Keywords: PCB || Printed Circuit Board || SMT Stencil || Multilayer PCB || Thick Copper PCB || PCB Warping
Why the PCB circuit board were required very flat?
First, in the SMT process, if the printed board is not smooth, it will cause the location to be inaccurate. The components can not be inserted into the hole of the board and the surface mount pad, and even the automatic plug machine will be damaged. Install components board after welding bending element, foot hard cut neatly. The board can not be installed in the case or in the machine socket. At present, the printed circuit board has entered the era of surface mounting and chip installation, and the requirements of the assembly plant on the board become more and more strict.
Standards warping and and testing methods
According to the identification and performance specifications of rigid PCB, the maximum allowable warping and distortion for surface mount printed boards is 0.75%, and various other boards allow 1.5%. At present, the warping of the electronic assembly plant, regardless of the double-sided or multilayer, 1.6mm thickness, usually 0.70 ~ 0.75%, a lot of SMT, BGA board, the requirement is 0.5%. Parts of the electronics factory are encouraging to increase the warping to 0.3%. The printed circuit board in the test platform, the test needle to warp the largest local, in order to test the diameter of the needle, divided by the PCB curved edge length, we can calculate the warping of printed circuit board.
Prevent the board from warping during the manufacturing process
1.Need attention points in design:
A. The arrangement of the inter-layer semi cured sheets shall be symmetrical, for example of 6-layers PCB, the thickness between the 1~2 and 5~6 layers and the number of the prepreg shall be the same, otherwise will caused warping after lamination.
B. For Multilayer, core material and prepreg shall be made using the same supplier's products.
C. the outer surface of the A and the B surface shall be as close as possible to the area of the line. If the A surface is large copper surface and the B surface is only a few lines, the printed board will easily warp after etching. If the area on both sides of the line is too wide, a few separate grids can be added to the thin side for balance.
2.Baking the board before blanking:
Baking the copper clad material (2 hours to 150 degrees Celsius, 8 + time) to remove water before blanking, and the resin plate is completely cured, to further eliminate the residual stress in the plate, the plate is helpful to prevent warping. At present, many double-sided and multilayer boards still adhere to the steps of cutting the material before or after the drying. But there are some plate factory exception, the current PCB factory drying board time regulations are inconsistent, from 4-10 hours have, it is recommended according to the production of printed board grades and customer warping requirements to decide. Cut into pieces after drying or the whole aniseed after drying of material, two kinds of methods are feasible, recommend cutting after baking plate. The inner panel should also be baked.
3. The longitude and latitude of prepreg:
When the prepreg is laminated, the warp and weft shrinkage is different, and the warp direction and weft direction must be distinguished when blanking and stacking. Otherwise, it is easy to make the finished board warp after laminating, and even if the pressure is added, the drying board is hard to correct. Many reasons for the warping of laminated plates are that the warp and weft of the prepreg are not clear when they are laminated.
How to distinguish between longitude and latitude? The roll of prepreg takes the direction of the warp, while the width is in the direction of the weft. For the copper plate, the long side is the weft direction and the short side is the direction. If it is not certain, it can be checked with the manufacturer or the supplier.
4.Post lamination stress relief:
After the completion of hot pressing and cold pressing, the multilayer board can be removed, cut or milled out of the rough edges, and then placed in the oven for 4 hours at 150 degrees Celsius, so that the stress in the plate is gradually released and the resin is completely cured. This step shall not be omitted.
5.The plates need to be straightened when they are electroplated:
0.4 ~ 0.6mm ultrathin multilayer plate surface plating and pattern plating should be made during the roller special, automatic electroplating line fly bar clamp plate, with a round stick skewer the pinch roll the whole fly bar, thus straightening roller all of the board, so after plating board no deformation. Without this measure, the sheet will bend and be difficult to repair after being plated with a twenty or thirty micron copper layer.
6. Cooling of board after hot air leveling:
PCB hot air leveling when the solder bath (about 250 degrees Celsius) of high temperature impact, and after removal, should be put on the flat marble or steel plate, natural cooling, sent to the aftertreatment machine for cleaning. This is good for the board against warping. Some factories in order to enhance the brightness of the tin surface board, hot air leveling immediately after the cold water, in a few seconds and then remove after the hot and cold shock, for certain types of board is likely to produce warping, delamination or blistering. In addition, the equipment can be equipped with air flotation bed for cooling.
7. warping board treatment:
Well managed factories, printed boards will be subject to a 100% flatness check at the time of final inspection. Any unqualified board will be picked out and placed in the oven. It will bake for 3~6 hours at 150 degrees Celsius and under heavy pressure and be cooled naturally under heavy pressure. Then remove the pressure to remove the board, in the flatness check, so that some of the boards can be saved, and some boards need to be two to three times the baking pressure to smooth. If the above involved warping prevention measures are not implemented, part of the board drying pressure is useless, and can only be scrapped.