PCB Tech - PCB circuit board warpage The cause and treatment of circuit board warpage

PCB circuit board warpage The cause and treatment of circuit board warpage

One: PCB circuit board warpage/circuit board warpage harm to circuit board processing (H2)
PCB circuit board warpage/circuit board warpage simply means that the circuit board is deformed, or the surface of the circuit board is uneven. PCB circuit board warpage / circuit board warpage exceeds a certain level, it will cause a lot of trouble in the subsequent process of circuit board processing, welding, assembly and other processes, which will seriously cause hidden dangers in the quality of electronic products. Therefore, PCB circuit board warpage/circuit board warpage should cause sufficient attention from circuit board manufacturers.

Circuit board warpage / circuit board warpage exceeding standard
A: (H3) PCB circuit board warpage/circuit board warpage exceeds the standard. Solder paste shift occurs when brushing solder paste. As the density of the circuit board continues to increase, the solder joints on the circuit board are getting smaller and smaller. Offset will cause the misalignment of solder paste and solder joints. A solder joint without solder paste will definitely lead to false soldering.

Circuit board warpage/circuit board warpage causes solder paste print offset
B: (H3) PCB circuit board warpage / circuit board warpage exceeding the standard will easily cause the device to throw away when the chip is mounted. The placement machine sets a certain distance between the components and the PCB circuit board when the placement machine is placed. When the circuit board is warped, some circuit board areas are far away from the suction nozzle of the placement machine, and some are close. When the magnitude of the distance change exceeds the tolerance range allowed by the placement machine, there will be material throwing during placement, that is, the device is not accurately placed on the circuit board. This situation directly leads to low yield after patch welding and high product quality hidden dangers.

Circuit board warping/circuit board warping leads to SMT chip throwing
C: (H3) PCB circuit board warpage / circuit board warpage exceeding the standard makes the relative position of the solder feet of the components on the circuit board deviate. When the legs of the components are very precise, the direct result is the solder joints on the circuit board Misplaced with the solder feet of the components. There are false welding in some welding feet.

Circuit board warpage/circuit board warpage causes component shift
D: (H3) PCB circuit board warpage / circuit board warpage exceeding the standard will cause the circuit board to fail to be assembled. As electronic products become more and more sophisticated, the housing of the assembled circuit board becomes more and more refined. If the circuit board warps excessively, the assembly hole position will shift. If the transfer is forcibly twisted, the physical external force is bound to be The components on the circuit board are stretched, and the consequence is that the quality of electronic products has hidden dangers.

Circuit board warping/circuit board warping causes assembly hazards
Two: PCB circuit board warpage/circuit board warpage acceptance criteria (H2)
Generally, circuit board manufacturers adopt the IPC-6012 circuit board warpage acceptance standard. The maximum warpage or twist of SMT circuit boards is 0.75%, and the warpage of other boards generally does not exceed 1.5%. (Calculation method of warpage=warpage height/curved edge length) In fact, as the density of integrated circuits increases, BGA and SMB packages are used in large quantities, and circuit board manufacturers also have standards for circuit board warpage. As a result, the warpage of some plants is even less than 0.3%. So for circuit board manufacturers, production control requirements are becoming more and more stringent.

Circuit board warpage/circuit board warpage acceptance criteria
Three: PCB circuit board warpage/circuit board warpage causes (H2)
A: (H3) The circuit board material is the main cause of PCB circuit board warpage/circuit board warpage. The base of the circuit board is generally made of FR-4 epoxy glass fiber cloth laminated with resin glue vertically and horizontally. In order to reduce costs, some copper clad laminate plants add fillers to the glass fiber cloth or use substandard glass fiber cloth materials, or there are problems with the process control of the pressing equipment. These are the reasons for the warpage of the circuit board plates. In order to reduce circuit board warpage/circuit board warpage, circuit board manufacturers must purchase genuine A-grade copper clad laminates when purchasing copper clad laminates. The warpage of circuit boards produced by such circuit board manufacturers will always be controlled at A reasonable range.

