Why does PCB board warp?

Damage of PCB board warp


In the automatic surface mounting line, if the circuit board is not flat, it will cause inaccurate positioning, the components cannot be inserted or mounted on the holes of the board and the surface mounting pad, and even damage the automatic insertion machine.

The circuit board with the component installed is bent after welding, and the component foot is difficult to cut flat and tidy. The board can not be installed on the chassis or the socket in the machine, so the assembly plant is also very worried about the board warping.

At present, the surface mount technology is developing towards high precision, high speed and intelligence, which puts forward higher requirements for flatness of PCB board as the carrier of various components.

It is specially pointed out in the IPC standard that the allowable deformation of PCB with surface mount devices is 0.75%, and that of PCB without surface mount devices is 1.5%.

In fact, in order to meet the requirements of high-precision and high-speed mounting, some electronic assembly manufacturers have more strict requirements on the deformation amount. If there is a requirement, the allowable deformation amount is 0.5%, and there are even individual requirements of 0.3%.

PCB board is composed of copper foil, resin, glass cloth and other materials. The physical and chemical properties of each material are different. After being pressed together, thermal stress residues will inevitably occur, resulting in deformation.

At the same time, in the process of PCB processing, various processes such as high temperature, mechanical cutting and wet treatment will also have an important impact on the deformation of the board. In short, the causes of PCB deformation are complex and diverse. How to reduce or eliminate the deformation caused by different material properties or processing has become one of the complex problems faced by PCB manufacturers.

Cause analysis of PCB board warp
The deformation of PCB board needs to be studied from several aspects, such as material, structure, pattern distribution, processing process, etc. The article will analyze and elaborate various reasons and improvement methods that may cause deformation.

The uneven area of the copper surface on the circuit board can worsen the bending and warping of the board.
Generally, a large area of copper foil will be designed on the circuit board for grounding. Sometimes, a large area of copper foil will also be designed on the Vcc layer. When these large areas of copper foil cannot be evenly distributed on the same circuit board, it will cause the problem of uneven heat absorption and heat dissipation speed.

Of course, the circuit board will also expand and shrink. If the expansion and contraction cannot be achieved at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached the upper limit of Tg value, the board will begin to soften and cause deformation.

The connection points (vias) of each layer on the circuit board will limit the expansion and contraction of the board.
Today’s circuit boards are mostly multilayer boards, and there will be rivet-like connection points (vias) between layers. The connection points are divided into through holes, blind holes and buried holes. The places with connection points will limit the effect of expansion and contraction of the board, and will also indirectly cause bending and warping of the board.

Reasons for PCB deformation:
(1) The weight of the circuit board itself will cause the board to sag and deform
Generally, the reflow furnace will use the chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board will be used as the fulcrum to support the whole board.If there are overweight parts on the board, or if the size of the board is too large, there will be a hollow in the middle due to its own weight, resulting in bending.

(2) The depth of V-Cut and connecting strip will affect the deformation of panelBasically, V-Cut is the prime culprit of the destruction of the plate structure. Because V-Cut is to cut grooves on the original large sheet, the V-Cut is prone to deformation.

Influence of pressing material, structure and pattern on plate deformation:
PCB board is formed by pressing core board, semi-solidified sheet and outer copper foil. The core board and copper foil are subject to thermal deformation during pressing, and the deformation depends on the coefficient of thermal expansion (CTE) of the two materials.

The coefficient of thermal expansion (CTE) of copper foil is about 17X10-6; The CTE in Z direction of common FR-4 substrate at Tg point is (50~70) X10-6; Above TG point, it is (250~350) X10-6, and CTE in X direction is generally similar to copper foil due to the presence of glass cloth.

Deformation caused by PCB processing:
The causes of PCB deformation during processing are very complex, which can be divided into thermal stress and mechanical stress.
The thermal stress is mainly generated in the process of pressing, and the mechanical stress is mainly generated in the process of stacking, handling and baking. The following is a brief discussion in the order of flow.

1. Incoming material of copper clad plate:
The copper clad plates are double-sided plates with symmetrical structure and no graphics. The copper foil and glass cloth CTE are almost the same, so there is almost no deformation caused by different CTE during the pressing process.
However, due to the large size of the copper clad laminate press and the existence of temperature differences in different regions of the hot plate, there may be slight differences in the resin curing speed and degree in different regions during the pressing process, as well as significant differences in the dynamic viscosity under different heating rates, which may also generate local stresses due to differences in the curing process.
Generally, this stress will maintain balance after pressing, but will gradually release and produce deformation in future processing.

2. Pressing:
PCB lamination process is the main process of generating thermal stress. Similar to copper clad laminate lamination, it will also generate local stress caused by curing process differences. Due to thicker thickness, diversified graphic distribution, more semi-cured sheets and other reasons, PCB thermal stress will also be more difficult to eliminate than copper clad laminate.The stress in the PCB is released in the subsequent drilling, shape or grilling processes, resulting in the deformation of the board.

3. Soldering resistance, characters and other baking processes:
Since solder resist inks cannot be stacked with each other during curing, the PCB boards will be placed vertically in the rack for curing. The solder resist temperature is about 150 ℃, which is just higher than the Tg point of the medium and low Tg materials. The resin above the Tg point is highly elastic, and the board is easy to deform under the action of dead weight or strong wind in the oven.

