# Why are PCB multilayer boards all even-numbered layers?

• It can be manufactured in a PCB factory. The four-layer board generally uses a core with one copper foil on each sideand a three-layer board  with one copper foil on one side. They must be pressed together.
• The process cost difference between the two is that the four-layer board has one more copper foil and bonding layer. The cost difference is not significant. When the PCB factory  makes a quote, they are generally quoted on an even number basis. Also, 3-4 layers are commonly quoted as a grade. (For example:If you design a 5-layer board, the other party will quote at the price of a 6-layer board. That is to say, the price you design for 3 layers is the same as the price you design for 4 layers.)
• In the PCB process technology, the four-layer PCB board is better controlled than the three-layer board, mainly in terms of symmetry. The warpage of the four-layer board can be controlled below 0.7%  (IPC600 standard),  but the size of the three-layer board is large. At that time, the warpage will exceed this standard, which will affect the reliability of the SMT assembly .and the entire product. Therefore, the designer shouldnot design the odd-numbered layer board. Even if the odd-numbered layer is necessary, it will be designed as a fake even-numbered layer. That is to design 5 layers into 6 layersand 7 layers into 8 Layers.

## Calculation Method of Multilayer PCB Stack-Up:

A: Thickness of inner layer      B: Thickness of PP sheet

E: Thickness of inner copper foil    F: Thickness of outer copper foil

X: Finished board thickness    Y: Finished PCB tolerance

### 1. Calculate the upper and lower limit of pressing:

Usually tin plate: upper limit -6MIL, lower limit-4MIL

Gold plate:: upper limit -5MIL, lower limit -3MIL

For example, tin plate: upper limit=X+Y-6MIL lower limit=X-Y-4MIL

Calculate the median = (upper limit + lower limit)/2

≈A+the area of the second layer of copper foil%*E+the area of the third layer of copper foil%*E+B*2+F*2

The inner cutting material of the above conventional four-layer board is 0.4MM smaller than the finished board, using a single 2116 PP sheet to press. For special inner layer copper thickness and outer layer copper thickness that more than 1OZ, the copper thickness should be considered when choosing the inner layer material.

### 2. Calculate the Pressing Tolerance:

Upper limit = Finished board thickness + Finished on-line tolerance value-[Plating copper thickness, green oil character thickness

(Conventional 0.1MM)]-The theoretically calculated thickness after pressing

Lower limit = finished board thickness-finished product off-line tolerance value- [electroplating copper thickness, green oil character thickness

(Regular 0.1MM)]-The theoretically calculated thickness after pressing

### 3. Commonly types of PP sheets:

PP
KB
KB
1080
0.07MM
0.065MM
2116
0.11MM
0.105MM
2116
0.11MM
0.105MM

Generally, do not use two PP sheets with high resin content together. If the inner layer of copper is too small, please use PP sheets with high resin content. 1080 PP sheets have the highest density and low resin content. Do not press single sheets as much as possible. Only 2 sheets of 2116 and 7630 PP sheets can be pressed into  thick copper plates above 2OZ. The layer cannot be pressed by a single sheet of PP. 7628 PP sheet can be pressed by a single sheet, 2 sheets, 3 sheets, or up to 4 sheets.

Explanation of theoretical thickness calculation of multilayer PCB board after pressing

Thickness after PP lamination = 100% residual copper lamination thickness-inner copper thickness*(1-Remaining copper rate%)

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