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Do you know why the expired PCB should be baked before it can be used in SMT?

2020-01-20 10:04:59

For many engineers, they may just not understand "Why must the PCB be baked before the SMT passes through the reflow oven after the PCB expires beyond the shelf life?"

The main purpose of PCB baking is to remove moisture and moisture, and remove moisture contained in or absorbed from the outside of the PCB, because some PCB materials are easy to form water molecules. In addition, after the PCB is produced and placed for a period of time, it has a chance Water vapor is absorbed into the environment, and "water" is one of the main killers that cause PCB popcorn or delamination.

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Because when the PCB is placed in an environment with a temperature of more than 100 ° C, such as reflow furnaces, wave soldering furnaces, hot air leveling or hand soldering, the "water" will turn into water vapor and then quickly expand its volume. The faster the heating speed on the PCB, the faster the water vapor will expand; when the temperature is higher, the volume of the water vapor will increase; when the water vapor cannot escape from the PCB immediately, it will have a chance to expand the PCB In particular, the Z direction of the PCB is the most fragile. Sometimes the PCB layer-to-layer vias may be broken, and sometimes it may cause PCB layer separation. More seriously, even the appearance of the PCB can be seen. I get blistering, swelling, bursting, etc .; sometimes, even though the above phenomenon is not visible on the PCB surface, it actually has internal injuries. As time goes by, it will cause the instability of electrical products or CAF and other problems. To cause the product to fail.

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Analysis of the Causes of PCB Explosion and Preventive Measures

The PCB baking process is actually quite troublesome. When baking, the original packaging must be removed before it can be placed in the oven, and then it must be baked at a temperature exceeding 100 ° C, but the temperature must not be too high to avoid the baking period. Excessive expansion of water vapor will explode the PCB. Generally, the temperature for PCB baking in the industry is generally set at 120 ± 5 ° C to ensure that water vapor can be eliminated from the PCB body before boarding on the SMT line. After soldering through a reflow oven, the baking time varies with the thickness and size of the PCB, and for thinner or larger PCBs, you must press the board with a heavy object after baking. This is for the purpose of It is necessary to reduce or avoid the tragedy of PCB bending and deformation due to stress release during cooling after baking. Once the PCB is deformed and bent, the problem of offset or uneven thickness will occur during SMT printing of solder paste. It will cause a lot of short-circuits or empty soldering during reflow.

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Precautions when PCB baking

1. The baking temperature cannot exceed the Tg point of the PCB, and the general requirement cannot exceed 125 ° C. In the early days, the Tg point of some leaded PCBs was relatively low, and now the Tg of lead-free PCBs is mostly above 150 ° C.

2. The PCB after baking should be used as soon as possible. If it is not used, it should be vacuum-packed as soon as possible. If exposed to the plant for too long, it must be rebaked.

3. Remember to install an air-drying oven in the oven, otherwise the water vapor from the oven will remain in the oven to increase its relative humidity, which is detrimental to PCB dehumidification.

4. From a quality point of view, the fresher the PCB used, the better the quality will be. After the expired PCB is used after baking, there will still be a certain quality risk.