The cause of PCB board burst and its solution

The main reasons for popcorn or Delamination of a PCB board are: Plate water absorption, α2/z-CTE is too big these two categories, and the "plate water absorption" caused by the explosion board accounted for 70% of the bad, other reasons such as PCB structure uneven growth and contraction, cold and heat imbalance, process injury and bad black... Although we can not rule out the possibility, but the ratio is not too high.

Why "water" is the main cause of PCB board burst?

The "water" has little effect on the bursting plate below 100°C. When the temperature exceeds 100°C, the "water" becomes the resin's plasticizer. If the resin absorbs more water, Tg will decrease (△Tg should be less than 5°C) and the rubber state will come earlier, it will cause instant Swelling (Swelling) in the Z direction of the plate and rapid cracking (it is most likely to happen between 100°C and Tg), and the pressure (psi) after the water vapor exceeds 100°C will increase in proportional series.

Generally, the CTE(expansion coefficient) of X and Y plates is relatively stable, between 15 and 16ppm/°C. In addition, the hidden water in the plate will also become plastic resin, and the external water together to help Zhou.

When the resin temperature exceeds the Tg point, it will change into rubber state. At this time, the "water" part of the explosive plate has become a supporting role, and at this time, most of the water has been evaporated into steam. Moreover, the rubber state is soft, and it is not easy to have explosive plate.

Where does the PCB "water" come from?

Since the "water" is so important to the explosion plate, then we have to study where the water comes from, on our general understanding and cognition, most of the "water" may come from the outside world, may be in the circuit board process inhalation attachment, or PCB storage from the environment gradually Diffusion (Diffusion) into; But the internal structure of the plate is easy to hide water is also one of the possible reasons; Another thing you might not expect is that PCB resins also have water molecules embedded in their formula, which, when heated, creates its own water. So in summary, the plate absorbs "water" and the water storage place is:

1. Resin molecules have similar polarity: water (resin molecules already have water molecules in them, and when they contain OH, water will form).

2, the interface between resin and glass fiber is easy to hide water (the board uses a resin and a glass fiber woven with warp and weft, if the weaving is not dense enough, there will be gaps, it is generally recommended to choose low permeability flat fiber cloth is not easy to hide water).

3, resin and copper foil interface is also easy to store water.

4, the hole of the plate will hide water.

PCB absorbent explosion board improvement program - baking

Since "water" is the main cause of PCB board explosion, so as long as the water removal in the PCB should be able to solve most of the problem of board explosion, and [baking] is the best way to remove the PCB board external water. Since the purpose of baking is to remove moisture, the baking conditions are best to meet the following requirements:

1, the baking temperature is heated to a little more than 100°C (105°C is recommended, because the temperature of the oven will have errors), so that the water can be turned into water vapor can be relatively easy to distribute.

2, when baking, it is best to put each piece of the board separately, so that the water is easier to volatilize. If PCBS are stacked on top of each other, water will not escape effectively.

3. The oven must have an exhaust device, otherwise it is useless to bake all the steam in the oven.

From the selection of PCB board and the process to control the water absorption conditions

Although baking is the best way to improve the burst board, baking not only wastes time, but also wastes equipment and manpower, and the Tg value will decrease after baking PCB is also a problem. A better method is to control the water absorption conditions from the selection of PCB circuit board and the manufacturing process.

1, plate itself if there is polarity, it is easy to absorb water. Try to choose not absorbent resin to prevent the plate from absorbing water.

2, can choose to open (flat) fiber cloth. Reduce the gap between the resin and the glass fiber interface to reduce the possibility of water accumulation.

3. Control PCB pressing process. After PCB has been pressed and baked, the Tg values of the samples can be measured twice by the same method and the same machine. If the △Tg of Tg2-Tg1 exceeds 2 ~ 3°C (Hardening including polymerization and cross-linking), the hardening reaction (including cross-linking) of the hardening process is not in the hardening position. These unseasoned slabs are prone to water absorption and rupture.

4. Measure the Tg value of the problem board according to TM-650 test manual Section 2.4.24.1 TMA method, and compare it with the specification value of the supplier. If the measured value is more than 5°C below the specification value, it indicates that the problem plate has water absorption lesions. The moisture in the resin acts as a plasticizer, which will not only lower Tg, but also make the rubber state arrive earlier.

Multilayer PCB stored for more than three months may exhibit stress concentration (from pressing) and water absorption (which will increase Z-expansion). Explosion-proof baking (105°C+24 hours) before welding is required, or 50 cell phone plates are pressed and baked in nitrogen gas at 185°C+70PSI for 2 hours using a leveler. The client side of the plate more than three months to bake before welding will reduce the plate explosion, baking not only increases the cost and adverse to OSP. When baking, it is necessary to separate a single piece of baking, so as to fully discharge the water.