The hazards of PCB warpage
In the automated surface mount line, the PCB warpage will cause inaccurate positioning, components cannot be inserted or mounted on the holes and surface mount pads of the board, and even the automatic insertion machine will be damaged.
The circuit board on which the components are installed is bent after soldering, and the component feet are difficult to cut neatly. The board cannot be installed on the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to encounter the circuit board warping.
The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for PCB boards that are home to various components.
In the IPC standard, it is specifically pointed out that the allowable warpage deformation of PCBs with surface mount devices is 0.75%, and the allowable warpage deformation of PCBs without surface mounting is 1.5%.
In fact, in order to meet the requirements of high-precision and high-speed placement, some electronic assembly manufacturers have stricter requirements on the amount of warpage. If required, the allowable amount of warpage is 0.5%, and even some require 0.3%.
The PCB 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 will inevitably occur and cause warpage.
At the same time, the PCB processing process will go through various processes such as high temperature, mechanical cutting, wet processing, etc., which will also have an important impact on the warpage of the board. In short, the reasons for the warpage of the PCB can be complex and diverse. How to reduce or eliminate the material characteristics Distortion or warpage caused by processing has become one of the complex problems faced by PCB manufacturers.
Analysis of the causes of PCB warpage
The warpage of the PCB needs to be studied from several aspects such as material, structure, pattern distribution, processing process, etc. The article will analyze and explain various reasons and improvement methods that may cause warpage.
The uneven copper surface area on the circuit board will worsen the bending and warping of the board.
Generally, a large area of copper foil is designed on the circuit board for grounding purposes. Sometimes there is also a large area of copper foil designed on the Vcc layer. When these large-area copper foils cannot be evenly distributed on the same circuit board When it is installed, 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 be performed at the same time, it will cause different stress and warpage. At this time, if the temperature of the board has reached the upper limit of the Tg value, the board will begin to soften and cause warpage.
The connection points (vias, vias) of each layer on the circuit board will limit the expansion and contraction of the board.
Today’s circuit boards are mostly multi-layer boards, and there will be rivet-like connection points (vias) between the layers. The connection points are divided into through holes, blind holes, and buried holes. Where there are connection points, the board will be restricted. The effect of expansion and contraction will also indirectly cause plate bending and plate warping.
Reasons for PCB warpage:
(1) The weight of the circuit board itself will cause the board to dent and deform
Generally, the reflow furnace uses a chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board are used as fulcrums to prop up the entire board.
If there are heavy parts on the board, or the size of the board is too large, it will show a depression in the middle due to the amount of the board, causing the board to bend.
(2) The depth of V-Cut and the connecting strip will affect the warpage of the panellization
Basically, V-Cut is the culprit that destroys the structure of the board, because V-Cut cuts grooves in the original large sheet, so the V-Cut is prone to warpage.
The influence of the material, structure, and graphics on the warpage of the board:
The PCB is formed by pressing the core board, the prepreg and the outer copper foil. The core board and the copper foil are deformed by heat when they are pressed together. The amount of warpage 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; while for ordinary FR-4 base material, the Z-direction CTE under the Tg point is (50~70)X10-6; above the TG point is (250~350)X10-6, X-direction CTE is generally similar to copper foil due to the existence of glass cloth.
Warpage caused during PCB processing
The reason for the warpage of PCB processing is very complicated and can be divided into thermal stress and mechanical stress.
Among them, thermal stress is mainly generated during the pressing process, and mechanical stress is mainly generated during the stacking, handling, and baking of the plates. The following is a brief discussion in the order of the process.
1. Incoming copper-clad laminate:
The copper-clad laminates are all double-sided, with symmetrical structure and no graphics. The CTE of copper foil and glass cloth is almost the same, so there is almost no warpage caused by the difference in CTE during the pressing process.
However, the size of the copper-clad laminate press is large, and the temperature difference in different areas of the hot plate will cause slight differences in the resin curing speed and degree in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so it will also produce Local stress due to differences in the curing process.
Generally, this kind of stress will maintain balance after pressing, but will gradually release and deform during future processing.
The PCB pressing process is the main process that generates thermal stress. Similar to the pressing of copper-clad laminates, it will also produce local stress caused by the difference in the curing process. Due to the thicker thickness, diverse pattern distribution, and more prepregs, the heat The stress will be more and more difficult to eliminate than the copper-clad laminate.
The stress in the PCB board is released during subsequent drilling, shape, or grilling processes, causing the board to deform.
3. Baking process of solder mask and characters:
Since the solder mask inks cannot be stacked on each other during curing, the PCB boards will be placed in a rack to bake the boards to cure. The solder mask temperature is about 150°C, which just exceeds the Tg point of medium and low Tg materials. The resin above the Tg point is highly elastic. The parts are easy to deform under the action of their own weight or the strong wind of the oven.
4. Hot-air solder leveling:
The temperature of the tin furnace is 225℃~265℃, and the time is 3S-6S during the leveling of the ordinary board hot-air solder. The hot air temperature is 280℃~300℃.
