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The BGA chip is very small, how to solve the wiring problem of the circuit board design
2021-08-29
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Author:Aure

The BGA chip is very small, how to solve the wiring problem of the circuit board design


Recently, I often encounter some purchases of circuit boards. When they find me, I am very entangled and told me: Many circuit board manufacturers cannot handle the PCBs designed by their engineers. They do not understand technology and PCB production process for purchasing, so they are in a dilemma. They don’t know how to communicate with PCB factories, and they can't express the designer's ideas and get effective suggestions for improvement...

Hearing this, my interest soared immediately. The first reaction was to ask them to send the drawings over. The BGA chip is very small, how to solve the wiring problem of the circuit board design

After reading the (PCB multi-layer board) drawings, I discovered that it is another common and old problem, but it is caused by many designers not understanding the PCB factory process!

First, let me talk about the problems I saw:

1. The BGA ball center pitch is 0.4mm, and the design engineer has designed a trace between the BGA pads.

2. Circuit board vias are designed between the four BGA pads.

Next, I will tell you why the PCB factory sees such a design and cannot make it:

A: The BGA ball center distance is 0.4mm, and the minimum BGA is 0.2mm, so that the BGA pad pitch is only 0.2mm.

If the wire is routed, the minimum limit of the wire is 3mil, which is 0.075mm. In this way, the distance between the line and the pad is only left:

(0.4-0.2-0.075)÷2=0.0625mm, only 2.46mil. This spacing will definitely not work!

Here comes the focus, I will emphasize the two extreme limitations of PCB production:

One: The minimum BGA clamping line is 3mil, and the minimum distance between the BGA clamping line and the pad is 3mil.

Two: The limit distance from the via to any graphic (line, pad, large copper):

The outer limit distance: 6mil. The limit distance of the inner layer of the circuit board: 8mil.

Someone may ask: Why is the inner layer spacing larger than the outer layer? Because, in the PCB production process, the deviation of the inner lamination and alignment will be greater!

Having said that, design engineers should be confused: Now that there are more and more 0.2mm BGA chips, is there no way?

The answer is yes: there must be! With the advent of the intelligent age, electronic devices are becoming more and more sophisticated. These are all problems that must be solved! Next, I will tell you the solution: design the hole in the disk and use the resin plugging process!

Hole in the disk, as the name implies: punch a hole on the pad! So what kind of circuit board manufacturing process is the hole in the disk? Here I will introduce it in detail!

The manufacturing process of the hole in the disk includes: drilling, electroplating, resin plugging, baking, and grinding. First, drill holes. The holes here are generally laser holes.

The minimum limit of the mechanical hole is 0.15mm, if it is 0.1mm, the laser must be used to drill the hole.

After the holes are drilled, the holes are electroplated and metalized, and then the holes are plugged with resin, baked, and finally polished to smooth them. The resin after the smoothing does not contain copper, so another layer of copper is needed to turn it into PAD, these processes are done before the original PCB drilling process, that is, the holes of the fortress hole are processed first, and then other holes are drilled, following the original normal PCB factory process.

Generally, the board with holes in the design plate will have the blind holes or buried holes we mentioned.

For example, a (PCB multi-layer board) four-layer board: 1 layer-2 layers to make holes in the disk, and then 2 layers-1 layer to route out, and then 1 layer to 4 layers can also be holed and routed.

In this way, a four-layer first-order HDI board of 1-2,1-4 or 1-2,1-4,3-4 is formed.