PCB News - How to avoid PCB board expansion and contraction for automobile antenna PCB factory

We must first understand the PCB board regulations of the car antenna PCB factory, and why it will expand and contract, what is the standard, and finally understand how to avoid it.

1, why the PCB circuit board regulations are very flat
On the automated technology plug-in network, if the PCB board of the car antenna PCB factory is not leveled, precise positioning will be prohibited, and electronic devices cannot be inserted into the holes of the board and the surface layer of the solder layer, and will even be damaged. machine. The wood board with electronic components will be bent after electric welding, and the component feet cannot be cut neatly. The wooden board cannot be installed on the main box or the power socket of the fuselage. Therefore, the assembly plant is very distressed when the board is warped. At this stage, PCB boards have entered the stage of surface installation and integrated ic installation, and the assembly plant's regulations on board warping will inevitably become more and more stringent.

2. Specifications and test standards for expansion and contraction

According to the British IPC-6012 (1996 edition) "Standards for the Evaluation and Characteristics of Stiffness PCB Boards", the allowable large expansion, shrinkage and distortion for surface mounting of PCB boards is 0.75%, and 1.5% for other kinds of boards. Compared with IPC-RB-276 (1992 edition), this has improved the requirement for surface mounting PCB board. At this stage, the degree of expansion and contraction approved by various electronic device assembly plants, whether double-sided or double-layered, is 1.6 mm thick, generally 0.70 to 0.75%, and for many SMT and BGA wood panels, the stipulation is 0.5%. Some electronic device processing factories have incited to increase the expansion and contraction specification to 0.3%, and the method of detecting expansion and contraction follows GB4677.5-84 or IPC-TM-650.2.4.22B. Put the PCB board on the service platform that has been verified by measurement, insert the detection needle into the area with larger expansion and contraction, and divide the diameter of the detection needle by the length of the curved side of the PCB board to calculate the expansion of the PCB board. Shrunk.

3, prevent board expansion and contraction during the whole production process

1. Architectural engineering design: common problems when designing PCB boards:
   A. The order of the semi-dry solid sheets of virtual beams should be symmetrical. For example, the thickness of the 1-2 and 5-6 virtual beams and the number of semi-dry sheets should be the same for six multilayer boards, otherwise it is very easy to expand and contract after laminating.

B. Solid wood multi-layer board blockboard and semi-dry solid sheet should be the products of the same dealer.
C. The total area of the road map type on side A and side B of the surface layer should be as close as possible. If the A surface is a large copper surface and the B surface only has two wires, this type of PCB is very easy to expand and contract after the etching process. If the total area of the two-sided route is too different, you can add some separate grids on the thin side to balance it.

2, baking plate before cutting:

The purpose of baking the copper clad laminate before opening (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and in addition to completely dry the epoxy resin in the board, and further remove the remaining ground stress in the board, This is helpful to avoid board expansion and contraction. At this stage, many double-sided, solid wood multilayer boards still persist in the process of baking before or after opening. However, there are also some plate processing plants listed. At this stage, the time requirements for baking plates of various PCB factories are inconsistent, ranging from 4-10 hours. It is recommended to base on the level of the PCB board produced and the customer's requirements for expansion and contraction. To make decisions. After cutting into a jigsaw method, or after baking a whole piece of star anise, the two methods are both feasible. The inner multilayer board should also be baked.

3. The direction of semi-dry solid tablets:

After the semi-dry solid sheet is laminated, the weft and weft shrinkage rates are different, and the weft and weft must be distinguished when cutting and laminating. Otherwise, it is very easy to cause the finished product board to expand and contract after lamination, and there is no way to correct it even if the board is under pressure. Because of the expansion and contraction of solid wood multi-layer boards, many of them are caused by the fact that the semi-dry solid sheets are not distinguished when they are laminated.
How to distinguish between warp and great directions? The turning direction of the semi-dry solid sheet in the plate is the latitude direction, and the total width direction is the latitude direction; for the copper board, the long side is the latitude direction, and the long side is the latitude direction. You can check with the manufacturer or distributor.

4. In addition to ground stress after lamination:

After pressing and cold extrusion, the solid wood multilayer board is removed, cut or milled to remove the burrs, and then placed flat in the drying oven at 150 degrees Celsius for 4 hours to slowly release the ground stress in the board and make the epoxy The resin is completely dry and this process cannot be omitted.

5. The metal sheet must be straightened during the electroplating process:

0.4～0.6mm thin solid wood multilayer board is used for surface electroplating process and pattern electroplating process to make a unique nip roller. After the metal sheet is clamped on the fly bus on the automatic electroplating process network, it is formed by a round bar. The nip rollers on the flying bus are strung together to straighten all the wood boards on the rollers, so that the wood boards after the electroplating process are not easily deformed. If there is no such countermeasure, after the copper layer of 20 to 30 μm is electroplated, the metal sheet will bend and cannot be saved.

6. Refrigeration of the board after the warm wind is leveled:
PCB board is usually subjected to high temperature impact of solder wire tank (about 250 degrees Celsius). After removal, it should be placed on a leveled natural marble or thick steel plate for refrigeration, and sent to the rear collector for cleaning. That's good for the board to prevent expansion and contraction. In order to improve the chromaticity of the lead-tin surface layer, some processing plants put funds into cold water immediately after the warm air is flattened, and then take them off after a few seconds to carry out the post-processing process. The plank is likely to cause expansion and contraction, layering or blistering. In addition, air-floating machine tools can be retrofitted on machinery and equipment to carry out refrigeration.

7. The solution to the expansion and contraction plank:
Management method in orderly processing factory, the PCB board of the car antenna PCB factory is 100% flatness inspection in the final inspection. All wooden boards that do not pass will be picked out, put into the drying oven, baked at 150 degrees Celsius and pressure for 3 to 6 hours, and of course refrigerated under pressure. Then the pressure was relieved and the planks were taken off and inspected for flatness, so that part of the planks could be saved, and some planks needed to be bake and pressed two or three times before they could be flattened.