The bow and twist issue on Printed Circuit Board can cause components and parts registration in PCB assembly process, when handling with SMT or THT, if the component pins are positioning not trim, which will bring a lot of difficulties on assembly and installation.
IPC-6012 standard define the maximum bow and twist 0.75% on circuit boards of SMB—SMT, other PCB bow and twist is generally not exceed 1.5%; Electronic assembly plants allow bow and twist (double/multilayer PCB) is usually 0.70—0.75%. As a matter of fact, many rigid PCB boards (board thickness 1.6 mm) require bow and twist is less than 0.5% such as SMB /BGA; Some factories even require less than 0.3%.
Prevention of bow and twist on electronic circuit boards:
1. PCB Design: PCB designers should know the difference of residual of layers’ copper will affect the bow and twist, so they need try to balance the residual copper of each layer, it’s high-efficiency.
2. Lamination: Prepreg between PCB layers arranged must be symmetrical if there are not special impedance to control,this is very important.And another factor is that multi-layer core and prepreg shall use the same vendor products,because different vendor’s products have different ingredients ratio. The patterning area of outer C/S can be as close as possible, independent grids can be used as well.
3. Baking before cut lamination.
4. Before multi-layer PCB pressing, which should pay attention to the longitude and latitude direction of prepreg.
5. Baking before PCB drilling: normally 150 degrees for 4 hours.
6. Don’t scrub by machine for thin PCB board, suggest using chemistry to clean; and using certain fixture to avoid PCB bending in PCB Plating process.
7. Rigid PCB bending measure: 150 degrees or heating 3-6 hours use smooth steel plate with heavy pressing, baking for 2-3 times.
8. After HASL process, put FR4 PCB on the smooth marble or steel plate to natural cooling down until room temperature.
The right way to deal with warping PCB Board: 150 degrees or heat pressing 3-6 hours, pressed by smooth plate and baking 2-3 times.
Unreasonable circuit layout of gerber files will also cause bad bow and twist.For example, if the difference of distribution about copper on each layer is too much,that will cause the different thickness of dielectric between layers when finished.As you know,a symmetric stack up is good to meet a nice PCBs’ bow and twist. So PCB designer should pay more attention to the stack up of PCBs.Times of laminations will also affect the warp, like the boards with blind and buried holes.
PCBA123 is a professional PCB manufacturer from China with high quality and good price.We can usually control the warp at 0.5%(max.) for the multi-layers board,moreover, if you have special requirement at the board’s warp, we also can meet the 0.3%(max.) by our special controlling.