Circuit board factory tells you the reason and prevention of PCB warpage

- Apr 25, 2020-

 

Circuit board factory tells you the reason and prevention of PCB warpage

   SMT is also called surface mount technology. During the manufacturing process, under a heating environment, the solder paste is melted by heat, so that the PCB pad is reliably combined with the surface mount components through the solder paste alloy. We call this process reflow soldering. When the circuit board passes through Reflow (reflow soldering), most of them are prone to board bending and warpage. In serious cases, it may even cause component empty soldering and tombstone. 

   In the automated insertion line, if the PCB of the circuit board factory is not flat, it will cause inaccurate positioning, the components cannot be inserted into the holes of the board and the surface mount pads, and even the automatic insertion machine can be damaged. The board on which the components are mounted bends after welding, and it is difficult for the component feet to be cut flat. The board can not be installed in the case or the socket in the machine, so the assembly factory is also very troubled when it encounters the board warp. At present, the printed board has entered the era of surface mounting and chip mounting, and the requirements of the assembly plant for board warping must be more and more strict.

 

   According to the US IPC-6012 (1996 Edition) "Identification and Performance Specifications of Rigid Printed Boards", the maximum allowable warpage and distortion for surface-mounted printed boards is 0.75%, and other various boards are allowed 1.5%. This raises the requirements for surface mounted printed boards compared to IPC-RB-276 (1992 edition). At present, the warpage permitted by various electronic assembly plants, regardless of double-sided or multi-layer, 1.6mm thickness, is usually 0.70 ~ 0.75%. Many SMT and BGA boards require 0.5%. Some electronics factories are encouraging to raise the standard of warpage to 0.3%. The method of testing warpage follows GB4677.5-84 or IPC-TM-650.2.4.22B. Put the printed board on the verified platform, insert the test pin to the place with the largest degree of warpage, and calculate the warpage of the printed board by the diameter of the test pin divided by the length of the curved edge of the printed board Curvature. A

What are the causes of board bending and board warping during the PCB manufacturing process? Let's discuss it below. A

The cause of each plate bend and plate warp may be different, but it should be attributed to the fact that the stress applied to the board is greater than the stress that the board material can bear. When the stress on the board is uneven or When the ability to resist stress is not uniform everywhere on the board, the results of board bending and board warping will occur. The following is a summary of the four major causes of plate bending and plate warping. A

1. The uneven copper surface area on the circuit board will worsen the board bending and board warping.

Generally, a large area of copper foil is designed on the circuit board for grounding. Sometimes, a large area of copper foil is also designed on the Vcc layer. When these large area copper foils cannot be evenly distributed on the same circuit board When it does, it will cause the problem of uneven heat absorption and heat dissipation speed. Of course, the circuit board will also expand and contract with heat. If the expansion and contraction cannot simultaneously cause different stress and deformation, then the temperature of the board has reached Tg At the upper limit, the board will begin to soften, causing permanent deformation. A

2. The weight of the circuit board itself will cause the board to sag and deform

Generally, the reflow furnace will use a chain to drive the circuit board in the reflow furnace, that is, use the two sides of the board as a fulcrum to support the entire board. It will show the phenomenon of depression in the middle because of its own species, causing the plate to bend. A

3. The depth of V-Cut and the connecting bar will affect the amount of panel deformation

Basically, V-Cut is the culprit of destroying the structure of the board. Because V-Cut cuts a V-shaped groove on the original large plate, so the V-Cut is prone to deformation. A

4. The connection points (vias) of each layer on the circuit board will limit the board's expansion and contraction.

Today's circuit boards are mostly multi-layer boards, and there will be rivet-like connection points between the layers. The connection points are divided into through holes, blind holes and buried holes. The connection points will limit the board. The effect of expansion and contraction will also indirectly cause bending and warping. A

So how can we better prevent the problem of board warpage during the manufacturing process, the following summarizes several effective methods, hoping to help everyone. A

1. Reduce the effect of temperature on the stress of the board

Since "temperature" is the main source of board stress, as long as the temperature of the reflow furnace is reduced or the speed of the board's heating and cooling in the reflow furnace is slowed, the occurrence of plate bending and warpage can be greatly reduced. However, other side effects may occur, such as solder short circuit. A

2. Using high Tg sheet

Tg is the glass transition temperature, that is, the temperature at which the material changes from glass to rubber. The lower the Tg value, the faster the board begins to soften after entering the reflow furnace, and the time to become soft rubber It will also become longer, and the amount of deformation of the board will of course become more severe. The use of higher Tg plate can increase its ability to withstand stress and deformation, but the relative material price is relatively high. A

3. Increase the thickness of the circuit board

In order to achieve a thinner and lighter target for many electronic products, the thickness of the board has been left 1.0mm, 0.8mm, or even 0.6mm. It is really a bit difficult for the thickness to keep the board from deforming after the reflow furnace. It is recommended that if there is no requirement for lightness and thinness, the board can preferably use a thickness of 1.6mm, which can greatly reduce the risk of bending and deformation of the board. A

