Thick-Copper PCB Fabrication
With the highly development of Electricity, power, energy, electronic industry, thick copper board is widely used by PCB design engineers. Thick copper PCB is mainly used for central power system and power electronic devices, which is as it enables the board to conduct higher current and decrease the product size.
Normally thick copper board mean the finished copper thickness of the board is more than 12 OZ, this is the most important factor for us to compare the thick copper PCB from those standard PCB with 1OZ or 2OZ copper.
Thick Copper PCB Material
The base material used for thick copper plate must have the following requirements.
1. High heat resistance resin system, high heat resistance.
2. Low expansion coefficient system.
3. Super high resin content and good porosity, so that it has very excellent filling capability.
4. Special glass fiber structure, which can eliminate stress points and improve reliability.
Thick Copper PCB Layout Rules
PCB has small volume, high density, many layers, complex layout, strong anti-interference ability and high manufacturability.
A. Uniformity and Symmetry of Inner Layer Copper Laying
1. Layout of PAD Location
Due to the superposition effect of inner solder pads and the limitation of resin flow during lamination of thick copper, it is easy to make the high residual copper ratio of the place after lamination thicker than that of the place with low residual copper ratio, resulting in the thickness uniformity of the plate, affecting the subsequent placement and assembly
2. Thick Copper layout of Inner Line Should Be Symmetrical
Due to the large thickness of copper, the difference between CTE of copper and CTE of base material is large, and the change of thickness under pressure and heating is quite large.
The problem of warpage is easy to occur due to the asymmetric distribution of inner copper.
3. Laying Copper Spots or Blocks on No Copper Area of Inner Layer
The large copper surface can be changed to copper point to optimize the layout of wiring, with uniform density and good consistency, so as to reduce the structural consistency after pressure and heating.
B. Increase Residual Copper Rate of Inner Layer for Thick Copper
When the copper thickness is increased, the line gap is deepened, and the residual copper rate is the same, the resin filling is increased, and the semi curing sheet is increased.
If the resin is too small, it is easy to lead to the lack of glue layering and uneven plate thickness.
If the residual copper rate is low, it needs a large number of resin filling, and the resin flow is limited.
Under the action of pressure, the thickness of the dielectric layer in the copper area, the line area and the base material area is different. The thickness of the dielectric layer between the lines is the thinnest, which is easy to cause the failure of Hi-Pot.
C. Increases Width and Spacing of Line for Thick Copper
1.Thick copper plate, increasing line width and line spacing helps to reduce the difficulty of etching.
2. It can improve the effect of laminating and filling glue, with large spacing and more glass fiber cloth filling, and large spacing can reduce the filling pressure of glue.
D. Inner PAD Layout Optimization of Thick Copper
1.The thickness of copper is large, Multi-layer stacking, and copper stacking is larger. The drilling tool is easy to wear due to long-time friction, which affects the quality of hole wall and the reliability of hole. It is suggested that the layout of inner layer non functional pad should be as few as possible, no more than four layers.
2. The pad should be designed as large as possible. The small pad will produce more stress in the drilling process, and the heat conduction speed will be fast in the process of treatment, which will easily lead to the copper corner crack of the pad.
3. Add Inner Layer Independent PAD to Hole Wall.
Increase the effective distance between the hole copper and the inner welding pad, reduce the micro short and caf failure caused by the hole wall quality.
Bigger Line Space Thick Copper Layout
Small Line Space Thick Copper Layout
Small PAD of inner layer lead to copper corner crack of the pad
Key Control Items for Thick Copper PCB on Fabrication Processes
After several times of etching, the problem is not filled with enough glue, the inner layer of the drill hole is cracked, the hole wall quality is not in control, the copper thickness is large, the side etching is increased, and the etching compensation coefficient needs rich experience.
Deep line gap, which need large amount of resin filling, high resin content, It is best for optimization with a few semi-cured sheets, more semi-cured sheets have more rivets, packing materials for lamination, suitable base materials for CTE, large heat is demand, but low actual heating rate, short duration of high temperature section, it is insufficient resin curing of semi-cured sheets, affecting reliability, it can appropriately increase the time of high temperature section.
The thickness of copper thick plate is increase, the use and control of drilling tool is very important, sectional drilling, feed speed, withdrawal speed, and all drilling parameters need rich experience to produce good holes.
When milling the target hole, X-ray energy gradually decreases with the increase of copper thickness, and the penetration force is easy to reach the upper limit. It is difficult to confirm the position of the first board when drilling a very thick PCB. For this point, you can set the offset confirmation target at different positions of the board edge, and mark the actual method on the plate data when cutting to prevent the position deviation.
It is difficult for the selection material to improve this problem, because the inner copper thick pad is cracked. You can increase the pad size, increase the peel strength of the material, and reduce the drilling speed. The concentric circle smaller than the hole diameter can be etched firstly to reduce the pulling force of the drilling hole.
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