In SMT chip factories, it can be seen from the PCB stackups that the classic stackup design is almost always an even number of layers rather than an odd number of layers. This phenomenon is caused by a variety of factors.
From the manufacturing process of SMT chip factory, it can be seen that the conductive surface in the PCBA circuit board is all stored on the core surface, and the material of the core surface is generally double-sided cladding. When the core plane is fully utilized, the number of conductive planes on a printed circuit board is an even number.
Even-numbered layer printed circuit boards have a cost advantage. Due to the absence of a dielectric and copper clad layer, the raw material cost of an odd-layer printed circuit board is slightly lower than that of an even-layer printed circuit board. However, because the odd-layer printed circuit board needs to add a non-standard laminated core planar bonding process on the basis of the core plane structure process, the processing cost of the odd-layer printed circuit board is significantly higher than that of the single-layer printed circuit board. Even-layer printed circuit boards. Compared with the ordinary core plane structure, adding copper pours outside the core layer structure will lead to a decrease in production efficiency and a longer production cycle. The outer core plane requires additional treatment prior to laminate bonding, which increases the risk of scratches and mismatches on the outer plane. The increased outer plane treatment will greatly increase the manufacturing cost.
In SMT processing, when the inner and outer planes of the printed circuit board are cooled after the multilayer circuit bonding process, different lamination tensions will cause the printed circuit board to bend to varying degrees. And as the thickness of the board increases, so does the risk of bending composite printed circuit boards with two different structures. Odd-numbered boards are prone to bending, and even-numbered boards can avoid board bending.
When designing, if you have an odd number of stacked layers, you can increase the number of layers in the following ways.
If the SMT chip processing plant designs the power plane of the printed circuit board with an even number and an odd signal plane, it can adopt the method of increasing the signal plane. The increased signal plane does not result in an increase in cost, but it can reduce processing time and improve the quality of the printed circuit board.
SMT chip manufacturers design printed circuit boards with an odd number of power planes and an even number of signal layers, and you can use the method of adding power planes. Another easy way is to add a ground plane in the middle of the stack without changing other settings, i.e., first route the printed circuit board on an odd-numbered layer and then duplicate a ground plane in the middle.
In microwave circuits and hybrid dielectric (dielectric with different dielectric constants) circuits, a blank signal plane can be added near the center of the PCB stack to minimize stack imbalance.