4. Hot air solder leveling:
The tin furnace temperature is 225 ℃~265 ℃ and the time is 3S-6S when the hot air solder for common plate is leveled. The temperature of hot air is 280 ℃~300 ℃.
When the solder is leveled, the plate enters the tin furnace from room temperature, and then carries out room temperature post-treatment and water washing within two minutes after it is discharged. The whole hot air solder leveling process is a sudden heating and cooling process.
Due to the different materials and uneven structure of the circuit board, thermal stress will inevitably occur in the process of cold and hot, resulting in micro-strain and overall deformation and warpage.

5. Storage:
PCB boards are generally vertically inserted in the shelf during the storage of semi-finished products. Improper adjustment of the shelf tightness or stacking of boards during storage will cause mechanical deformation of the boards. Especially for the plates below 2.0mm, the impact is more serious.
In addition to the above factors, there are many factors affecting PCB deformation.

Prevention of PCB board warp
Warping of the circuit board has a great impact on the production of the printed circuit board. Warping is also one of the important problems in the production process of the circuit board. The board with components is bent after welding, and the assembly feet are difficult to be neat.
The board cannot be installed on the chassis or socket in the machine, so the warping of the circuit board will affect the normal operation of the whole post-process.
At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the process requirements for the warpage of the circuit board are increasingly high. So we need to find out the reason why the circuit board is warped.

1. Engineering design:
Precautions for printed board design:
A. The arrangement of interlayer semi-cured sheets should be symmetrical, for example, the thickness between layers 1 to 2 and 5 to 6 should be consistent with the number of semi-cured sheets, otherwise it is easy to warp after lamination.

B. The products of the same supplier shall be used for multilayer core boards and semi-cured sheets.
C. The line graphic area of outer A and B sides should be as close as possible. If side A is a large copper surface and side B only runs a few wires, the printed board is easy to warp after etching. If the line areas on both sides are too different, some independent grids can be added on the sparse side for balance.

2. Drying plate before blanking:
The purpose of baking (150 ℃, time 8 ± 2 hours) before blanking the copper clad plate is to remove the moisture in the plate, at the same time make the resin in the plate completely solidified, and further eliminate the residual stress in the plate, which is helpful to prevent the plate from warping.
At present, many double-sided and multi-layer boards still adhere to the process of drying boards before or after blanking. However, there are also some exceptions in the plate factory. At present, the drying time of each PCB factory is also inconsistent, ranging from 4 to 10 hours. It is recommended to determine according to the grade of the printed board produced and the customer’s requirements for warpage.The two methods are feasible, namely, drying after cutting into panels or drying after the whole piece of large material. It is recommended to dry the panels after cutting. The inner plate should also be baked.

3. Longitudinal and latitudinal directions of semi-cured sheet:
The longitudinal and latitudinal shrinkage of the prepreg after lamination are different, so the longitudinal and latitudinal shrinkage must be distinguished when blanking and laminating. Otherwise, it is easy to cause warpage of the finished plate after lamination, and it is difficult to correct even if the plate is dried under pressure.
Many of the reasons for the warpage of the multilayer board are caused by the indistinct longitudinal and latitudinal directions of the semi-cured sheet and the disorderly stacking during the lamination.How to distinguish longitude and latitude? The rolling direction of the rolled semi-solidified sheet is longitude, while the width direction is latitude; For copper foil, the long side is the weft direction, and the short side is the warp direction. If you are unsure, you can check with the manufacturer or supplier.

4. Stress removal after lamination:
After hot-pressing and cold-pressing, take out the multilayer board, cut or mill the burr, and then lay it flat in the oven at 150 ℃ for 4 hours to gradually release the stress in the board and completely cure the resin. This step cannot be omitted.

5. Straightening is required for sheet plating:
When 0.4~0.6mm ultra-thin multilayer plates are used for plate surface plating and graphic plating, special pinch rolls shall be made. After the thin plates are clamped on the flying bar on the automatic plating line, a round rod shall be used to string the pinch rolls on the whole flying bar, so as to straighten all the plates on the roll, so that the plated plates will not be deformed.
Without this measure, the thin plate will bend after electroplating a copper layer of 20 to 30 microns, and it is difficult to remedy.

6. Cooling of plate after hot air leveling:
When the printed board is leveled by hot air, it is subject to the high temperature impact of the soldering bath (about 250 ℃). After being taken out, it should be placed on the flat marble or steel plate for natural cooling, and then sent to the post-processor for cleaning. This is good for preventing warpage of the board.
In some factories, in order to enhance the brightness of the lead tin surface, the plates are immediately put into cold water after being leveled by hot air, and then taken out for post treatment after a few seconds. This hot cold impact is likely to cause warping, delamination, or blistering on certain types of plates.
In addition, air floating bed can be installed on the equipment for cooling.

7. Treatment of warped board:
In a well-managed factory, 100% flatness inspection will be conducted for printed boards during final inspection. All unqualified boards will be picked out, put into the oven, dried at 150 ℃ and under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure.
Then remove the board by pressure relief and perform flatness inspection. This can save some boards. Some boards require two to three times of drying and pressing to be leveled. If the anti-warping measures mentioned above are not implemented, some boards are useless for drying and pressing, and can only be scrapped.

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