6L Multilayer PCB with backdrill via

What is a backdrill via in PCB?

Backdrill via is actually a special kind of deep control drill. In the production of multi-layer circuit boards, such as 12-layer circuit boards, we need to connect the first layer to the ninth layer. Usually, we drill through holes (once) and then Chen copper. So the first layer connects directly to the twelfth layer, but actually, we only need the first layer to the ninth layer, and the tenth to twelfth layers are like a pillar because there is no line to connect them.

This pillar affects the signal pathway and causes signal integrity problems in communication signals. So drill this extra column (called STUB in the industry) off the back (second drill). So it's called backdrill, but it's not usually as clean as it is because a little bit of copper will be electrolyzed and the drill tip itself is sharp. Therefore, PCB manufacturers will leave a small point, the length of STUB left behind is called B value, generally in the range of 50-150 UM is good.

Benefits of backdrill via

1) Reduce noise interference.

2) Improve signal integrity.

3) Local circuit board thicker and smaller.

4) Reduce the use of buried blind hole and PCB fabrication more difficult.

If the number of layers of a PCB exceeds 4, the PCB's route from one of the two layers (except from the surface to the surface) always produces stubs similar to the following: the extra copper plated part is generated, when the frequency of the circuit signal increases to a certain height, the extra copper plated part is equivalent to the antenna, and the signal radiation causes interference to other signals around it. Serious cases will affect the normal operation of the line system, the role of Backdrill is to remove such EMI problems by drilling out excess copper-plated holes.

How PCB back drilling overcomes signal integrity issues

By using a slightly larger drill bit to re-remove these stubs after the plated through-hole has been fabricated.Backdrill holes to a predetermined and controlled depth that is close to, but not touching, the last layer used for the hole. Ideally the remaining stubs should be less than 10 mils. the diameter of the back-drilled hole is greater than the diameter of the plated through-hole. Typically, the diameter of the bit being drilled is 8 mils to 10 mils larger than the diameter of the original bit. the reason for this is that the clearance of the alignments and planes must be large enough to avoid accidentally drilling through the alignments and planes adjacent to the backdrilled hole during backdrilling. the diameter of the hole should be greater than the diameter of the plated through-hole.

What is the purpose of the backdrill hole?

The function of backdrill is to drill through the holes which do not play any role in connection or transmission, avoid the reflection (scattering) delay of high-speed signal transmission, etc. and bring "distortion" to the signal. Research shows that the main factors that affect the signal integrity of the signal system are the design of) circuit board material) transmission line) connector) chip packaging, etc. The through hole has a great influence on the signal integrity.

Under what circumstances does PCB need to do backdrill via

The stub of the inner line has a more obvious effect on the signal when the signal rate is higher than a certain level (usually above 5G). In order to reduce this effect, the stub of the inner line must be as short as possible and the shorter the effect is, the smaller the effect. There are two ways to solve this problem: First, the shortest level of stub should be given priority when traveling on high-speed lines, and the effect of stub is negligible if it is short enough. Second, when the inner line stub is too long, the backdrill process can be used to drill out the stub, but backdrill will increase the cost!

Backdrill production process?

1) Provide a PCB, which is equipped with a positioning hole that is used to drill and locate the PCB.

2) Place electroplating on the PCB after drilling a hole, and dry film seal the positioning hole before electroplating.

3) Make outer graphics on the plated PCB.

4) Graphic electroplating is performed on the PCB after the outer graphics are formed, and the positioning holes are dry-film sealed before the graphics electroplating.

5) Use the positioning holes used by one drill for backdrill positioning, and use a drill to backdrill the electroplated holes that need backdrill.

6) Wash the backdrill via after backdrill to remove the remaining drill debris from the backdrill via.

What are the technical features of backdrill via circuit board?

1) Most back circuit boards are hard circuit boards

2) Layers are generally 8 to 50

3) PCB thickness: 2.5mm or more

4) Thicker diameter ratio

5) Large printed circuit board size

6) Minimum bore diameter >=0.3mm for general head drill

7) Fewer outer lines, mostly square arrays with crimping holes

8) The backdrill via is usually 0.2MM larger than the hole that needs to be drilled

9) Backdrill depth tolerance: +/-0.05MM

10) If backdrill requires drilling into layer M, the minimum thickness of the medium from layer M to layer M-1 (the next layer of layer M) is 0.17M M

What are the main applications of backdrill via circuit board?

Backdrill circuit boards are mainly used in communication devices, large servers, medical electronics, military, aerospace and other fields.