Genuine copper clad laminate material can reduce circuit board warpage/circuit board warpage
B: (H3) The circuit board pressing process is another cause of PCB circuit board warpage/circuit board warpage exceeding the standard. The circuit board pressing is to press the inner layer of the circuit board together with PP material, and the pressing process is directly related to the subsequent circuit board warpage.
C: (H3) The moisture content of the glass fiber cloth in the circuit board is also an important factor causing circuit board warpage/circuit board warpage. Fiberglass cloth is water-absorbing. When the circuit board is damp, the high and low temperatures during the welding of the circuit board will cause the circuit board to deform.
D: (H3) The design of circuit board traces may also be a factor of circuit board warping/circuit board warping. When designing wiring on a circuit board, there are some reasons for the layout of components and the functions of electronic products. The wiring on the circuit board is uneven on the top and bottom layers, or one side is vertical and the other side is horizontal, or one side has a large area. There is no copper on one side. The expansion and contraction of copper foil is different from the expansion and contraction of glass fiber cloth. When the copper foil on the surface of the circuit board is unevenly distributed, the copper foil stretches the surface of the circuit board and causes the circuit board to be twisted and deformed.
E: (H3) The operation in the circuit board production process will also cause circuit board warpage/circuit board warpage to exceed the standard. In the circuit board manufacturing process, the electroplating process must be in the solution, and the solder mask and the curing of the white letters must be baked at a high temperature. From one process to another, it must be washed and dried. These frequent high and low temperatures, if the circuit board is placed unevenly during the production of the circuit board, there will be circuit board warping/circuit board warping.

Four: How to prevent and avoid PCB circuit board warpage/circuit board warpage (H2)
A: (H3) Choosing high-quality boards is an important option to avoid circuit board warping/circuit board warping.
High-quality boards have the following characteristics: good brand reputation, formal channels, quality assurance certificates at the time of purchase, strong after-sales service and R&D capabilities. Because of the intensified competition among circuit board manufacturers, some circuit board manufacturers choose some copper clad laminates with questionable origins in order to reduce costs. Due to the characteristics of the use of circuit boards and the wide range of product applications, a considerable number of electronic products are not obvious to the hidden dangers of the board. . In order to reduce costs, some electronic product manufacturers blindly reduce the purchase price of circuit boards. In order to meet the needs of the market, circuit board manufacturers will naturally choose some problematic and low-cost plates. This kind of problem cannot be seen on the surface in a short period of time. In the long run, once the circuit board warpage/circuit board warpage exceeds the standard due to the problem of the board, the consequences will be catastrophic.
B: (H3) The engineering design of multilayer boards in circuit board production can reduce circuit board warpage/circuit board warpage as much as possible. For example: the arrangement of the prepregs between the layers of the circuit board corresponds to that of the multilayer board core board and the prepreg should use the same board supplier's copper clad laminate, and the outer C/S surface pattern area of the multilayer circuit board should be as close as possible, and an independent network grid. These methods can minimize the possibility of circuit board warpage/circuit board warpage.
When cutting the material before the circuit board is manufactured, the baking of the circuit board can reduce the warpage of the circuit board/circuit board. Circuit board baking conditions: generally 150 degrees for 6-10 hours, the circulating air in the oven is turned on to remove the water in the board, further curing the resin completely, and eliminating the stress in the board; no matter the inner layer or the double-sided board all need.
C: (H3) When the multilayer board is laminated, the warp and weft direction of the cured film can reduce the possibility of circuit board warpage/circuit board warpage exceeding the standard. The multi-layer board warp and weft take-up ratio is different. Generally, the direction of the cured sheet roll is the warp direction, and the long direction of the copper clad laminate is the weft direction. Such a distribution can increase the strength of the circuit board and reduce the internal stress, so as to reduce the warpage of the circuit board/circuit board.
D: (H3) After spraying tin, it is cooled on the air flotation bed and then cleaned. It can be seen that the warpage of the circuit board/circuit board caused by the high temperature of the tin spray can be reduced. The temperature of spraying tin is generally about 300 degrees. Under the pressure of the air knife, 300 degrees of molten tin liquid is sprayed onto the circuit board. At this time, the circuit board becomes very soft under the action of high temperature. During the cooling process of the circuit board, if it is placed unevenly, the circuit board after cooling will be deformed, and its structure is circuit board warpage/circuit board warpage. Therefore, the circuit board that has been sprayed with tin must be completely cooled on the air flotation bed before cleaning, so that the flatness of the circuit board will be greatly improved.

Five: PCB circuit board warpage / circuit board warping after the degree of warpage exceeds the standard (H2)
Once the circuit board is made into a finished product, there is circuit board warpage/circuit board warpage, and it is very difficult to process it in hopes of smoothing. Therefore, to control the warpage of the circuit board / the warpage of the circuit board must be controlled at the front end, and subsequent corrections are expensive, and the effect may not be very good.

Circuit board warping / circuit board warping board processing conditions, two flat 5 cm thick steel plates are used to make a fixture, and the steel plates are tightened with screws. Place the warped circuit board in the middle of the steel plate, and use paper space between each circuit board. The purpose is to expand the circuit board at high temperatures, and there is paper in the middle to avoid physical damage to the circuit board from being squeezed. Put it in the oven and bake it at 150°C or hot pressing for 3-6 hours, repeated 2-3 times. Place it to cool naturally and open the package.