PCB Blog - Common causes and avoidance methods of PCB warping

PCB warping is a term used in the industry. In fact, it refers to the bending of a flat PCB, which is also called warping. In severe cases, the warping is a bit like an arch bridge.

In actual production, PCB is not 100% flat and somewhat bent. We can judge the warpage of PCB by "warpage".

According to IPC standards, the PCB warpage to be mounted shall be ≤ 0.75%, which is a qualified product. That is, if the PCB warpage exceeds 0.75%, it will be judged as warped and unqualified. PCB boards do not need to be mounted (only contain plug-in components), the requirements for flatness are lower, and the warpage can be relaxed to ≤ 1.5%.

In fact, in order to meet the requirements of high-precision and high-speed mounting, some manufacturers have more stringent requirements on PCB warpage, with requirements for warpage ≤ 0.5%, and even for individual requirements ≤ 0.3%.

How to calculate the PCB warpage?

1. Warpage=Warpage height/edge length

According to this calculation formula, we generally use the following two methods to detect PCB warpage:

The first method is the common detection method - marble detection method. It is directly measured with marble, because the marble is relatively flat (or use glass plates with a thickness of ≥ 5mm). The specific measurement operation is to place the PCB flat on the marble, with the four corners touching the ground, measure the arching height in the middle of the PCB and the diagonal length of the PCB, and then divide the arching height by the diagonal length of the PCB to get the PCB warpage.

The second method is the most advanced measurement method - optical detection method. The equipment used is the flatness meter. It uses the principle of optical interference to measure PCB warpage, and the accuracy can reach 0.1mil (2.54 μ m）。

2. What are the hazards of PCB warping?

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

For PCB boards without patch, the board warping will cause the pin pin of plug-in components on the board to be difficult to cut flat, and the PCBA board will naturally be unable to be installed on the corresponding chassis or socket in the machine, which will adversely affect the reliability of the whole machine. Therefore, the assembly plant is very tired of encountering plate warping.

For PCB boards requiring placement, the board warping will not only affect the placement quality, but also may damage the SMT equipment. In the automated SMT production line, if the PCB board is not flat, it will be difficult to brush or cannot brush solder paste, and it will also cause inaccurate positioning, which will lead to the components can not be mounted on the bonding pad, or even damage the automatic insertion machine.

3. How is the warping caused? 8 common causes

There are many reasons for plate warping. Generally speaking, it is mainly caused by the manufacturer and the design side.

There are many reasons for the plate warping caused by the manufacturer, and the common causes are as follows:

1. No baking board is made after the material is opened, or the baking board time is not enough. 2. The V-CUT is too deep, causing the V-cut on both sides to be warped. 3. The TG value of the plate is too low, and the plate is easy to soften, resulting in unacceptable high temperature and plate warping.

4. The plate thickness is less than 1.0mm, and the cold pressing plate warping process is not mature before shipment, resulting in plate warping.

The common causes of plate warping at the engineering design end are as follows:

1. The area of copper surface on the circuit board is uneven, with more on one side and less on the other. The surface tension in places with sparse lines will be weaker than that in places with dense lines, which will lead to plate warping when the lines are too hot. 2. Due to the special medium or impedance relationship, the laminated structure may be asymmetric, leading to warping. 3. The hollowed out 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 boards is too many and the space between boards is hollow, especially for rectangular boards, which are easy to warp.

4.  To avoid warping, PCB design engineers should do this!

Dry goods!

According to the feedback of Huaqiu customers, there are several common methods to improve or avoid plate warping at the design end:

Method 1: Lay copper in the board to increase the tension on the board surface

When the board length exceeds 80mm, and there is no copper in the board, and the board is thin (the board thickness is less than 1.0mm), the board will be warped. (This is about FR4 material)

Before the board is paved with copper

The editor's suggestion is to lay copper in the board to increase the tension on the board surface without affecting the function. If copper cannot be laid in the board and the board cannot be thickened, the pressing plate can only be used for warping. (The materials are drawn by the author at random, for reference only)

After the board is paved with copper

Method 2: Lay copper on the hollowed out area and process the edge

When the board has too many hollow positions and the board is too large, excessive reflow soldering will easily bend.

The hollow is not paved with copper

The editor's suggestion is: lay copper in the hollow area to reduce the warpage of the board; In addition, under the condition that the function of the board is not affected internally, copper is also paved; Finally, it is suggested that the process edge should be paved with copper.

After laying copper on the hollow

Method 3: The core plate and PP sheet are of the same brand

The core plate and PP sheet of the multilayer board shall be of the same brand, otherwise the board will be warped.

For example, the six ply boards have asymmetric pp sheets: 2-3 core boards have thin pp sheets, 4-5 core boards have thick pp sheets, and the boards will warp when pressed out. Therefore, the core plate and PP sheet shall be of the same brand to ensure the same thickness and ensure the symmetry of PP sheet of multilayer PCB.

5. Treatment of warped PCB Due to various reasons, such as unreasonable design and unfulfilled warping prevention measures, the PCB finally appears warping. What should we do?

The manufacturer can put unqualified boards into an oven, bake them for 3-6 hours at 150 ℃ and under heavy pressure, and cool them naturally under heavy pressure; Then, take out the boards by pressure relief, and check the flatness, so that some boards can be saved. Some boards need two or three times of baking and pressing to be leveled. If the board is still warped after repeated baking and pressing, it can only be scrapped.

Board warping is an unavoidable problem in PCB production, especially in mass production. In order to reduce the probability of plate warping, designers can consider laying copper as much as possible under the premise of cost permitting; However, manufacturers can select high-quality A-grade plates and optimize the production process to avoid warping.

As a well-known PCB manufacturer in China, iPCB Circuit adopts domestic well-known brands of PCB boards, and most of them use high-quality TG150 PCB boards to avoid board warping during wave soldering. Even if the customer chooses TG130, iPCB still uses TG150 PCB boards, but charges according to the price of TG130, and does a good job at any cost!

In addition, in order to ensure high quality, all PCBs in iPCB will be baked for at least 4 hours after opening. Due to the design of the board itself, PCB warping occurs. Huaqiu also has a mature solution, especially its cold pressing process is very mature.