When the solder is leveled, the board is put into the tin furnace from room temperature, and the post-treatment water washing at room temperature is carried out within two minutes after being out of the furnace. The entire hot-air solder leveling process is a sudden heating and cooling process.
Due to the different materials of the circuit board and the uneven structure, thermal stress will inevitably appear during the cooling and heating process, leading to microscopic strain and overall deformation and warping area.
The storage of PCB boards in the semi-finished stage is generally firmly inserted in the shelf, and the tightness of the shelf is not adjusted properly, or the stacking of the boards during the storage process will cause the board to be mechanically deformed. Especially for thin plates below 2.0mm, the impact is more serious.
In addition to the above factors, there are many factors that affect the deformation of the PCB board.
Prevention of PCB warpage and deformation.
Circuit board warpage has a great influence on the production of printed circuit boards. Warpage is also one of the important problems in the production process of circuit boards. The board with components is bent after soldering, and the component feet are difficult to be neat.
The board cannot be installed on the chassis or the socket inside the machine, so the warpage of the circuit board will affect the normal operation of the entire subsequent process.
At this stage, the printed circuit board has entered the era of surface mounting and chip mounting, and the requirements of the process for the warpage of the circuit board can be said to be higher and higher. So we have to find the reason for the warping of the halfway help.
1. Engineering design:
Matters needing attention when designing the printed circuit board:
A. The arrangement of the interlayer prepregs should be symmetrical, for example, for six-layer boards, the thickness between 1-2 and 5-6 layers and the number of prepregs should be the same, otherwise, it will be easy to warp after lamination.
B. Multi-layer core board and prepreg should use the same supplier’s products.
C. The area of the circuit pattern on side A and side B of the outer layer should be as close as possible. If the A surface is a large copper surface, and the B surface is only a few lines, this kind of printed board is easy to warp after etching. If the area of the lines on the two sides is too different, you can add some independent grids on the thin side for balance.
2. Baking board before cutting:
The purpose of baking the board before cutting the copper-clad laminate (150 degrees Celsius, time 8±2 hours) is to remove the moisture in the board, and at the same time make the resin in the board completely solidify, and further eliminate the remaining stress in the board, which is useful for preventing the board from warping.
At present, many double-sided and multi-layer boards still adhere to the step of baking before or after the blanking. However, there are exceptions for some board factories. At present, the drying time of PCB factories is 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.
Bake after cutting into a jigsaw or blanking after the whole block is baked. Both methods are feasible. It is recommended to bake the board after cutting. The inner board should also be baked.
3. The latitude and longitude of the prepreg:
After the prepreg is laminated, the warp and weft shrinkage rates are different, and the warp and weft directions must be distinguished during blanking and lamination. Otherwise, it is easy to cause the finished board to warp after lamination, and it is difficult to correct it even if the pressure is applied to the baking board.
Many reasons for the warpage of the multilayer board are that the prepregs are not distinguished in the warp and weft directions during lamination, and they are stacked randomly.
How to distinguish latitude and longitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the weft direction; for the copper foil board, the long side is the weft direction, and the short side is the warp direction. If you are not sure, you can check with the manufacturer or supplier.
4. Stress relief after lamination:
After the multi-layer board is hot-pressed and cold-pressed, it is taken out, cut, or milled off the burrs, and then placed flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and fully cure the resin. This step cannot be omitted.
5. The thin plate needs to be straightened during electroplating:
When the 0.4～0.6mm ultra-thin multilayer board is used for surface electroplating and pattern electroplating, special clamping rollers should be made. After the thin plate is clamped on the flybus on the automatic electroplating line, a round stick is used to clamp the entire flybus. The rollers are strung together to straighten all the plates on the rollers so that the plates after plating will not be deformed.
Without this measure, after electroplating a copper layer of 20 to 30 microns, the sheet will bend and it is difficult to remedy it.
6. Cooling of the board after hot air leveling:
When the printed board is leveled with hot air, it is impacted by the high temperature of the solder bath (about 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate for natural cooling, and then sent to a post-processing machine for cleaning. This is good for preventing the warpage of the board.
In some factories, in order to enhance the brightness of the lead-tin surface, the boards are put into cold water immediately after the hot air is leveled, and then taken out after a few seconds for post-processing. This kind of hot and cold impact may cause warping on certain types of boards. Twisted, layered, or blistered.
In addition, an air flotation bed can be installed on the equipment for cooling.
7. Treatment of warped board:
In a well-managed factory, the printed board will be 100% flatness checked during the final inspection. All unqualified boards will be picked out and placed in an oven, baked at 150 degrees Celsius under heavy pressure for 3-6 hours, and cooled naturally under heavy pressure.
Then relieve the pressure to take out the board, and check the flatness, so that part of the board can be saved, and some boards need to be bake and pressed two or three times before they can be leveled. If the above-mentioned anti-warping process measures are not implemented, some of the boards will be useless and can only be scrapped.
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