The design of a multilayer PCB (printed circuit board) can be very complicated. The fact that the design even requires the use of more than two layers means that the required number of circuits will not be able to be installed only on the top and bottom surfaces. Even when the circuit does fit in the two outer layers, the PCB designer can decide to add power and ground layers internally to correct performance defects.

From thermal issues to complex EMI (Electromagnetic Interference) or ESD (Electrostatic Discharge) issues, there are many different factors that can lead to suboptimal circuit performance and need to be resolved and eliminated. However, although your first task as a designer is to correct electrical problems, it is equally important not to ignore the physical configuration of the circuit board. Electrically intact boards may still bend or twist, making assembly difficult or even impossible. Fortunately, attention to PCB physical configuration during the design cycle will minimize future assembly troubles. Layer-to-layer balance is one of the key aspects of a mechanically stable circuit board.

01
Balanced PCB stacking

Balanced stacking is a stack in which the layer surface and cross-sectional structure of the printed circuit board are both reasonably symmetrical. The purpose is to eliminate areas that may deform when subjected to stress during the production process, especially during the lamination phase. When the circuit board is deformed, it is difficult to lay it flat for assembly. This is especially true for circuit boards that will be assembled on automated surface mount and placement lines. In extreme cases, deformation can even hinder the assembly of the assembled PCBA (printed circuit board assembly) into the final product.

IPC’s inspection standards should prevent the most severely bent boards from reaching your equipment. Nevertheless, if the PCB manufacturer’s process is not completely out of control, then the root cause of most bending is still related to the design. Therefore, it is recommended that you thoroughly check the PCB layout and make necessary adjustments before placing your first prototype order. This can prevent poor yields.

02
Circuit board section

A common design-related reason is that the printed circuit board will not be able to achieve acceptable flatness because its cross-sectional structure is asymmetric about its center. For example, if an 8-layer design uses 4 signal layers or copper over the center covers relatively light local planes and 4 relatively solid planes below, the stress on one side of the stack relative to the other may cause After etching, when the material is laminated by heating and pressing, the entire laminate will be deformed.

Therefore, it is good practice to design the stack so that the type of copper layer (plane or signal) is mirrored with respect to the center. In the figure below, the top and bottom types match, L2-L7, L3-L6 and L4-L5 match. Probably the copper coverage on all signal layers is comparable, while the planar layer is mainly composed of solid cast copper. If this is the case, then the circuit board has a good opportunity to complete a flat, flat surface, which is ideal for automated assembly.

03
PCB dielectric layer thickness

It is also a good habit to balance the thickness of the dielectric layer of the entire stack. Ideally, the thickness of each dielectric layer should be mirrored in a similar way as the layer type is mirrored.

When the thickness is different, it may be difficult to obtain a material group that is easy to manufacture. Sometimes due to features such as antenna traces, asymmetric stacking may be inevitable, because a very large distance between the antenna trace and its reference plane may be required, but please make sure to explore and exhaust all before proceeding. Other options. When uneven dielectric spacing is required, most manufacturers will ask to relax or completely abandon bow and twist tolerances, and if they cannot give up, they may even give up work. They don’t want to rebuild several expensive batches with low yields, and then finally get enough qualified units to meet the original order quantity.

04
PCB thickness problem

Bows and twists are the most common quality problems. When your stack is unbalanced, there is another situation that sometimes causes controversy in the final inspection-the overall PCB thickness at different positions on the circuit board will change. This situation is caused by seemingly minor design oversights and is relatively uncommon, but it can happen if your layout always has uneven copper coverage on multiple layers in the same location. It is usually seen on boards that use at least 2 ounces of copper and a relatively high number of layers. What happened was that one area of ​​the board had a large amount of copper-poured area, while the other part was relatively free of copper. When these layers are laminated together, the copper-containing side is pressed down to a thickness, while the copper-free or copper-free side is pressed down.

Most circuit boards using half ounce or 1 ounce of copper will not be affected much, but the heavier the copper, the greater the thickness loss. For example, if you have 8 layers of 3 ounces of copper, areas with lighter copper coverage can easily fall below the total thickness tolerance. To prevent this from happening, make sure to pour the copper evenly into the entire layer surface. If this is impractical for electrical or weight considerations, at least add some plated through holes on the light copper layer and make sure to include pads for holes on each layer. These hole/pad structures will provide mechanical support on the Y axis, thereby reducing thickness loss.

05
Sacrifice success

Even when designing and laying out multi-layer PCBs, you must pay attention to both electrical performance and physical structure, even if you need to compromise on these two aspects to achieve a practical and manufacturable overall design. When weighing various options, keep in mind that if it is difficult or impossible to fill the part due to the deformation of the bow and twisted forms, a design with perfect electrical characteristics is of little use. Balance the stack and pay attention to the copper distribution on each layer. These steps increase the possibility of finally obtaining a circuit board that is easy to assemble and install.