PCB Tech - PCB additive method can reduce the wiring width by half

With the advent of PCB semi-additive technology, the trace width can be reduced by half to 1.25 mils, so the circuit assembly density can be maximized. According to a report on the EETimes website, the current continuous advancement of integrated circuits has shifted from the semiconductor IC lithography process (Lithography) to the PCB process in the past.

The most commonly used subtractive PCB process in the industry at present, the minimum tolerance of the wiring width can be within 0.5mil. Analysts pointed out that for those with a wiring width of more than 3 mils and a relatively low signal edge rate, although the 0.5 mil change value is not obvious, it has a significant impact on the impedance control of thinner wiring.

First of all, the PCB manufacturing process basically covers one or both sides with a copper-containing substrate material, which is the so-called core. The copper substrate material and thickness used on the substrate produced by each manufacturer are different, so the insulation and mechanical characteristics are also different.

After pressing the copper foil and the substrate material to form a substrate, start to cover the substrate with an anti-corrosion agent before exposure, and then etch the unexposed anti-corrosion agent and copper in an acid bath to form wiring. The purpose of this method is to allow the wiring to form a rectangular section, but during the acid bath process, not only the vertical copper will be eroded, but also part of the horizontal wiring wall will be dissolved.

The subtractive method under strict control allows the wiring to form a trapezoidal cross-section of almost 25 to 45 degrees, but if it is not properly controlled, it will cause the upper half of the wiring to be over-etched, resulting in a narrow top and thick bottom. If the height of the etched wiring is compared with the depth of the upper half of the wiring being eroded, the so-called etch factor will be obtained. The larger the value, the more rectangular the wiring section is.

Once the wiring can be rectangular, it means that its impedance (Impedance) is more predictable, and it can be repeated at an almost vertical angle, which means that the circuit assembly density can reach the highest. From the perspective of signal integrity, PCB copy board manufacturing yield can also be improved .

The same method that can achieve this result is semi-additive. The substrate of this method is laminated with a thinner copper foil of 2 or 3 microns (µm), and then the via hole is drilled and covered with electroless copper.

Then, an anti-corrosion agent is added in a specific range for exposure to form the required wiring. After the exposed areas are stacked, the remaining copper is etched. Therefore, this method is basically the opposite of the subtractive method. Compared with the subtractive method that uses chemical principles, the partial additive method wiring basically uses Photolithography. Therefore, the wiring width formed by the latter is more in line with the original design.

Under extremely strict tolerances, its wiring width can maintain a level of 1.25mils and have a certain level of impedance control. Through actual measurement, it is found that the impedance change measured by the entire PCB copy board will not exceed 0.5 ohm, which is one-fifth of the subtraction method.

Analysis pointed out that accurate impedance control is indispensable to meet the requirements of high-speed digital systems and microwave applications, which can also be achieved through partial additive methods. Moreover, it can achieve almost vertical wiring design characteristics, which can maximize the circuit assembly density.