What is the PCB warpage standard? What causes PCB warpage?
PCB is a printed circuit board, and PCB plays a role in any electronic product. To enhance everyone’s understanding of PCB, this article will introduce PCB warpage, PCB warpage standards, and the causes of warpage during PCB processing. If you are interested in PCB or the content of this article, you may wish to continue reading with the editor.
1.What is PCB warpage?
In order to properly place SMT components, the PCB must remain completely flat. For accurate placement, the placement machine must release the SMT components to the same height above the circuit board for all components.
If the PCB is warped, that is, not flat, the machine cannot maintain a constant height when releasing the components when placing them on the circuit board – this affects the accuracy of placement, especially for fine-pitch components.
In addition, flat PCBs help keep SMT components in place during reflow. If the high temperature in the reflow oven causes the flatness of the circuit board to change, the SMT components may slide out of place because they float on the molten solder, resulting in solder bridging and open circuits.
PCBs are usually made of glass fiber and some other composite materials. Most PCBs are laminated only once and are very simple.
PCB warpage means that the shape of the PCB has changed.
2.What is the standard for PCB warpage?
According to the IPS standard, the warpage degree (WD) of the required mounting PCB should be less than or equal to 0.75%. That is to say, when the WD is greater than 0.75%, it should be judged as a warp board, or a defective product.
In fact, when only plug-ins are required without installing components, the flatness requirements of the board are lower, and the WD standard can be less than or equal to 1.5%.
Of course, in order to meet higher customer needs, some manufacturers can pursue stricter standards. Some WD standards need to be less than or equal to 0.5%, and even this requirement reaches less than or equal to 0.3%.
3.Causes of warpage during PCB processing
The causes of PCB processing warpage are very complex 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 board.
1) PCB warpage caused by incoming copper clad laminates
Copper clad laminates are all double-sided, symmetrical in structure, and have no graphics. The CTE of copper foil and glass cloth are almost the same, so there is almost no warpage caused by different 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 curing speed and degree of resin in different areas during the pressing process. At the same time, the dynamic viscosity at different heating rates is also quite different, so local stress will also be generated due to the difference in the curing process.
Generally, this stress will remain balanced after pressing, but will gradually release and deform during the subsequent processing.
2) PCB warpage caused by PCB pressing process
The PCB pressing process is the main process for generating thermal stress. Similar to the pressing of copper clad laminates, local stress will also be generated due to different curing processes. Due to the thicker thickness, diverse pattern distribution, and more prepreg, thermal stress will be more difficult to eliminate than copper clad laminates.
The stress in the PCB board is released during the subsequent drilling, forming or baking process, causing the board to deform.
3) PCB warpage caused by solder mask and silk screen baking process
Since the solder mask ink cannot be stacked on each other during the curing process, the PCB board will be placed in the rack to bake the board for curing.
The solder resist temperature is around 150℃, which exceeds the Tg value of the copper clad board. The PCB is easy to soften and cannot withstand high temperatures. Manufacturers must heat both sides of the substrate evenly while keeping the processing time as short as possible to reduce the warping of the substrate.
4) PCB warping caused by PCB cooling and heating process
The tin furnace temperature is 225℃-265℃, and the hot air solder leveling time for ordinary boards is 3s-6s. The hot air temperature is 280℃-300℃.
After the solder is leveled, the board is put into the tin furnace at room temperature, and the normal temperature post-treatment water washing is carried out within two minutes after it comes out of the furnace. The entire hot air solder leveling process is a process of sudden heating and cooling.
Due to the different materials and uneven structures of the circuit board, thermal stress will inevitably occur during the cooling and heating process, resulting in microscopic strain and overall deformation warping area.
5) PCB warping caused by improper storage
PCB boards are generally firmly inserted into the shelves in the semi-finished stage. The tightness of the shelves is not adjusted properly, or the stacking of boards during storage will cause mechanical deformation of the boards.
Especially for thin boards below 2.0mm, the impact is more serious.
The above is the PCB-related content brought by the editor this time. Through this article, I hope everyone has a certain understanding of PCB. If you like this article, you may wish to continue to pay attention to our website. The editor will bring more exciting content later. Finally, thank you very much for reading, have a nice day!