There are 7 ways to avoid the warpage of PCB
The arrangement of inter-layer prepreg should be symmetrical, such as the 6 layers whose the thickness of the 1~2 and 5~6 layers and the number of the prepreg should be the same, otherwise the laminates will be warped easily; the multilayer core plate and the prepreg should use the same supplier’s product.
Baking board before loading
The purpose of the drying plate (150 degrees centigrade, time 8 + 2 hours) for the copper clad plate is to remove the water in the plate, and to solidify the resin in the plate at the same time, to further eliminate the residual stress in the plate, which is helpful to prevent the warpage.
At present, many double-sided, multilayer boards still adhere to the pre-or post-drying process. However, there are some exceptions to the factory. At present, the drying time of each PCB factory is inconsistent of the range of 4 to 10 hours. It is suggested according to the grade of the PCB and the requirements of the customer for warpage. These two methods are feasible. It is suggested that the board should be dried after shearing. Inner laminates should also be baked.
The pregreg of longitude and weft direction
The meridional and latitudinal shrinkage of prepreg are different from that of the latter, and the meridional and zonal shrinkage must be distinguished between the blanking and lamination. Otherwise, the laminated plate warpage easily, even if the pressure baking board is difficult to correct. Many of the causes of multilayer warpage are due to the warping of the prepreg.
How to distinguish longitude and latitude? The winding of the curable sheet is in the direction of meridian direction and the direction of width is zonal direction; for the copper foil plate, the long edge is the zonal direction, the short edge is the meridional direction, and the manufacturer or supplier can be consulted if it is not certain
Stress relieved after lamination
The multilayer plate is removed after hot pressing, cut or milled off the edges, and then laid flat in the oven for 150 degrees Celsius for 4 hours, so that the stress in the plate is gradually released and the resin is cured completely. This step must not be omitted.
It is need to straighten out sheet electroplating
When 0.4~0.6mm ultra-thin multilayer board is used for plate plating and graphic electroplating, special clamping rollers should be made. After the thin plate is clamped on the flying bar on the automatic electroplating line, the clamping roller on the whole flying bar shall be strung together with a round stick to straighten all the boards on the roll. So that the plate after plating will not deform. Without this measure, the sheet will bend after electroplating 20 to 30 microns of copper, and it is difficult to remedy.
Cooling of Plate after Hot Air leveling
The hot air of printed circuit board should be hit by soldering tank (about 250 degrees Celsius) at ordinary time. After taking it out, it should be cooled naturally on the flat marble or steel plate and cleaned on the machine after sending it to the machine. This is good for the board to prevent warpage. In order to enhance the brightness of lead-tin surface, some factories put the hot air into cold water immediately after leveling the plate, and take it out after a few seconds for post-treatment. This kind of hot and cold impact may warp, delaminate or bubble some types of boards. In addition, air floatation bed can be added to the equipment for cooling.
Treatment of warping Board
In a well-managed factory, PCB will be checked for 100% flatness at final inspection. Any unqualified board will be picked out and placed in the oven, dried at 150 degrees Celsius and under heavy pressure for 3 hours, and naturally cooled under heavy pressure. Then take out the board and check the flatness, which can save part of the board, some of which need two to three times of baking to smooth. If the above-mentioned warpage prevention measures are not implemented, part of the board baking pressure is useless, can only be scrapped.