4. Reduce the size of the circuit board and reduce the number of puzzles

Since most reflow furnaces use chains to drive the circuit board forward, the larger the size of the circuit board will be deformed in the reflow furnace due to its own weight, so try to put the long side of the circuit board as the board edge. On the chain of the reflow furnace, the depression deformation caused by the weight of the circuit board itself can be reduced, and the number of pieces is reduced for this reason, that is to say, when passing the furnace, try to use the narrow side to pass the direction of the furnace vertically as far as possible The amount of depression deformation. A

5. Use the oven tray fixture

If the above methods are difficult to achieve, the last thing is to use a reflow carrier / template to reduce the amount of deformation. The reason why the reflow carrier / template can reduce the bending of the plate is that it is because whether it is thermal expansion or cold contraction, I hope The tray can hold the circuit board until the temperature of the circuit board is lower than the Tg value and begins to harden again, it can still maintain the original size.

If the single-layer tray can not reduce the deformation of the circuit board, you must add another layer of cover and clamp the circuit board with the upper and lower two-layer tray. However, this kiln tray is expensive, and manual labor is required to place and recycle the tray. A

6. Use Router instead of V-Cut board

Since V-Cut will destroy the structural strength of the panel between the circuit boards, try not to use the V-Cut sub-board or reduce the depth of the V-Cut. A

7. Three points run through in engineering design:

A. The arrangement of prepregs between layers should be symmetrical. For example, for a six-layer board, the thickness between 1 ~ 2 and 5 ~ 6 layers and the number of prepregs should be the same, otherwise it will easily warp after lamination. A

B. Multi-layer board core board and prepreg should use the products of the same supplier. A

C. The pattern area of the outer layer A and B should be as close as possible. If the A side is a large copper side, and the B side only takes a few lines, this printed board is easily warped after etching. If the line area of the two sides is too different, you can add some independent grids on the thin side to balance. A

8. The latitude and longitude direction of the prepreg:

The shrinkage of warp direction and weft direction are different after lamination of prepreg. When cutting and laminating, the warp direction and weft direction must be distinguished. Otherwise, it is easy to cause warpage of the finished board after lamination, and it is difficult to correct even if the pressure drying board is pressed. Many of the reasons for the warpage of the multilayer board are caused by the lamination of the prepreg in latitude and longitude during lamination. A

The method of distinguishing the warp and weft directions: the roll of the prepreg rolled up is the warp direction, and the width direction is the weft direction; for copper foil, the long side is the weft direction, and the short side is the warp direction. Or supplier inquiries. A

9. Baking board before cutting:

The purpose of pre-baking the copper clad laminate (150 degrees Celsius, time 8 ± 2 hours) is to remove the moisture in the board and at the same time completely cure the resin in the board, further eliminating the remaining stress in the board, which is useful for preventing the board from warping Help. At present, many double-sided and multi-layer boards still adhere to the step of pre-baking or post-baking. However, there are exceptions to some sheet metal factories. At present, the time requirements for baking boards in various PCB factories are also inconsistent, ranging from 4 to 10 hours. It is recommended to decide according to the grade of the printed board produced and the customer's requirements for warpage. Either of the two methods is feasible after cutting into a panel and then baking or cutting the whole block and then cutting. It is recommended to bake the board after cutting. The inner board should also be baked ...

10. Stress relief after lamination:

The multilayer board is taken out after hot and cold pressing, cut or milled off the burrs, and then placed in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and completely cure the resin. This step cannot be omitted. A

11. Straightening of the thin plate during electroplating:

When 0.4 ~ 0.6mm ultra-thin multi-layer board is used for plate surface plating and pattern plating, special clamping rollers should be made. After clamping the thin plate on the flybar on the automatic plating line, use a round bar to clamp the entire flybar Rollers are strung together to straighten all the boards on the rollers so that the plated board will not deform. Without this measure, after plating a copper layer of 20-30 microns, the sheet will bend and is difficult to remedy. A

12. Cooling of the board after hot air leveling:

The printed board is impacted by the high temperature of the solder bath (about 250 degrees Celsius) during hot air leveling. After being taken out, it should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processing machine for cleaning. This is very good for the board to prevent warpage. In order to enhance the brightness of the surface of lead and tin in some factories, the hot air of the board is immediately put into cold water after being leveled. After a few seconds, it is taken out for post-processing. This kind of hot and cold impact may cause warping to some types of boards. Song, layering or blistering. In addition, an air floating bed can be installed on the equipment for cooling. A

13. Treatment of warped board:

In order to manage the factory in order, the printed board will be checked for 100% flatness during the final inspection. All unqualified boards will be picked out and placed in an oven, baked at 150 degrees Celsius and heavy pressure for 3 to 6 hours, and naturally cooled under heavy pressure. Then remove the pressure and take out the board to check the flatness, which can save some of the board, and some boards need to be baked and pressed two to three times to be leveled. If the above-mentioned anti-warping process measures are not implemented, some boards are useless and can only be scrapped.

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