roller pressure leveling machine, then put into the next process. For the final product, if there are still relatively poor warpage products, but also into the roll-type small flattening machine to leveling again, this approach is effective for relatively thin, small warpage deformation of the PCB board.

Pressing machine leveling method

For PCBs that have been finished and warpage significantly, and can not be leveled with a roller pressing machine. Some PCB plants place it in a small pressing machine (or in a similar fixture), and the warpage PCB board was pressed a few hours to ten hours then cold leveling, from the practical application of observation, the effect of this approach is not very obviously. First, the leveling effect is not large, the other is the leveling plate is easy to rebound (ie, recovery of warping).

Some PCB plants will heat small pressing machine to a certain pressure firstly, then have hot pressing to warpage PCB plate, the effect will be better than the cold pressure, the wire will be pressed deformation or even into the substrate if the pressure is too big; If the temperature is too high, will make rosin water discolored, or even substrate discolored, and so defects. And whether it is cold pressing leveling or hot pressing leveling need a longer time (a few hours to ten hours) to see the results.   

Bowing mold leveling method

For the finished PCB board has been warped, not out of the goods, using other leveling method has no effect, “if there is better leveling method “, according to the mechanical properties of polymer materials and author’s working practice, it is recommended to take bow mold hot-press leveling method.

According to the area of the PCB board, to make a number of very simple bow mold (see Figure 1), where recommended two leveling operation method:

Will warpage PCB board into the bow mold, then into the oven bake leveling:

The bowed surface of the warped PCB board is directed against the curved surface of the mold (ie, the convex surface is opposite), and the screw of PCB board is adjusted and deform the PCB board slightly in the opposite direction of its bow. And then put the mold of PCB board into the oven has been heated to a certain temperature, baking for some time.

In the heated state, the substrate stress gradually relaxed, so that making the deformation of the PCB board return to a flat state. But the baking temperature should not be too high, avoiding discoloration of rosin or substrate yellowing, but the temperature should not be too low, at lower temperatures, it take a long time to relax stress completely, usually glass substrate temperature can be used as baking of the reference temperature, the glass temperature is resin transition point. At this temperature the polymer chain can rearrange the orientation, so that the substrate stress is relaxed completely, so the leveling effect is obvious, the advantage that using the bow mold leveling is that the investment is very small, the PCB factories have the ovens, the leveling operation is very simple , If the number of the plate warpage is more, it will be ok if making several pieces bow mold , and put several pieces molds to the oven one time, and drying time is relatively short (dozens of minutes or so), so the leveling efficiency is relatively high .

Firstly baking PCB board soft and clamped into the bow mold, then pressing leveling:
For the PCB board that the warpage deformation is relatively small, you can firstly put the PCB board to be leveled to the oven that have been heated to a certain temperature (the temperature can be set to refer to the substrate glass transition temperature, and after the substrate in the oven baked a certain time, observing softening and discoloration to determine the situation), usually the baking temperature of glass fiber cloth substrate is higher, a little longer time ; the baking temperature of paper substrate can be lower; the baking temperature of thick plate can be slightly higher, the time is slightly longer; the baking temperature of thin plate can be slightly lower; for the baking temperature of PCB board that has been sprayed pine fragrance should not be too high, to prevent the color of rosin perfume. Baking a certain time, remove a few sheets to more than a dozen, clip into the bow mold, adjust the pressure screw, making the PCB board slightly deformation to the warping ’ s opposite direction, after the plate cooled and finalized, you can unload the mold, remove the flattened PCB board. Some users do not understand the substrate glass temperature, where recommended baking reference temperature, the baking temperature of paper substrate is 110℃~130℃, FR-4 is 130℃~150℃. When leveling, making several small tests to the selected baking temperature and baking time, to determine the leveling baking temperature and baking time, the baking time is longer, the substrate baked thoroughly, the leveling effect is better , the warping rebound of the PCB board is less after leveling. the warpage resilience of the PCB board is low after the bow mold leveling; it can still remain flat state even after the wave; it has less influence on the appearance of PCB board color.

Distortion of the PCB board, the difficulty degree of leveling is larger than the plate of the bow bending deformation, but as long as the operation is proper, but also get a good leveling effect.
The most critical point is to twist the PCB board’s diagonal line with deformation to into the center position of the bow mold (PCB board to be leveling and mold stagger 45℃angle), to be level according to the above method, twist value can be significantly reduced.

For laminated board, and even thicker thickness laminated board, it will have effect using the above practices, but it is not enough to depend on the power of the screw , must press using pressing machine.
In the leveling process, in particular to remind that: when mold production, arc must be round and smooth; temperature and heating time to be appropriate, so as to avoid the damage for appearance of the leveling laminated board surface.
The size of the mold depends on the leveled product, a small board, you can use the mold to level, a large plate can also be leveled using the mold, when hot leveling the large areas of the laminated board with large pressing machine , the pressure should not be too Large, so as the laminated board produce other deformation.

Quality control system 

Secondary test method
Part of the PCB manufacturers adopt the “Secondary test method” to improve and found the breakdown defect rate through the first time high voltage.

Bad board foolproof test system
More and more PCB manufacturers installed “good board marking system” and “bad board anti-wrong box” in the light board test machine in order to avoid the loss of man-made effectively. Good board marking system marked the PASS board logo has been passed test for the test machine, which can effectively prevent the board or a bad board flow to the hands of customers. In the test process, the bad board anti-wrong box can test the PASS board, the test system output box open signal. the other hand, when test bad board, the box is closed, so that the operator placed the circuit board has been tested correctly.

Establishment of PPm quality system
At present, PPm (Partspermillion, the defect rate of parts of per million) quality system in PCB manufacturers began to be widely used. Among the many customers in the company, it is the most worth learning of its application and the effectiveness for in Singapore’s HitachiChemICal. In the factory there are more than 20 people responsible for the quality of online PCB abnormalities and abnormal return of PCB quality statistical analysis of the work. Using statistical analysis way of SPC production process, having statistical analysis to each piece of bad board and each piece of defective plate after the return, also combined with micro-slices and other auxiliary tools to analyze the production process in which the production of bad and defective plate. According to the statistical data results, purposefully to solve the problems on the process.

Comparison test method
Some customers use two different brands of PCB models to compare the test, and track the corresponding batch of PPm situation, in order to understand the performance of the two test machines to choose a better performance test machine to test automotive PCB.

Improving test parameters
Selecting higher test parameters to rigorously inspect such PCBs. Therefore, if you choose a higher voltage and threshold, increase the number of high-voltage reading leakage, can improve the detection rate of PCB defects. For example, a large Taiwan-funded PCB enterprises in Suzhou using 300V, 30M, 20 Europe for testing automotive PCB.
Checking the test machine parameters regularly
Test machine in the long-term operation, internal resistance and other related test parameters will be biased. And therefore need to regularly adjust the machine parameters to ensure the accuracy of test parameters. Test equipment in a considerable part of the large-scale PCB companies are six months or a year for machine maintenance, adjust the internal performance parameters. The pursuit of “zero defect” automotive PCB PCB has been for the majority of the efforts of the direction, but by the process equipment, raw materials, and many other restrictions, PCB hundred enterprises in the world are still exploring ways to reduce PPm.