PCB bending or warping can lead to a whole range of issues. If you are lucky, the impact of warping may be so small that it isn’t noticed. On the other hand it might be so intense that it could cause immediate electrical issues.
Warping can often be a result of Reflow soldering, as a process. In severe cases it can even lead to a situation where the components become erect and it leads to empty soldering. Warping can also be the result of a thermal mismatch as different materials experience thermal expansion at different rates.
While the reasons for warping and bending could be many, primarily it is caused by high stress on the plate. This could be on account of the following:
- Copper Surface Area- When the area of the copper surface is uneven, it can lead to uneven heating and cooling, leading to deformation.
- Weight of the board- The weight of the board itself can lead to sagging
- V- Cut- The depth and connection of the v-cut can lead to deformation
- Connection Points- The Vias of each layer can limit the expansion and contraction of the board.
The maximum permissible wrap for surface mounting of PCB is 0.75% as per IPC-6012. It is therefore extremely important to look for effective methods to ensure that warping doesn’t happen. Here are a few:
A large amount of stress on the plate is caused on account of temperature. It is therefore imperative that the temperature of the furnace is reduced or the rate of increase in temperature and the subsequent cooling is adjusted.
The transition temperature of the glass, which is the temperature at which the glass changes its state, affects warping. The lower the Tg value of the material, the more serious the deformation is likely to be. Use of sheets with higher Tg, therefore, can improve ability to withstand stress, even though the material could come at a higher cost.
Thickness of the board
The thinness of electronic products required these days, affects the board and often causes deformity after passing through a reflow oven. If it is possible to keep the thickness at 1.6mm, the probability of deformation of the board is significantly reduced.
Size and number of boards
Most reflow ovens use chains. Large sized boards, therefore are likely to be deformed in the oven on account of their own weight. It is therefore advisable to put the long side of the board as a board edge.
Use of a stove tray can also reduce deformation. In case a single tray cannot reduce deformation, adding a cover can also do the trick.
Use of router
Use of a router instead of a v-cut divider can also ensure that the structural strength of the boards isn’t destroyed.
The semi-solidified sheets should be arranged in a symmetrical manner, otherwise there are chances of warping. For example, in an 8-layer board, the thickness and numbers of prepreg should be the same in 1~2L and 7~8L. Otherwise, there are chances of the board bending. It is also advisable to use products from the same supplier when it comes to the multi layered board as well as the semi-cured sheets.
Latitude and longitude of semi-cured tablets
The warp and weft shrinkage rates of semi-cured sheet after lamination are different. It is advisable to roll the semi-solidified sheet in the direction of the longitude and the width in the direction of the latitude.
De-stress after lamination
Post the hot and cold pressing, the laminates need to be taken out and laid flat in the oven to release the stress in the plate. Baking the laminate for about 8 hours at 150 degree Celsius can remove excess water and solidify the resin.
Straightening of plate
When thin multilayer plates are used for surface and pattern plating, it is important to use special clamping rolls. Once the plates are clamped on the plating line, the clamping rolls need to be stringed so that all the plates are straightened. If this process is missed the plate can bend and the deformity can be hard to correct.
Cooling of board after hot air leveling
The printed board has to bear the high temperature of the solder groove. It is important that it should be allowed time for natural cooling by putting it on flat marble or steel plate.
Treatment of boards
All PCBs are inspected for flatness. The boards that do not make the cut are put in the oven for between 3-6 hours at a temperature of 150 degree Celsius and under pressure. Sometimes a part of the plate can be saved on account of this process of anti-warping.