Electronic Design - How to strengthen BGA from the PCB design side?

The stress sources of PCB circuit board parts falling or tin cracking are as follows:

Stress comes from internal creep

For example, when the circuit board or BGA package undergoes reflow high temperature deformation, the stress will be released until it reaches a balance point. This balance point may also be when the solder ball cracks.

Stress comes from external impact or pressure

Take the mobile phone as an example. The most likely external stress is the bending in the pocket (iPhone6 plus bending door event), or the impact caused by accidentally falling on the ground.

Stress comes from thermal expansion and contraction caused by environmental temperature changes

In some areas, it freezes outdoors in winter. When the product moves from a heated indoor environment to the outdoors, drastic temperature changes will occur; in tropical areas, where there is air-conditioning indoors, huge changes in temperature will occur when the product moves from indoor to outdoor. Not to mention accidentally or deliberately putting the product in the car, the temperature rises in the sun during the day, and the temperature drops rapidly at night. The reason why temperature is important is that different materials have different expansion coefficients. The expansion coefficient of circuit board plates must be different from solder balls, and the materials of BGA packages are also different. Just imagine that ordinary roads and bridges will be designed. Expansion joints are used to reduce the risk of low thermal expansion and contraction of materials, but it seems that electronic materials can only try to find materials with relatively small expansion coefficients.

Here are a few articles related to BGA welding and dropping, it is recommended that you read it first:

How to judge whether the BGA drop is an SMT factory process or design problem?

Increasing the amount of solder paste can improve BGA soldering defects?

After understanding that the falling of BGA parts or solder ball cracking is absolutely inseparable from "stress", then we can talk about how to strengthen the BGA from the design side and try to prevent cracking. The method is not worthwhile if it is broken. Don't make a choice, think in two directions, the first is to find a way to reduce the impact of stress, and the second is to strengthen the ability of BGA to resist stress.

Here are a few ways to strengthen BGA to prevent cracking:

1. Enhance PCB's anti-deformation ability

Circuit board deformation usually comes from rapid heating and rapid cooling (thermal expansion and contraction) caused by high temperature reflow (Reflow), coupled with uneven distribution of parts and copper foil on the circuit board, worsening the circuit board The amount of deformation.

The methods to increase the resistance of the circuit board to deformation are:

1. Increase PCB thickness. If you can, it is recommended to use a circuit board with a thickness of 1.6mm or more. If you still have to use 0.8mm, 1.0mm, 1.2mm thickness boards, it is recommended to use furnace fixtures to support and strengthen the deformation of the board when passing the furnace. Although you can try to reduce

2. Use high Tg PCB material. High Tg means high rigidity, but the price will increase accordingly. This must be a trade-off.

3. Add steel bars around the BGA. If there is space, you can consider building a supportive iron frame around the BGA to strengthen its ability to resist stress, just like building a house.

4. Pouring Epoxy glue (potted) on the circuit board. You can also consider pouring glue around the BGA or on the back of the corresponding circuit board to strengthen its stress resistance.

Second, reduce the deformation of the PCB

Generally speaking, when the circuit board (PCB) is assembled into the case, it should be protected by the case, but because today’s products are getting thinner and thinner, especially hand-held devices, they often suffer from external force bending or falling impact. The resulting circuit board is deformed.

To reduce the deformation of the circuit board caused by external forces, there are the following methods:

1. Increase the buffer design of the mechanism to the circuit board. For example, designing some cushioning materials, even if the case is deformed, the internal circuit board can still remain unaffected by external stress. But the life and capacity of the buffer must be considered.

2. Add screws or positioning and fixing mechanisms around the BGA. If our purpose is only to protect the BGA, we can force the organization near the BGA to be fixed so that the vicinity of the BGA is not easily deformed.

3. Strengthen the shell to prevent its deformation from affecting the internal PCB.

3. Enhance the reliability of BGA

1. Fill the bottom of the BGA with glue (underfill).

2. Increase the size of the BGA solder pads on the circuit board. This will make the wiring of the circuit board difficult, because the gap between the ball and the ball that can be routed becomes smaller.

3. Use SMD (Solder Mask Designed) layout. Cover the solder pads with green paint.

4. Use Vias-in-pad (VIP) design. However, the vias on the solder pads must be filled with electroplating, otherwise there will be bubbles generated during reflow, which will easily cause the solder balls to break from the middle. This is similar to building a house and piling the ground. Please refer to [Principle of Vias-in-pad processing]

5. Increase the amount of solder. But it must be controlled under the condition that no short circuit is allowed.