About PCB warpage, standard, handle method, and how to avoid it?

1. What is PCB warpage? and, What is warpage standard?

Board warped refers to the bending of a flat PCB board, also known as warpage. Serious warpage is a bit like an arch. PCB isn’t 100% flated at the actual manufacturing. We can judge the warpage degree of PCB by “warpage degree”.

According to IPS standard, the needed mount PCB’s warpage degrees(WD) should be less or equal to 0.75%. That’s to say, it should be judged as the warped board, or defect product when WD is more than 0.75%. Actually, board flat degrees require be less when it doesn’t mount components and just needs to plug-in parts, WD standard could be less or equal to 1.5%. Of course, some factory for match more higher customer demand, they could pursuit for more stracit stanard, some WD stanard need less or equal 0.5%, even this require is reached to less or eqaul 0.3%.

How to calculate board warped? The warped degree is equal to warped height divided by curved edge length.

According to this formula, we have two methods that could test board wrapage degrees.

First, standard flat inspection is a common measure method. It must have a standard flat apply to measure data. Specific operation is to put PCB to the standard flat, four corners touch down the ground, measure PCB’s arch height and its diagonal length, finally, this height divided by length will get PCB warped degreed.

Second, optic inspection for wrapped degree, which is the higher precision method. It uses the principle of light interference to measure the warpage of the PCB, and the accuracy can reach 0.1mil (2.54μm).

2. Which demage is PCB warpage?

Excessive warpage will not only affect the placement, but also affect the reliability of the SMT placement machine.

For that no needs mount PCB, warpage will lead to plug-in parts’ pin is hard to cut, and then the PCBA board cannot be installed on chassis or socket in the machine. That it will lead whole machine stability isn’t good, this is nightmare to assembly product line.

Fore that need to mount components’ PCB, it is not only affect mount product qulity and also damage SMT equipment. Solder is diffcult to paste to PCB as it isn’t flat, in automatic SMT line, it will lead to error placement, and components can’t mounted to pcb pad.

3. What is cause warped board?

General, it think to be have 8 causes. It have come from manufacture, another is derived from design.

Manufacturer’s cause is more, common four below:

  • 1. After the board are cutted, the baking is not made, or the baking time is not enough.
  • 2. V-CUT is too deep, leads to two sided V cut place warped.
  • 3. Board material’s TG vaule too low, board easy soften and lead to can’t endurance high temperature, and happed warped board.
  • 4. The plate thickness is less than 1.0mm, and the cold-pressed plate warping process is immature before shipment, resulting in plate warping.

Engineering design common causes are as follows:

  • 1. The copper surface area on the circuit board is uneven, with more on one side and less on the other. Where the lines are sparse, the surface tension will be weaker than that in dense places, resulting in warping of the board when the temperature is too high.
  • 2. The laminate structure may be asymmetrical due to the special medium or impedance relationship, resulting in warping of the board.
  • 3. The hollow position of the board itself is too large and many, and it is easy to warp when the temperature is too high.
  • 4. The number of panels on the board is too large and the spacing between the panels is hollow, especially the rectangular board, which is also easy to warp.

4. Avoid warped board, PCB design engineer should do this!

Common methods to improve or avoid warpage at the design end are as follows:

  • 4. 1 Copper is laid in the board to increase the tension on the surface of the board. When board length is more than 80mm and no cooper, and board thickness is less than 1.0mm, this situation will be lead to warped board.

Before copper

suggestion: If board can’t to cooper, and as can’t add thickness, so just adpt heavy weight pressure warped board.

After copper-coated

  • 4. 2 The hollow area is covered with copper and added with craftsmanship. When the board has too many hollow positions and the board is too large, it will be easy to bend after reflow soldering

Before copper is laid in the hollow

Suggestion: coppered in the hollow area to reduce the warpage of the board; in addition, if the inner part of the board does not affect the function, the copper is also laid; the last suggestion is to add the craft edge and do the copper laying while the craft is.

After copper plating

  • 4. 3 The core board and PP sheet are of the same brand. The core board and PP sheet of the multi-layer board need to be of the same brand, otherwise the board will warp.

For example, the pp sheet of the 6-layer board is asymmetric: the pp sheet of the 2-3 core board is thin, and the pp sheet of the 4-5 core board is thick, so that it will warp when it is pressed out. Therefore, the core board and the PP sheet must be of the same brand to ensure the same thickness and to ensure the symmetry of the PP sheet of the multi-layer board.

5. Warped board handle method.

Duo to many different caused, for instance, design defect, fixed warped process isn’t work, and still happen warped, what should do it?

Manufacture could put defects product into oven to baking, and under 150 celsius degree to baking it for 3 to 6 hours. Let it natural cool down under higher weight pressure. And then release weight presure take out these board, check it again. The process maybe could save some board, and if it still have bad warped degree, so that have to scrapped it.