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Several basic factors affecting the process of electroplating and filling holes in PCB production
2021-09-04
View:34
Author:Belle

The output value of the global electroplating PCB industry accounts for a rapid increase in the proportion of the total output value of the electronic component industry. It is the industry with the largest proportion in the electronic component industry and occupies a unique position. The annual output value of electroplated PCB is 60 billion US dollars. The volume of electronic products is becoming lighter, thinner, shorter and smaller, and direct stacking of vias on blind vias is a design method to obtain high-density interconnection. To do a good job of stacking holes, the bottom of the hole should be flat. There are several ways to make a typical flat hole surface, and the electroplating hole filling process is one of the representative ones.

In addition to reducing the need for additional process development, the electroplating and filling process is also compatible with current process equipment, which is conducive to obtaining good reliability.

Electroplating hole filling has the following advantages:

(1) Conducive to the design of stacked holes (Stacked) and on-disk holes (Via.on.Pad);

(2)Improve electrical performance and help high-frequency design;

(3) Contribute to heat dissipation;

(4) The plug hole and electrical interconnection are completed in one step;

(5) The blind holes are filled with electroplated copper, which has higher reliability and better conductivity than conductive glue.

Physical influence parameters

The physical parameters that need to be studied are: anode type, anode-cathode spacing, current density, agitation, temperature, rectifier and waveform, etc.

(1) Anode type. When it comes to anode types, there are nothing more than soluble anodes and insoluble anodes. The soluble anode is usually a phosphorous copper ball, which is easy to produce anode mud, contaminate the plating solution, and affect the performance of the plating solution. Insoluble anodes, also known as inert anodes, are generally composed of titanium mesh coated with mixed oxides of tantalum and zirconium. Insoluble anode, good stability, no anode maintenance, no anode mud generation, pulse or DC electroplating is applicable; however, the consumption of additives is relatively large.

(2) The distance between cathode and anode. The design of the spacing between the cathode and the anode in the electroplating hole filling process is very important, and the design of different types of equipment is not the same. However, it needs to be pointed out that no matter how the design is, it should not violate Fara's first law.

(3) Stirring. There are many types of stirring, including mechanical shaking, electric shaking, air shaking, air stirring, and jet (Eductor).

For electroplating and filling holes, it is generally inclined to increase the jet design based on the configuration of the traditional copper cylinder. However, whether it is a bottom jet or a side jet, how to arrange the jet tube and the air stirring tube in the cylinder; what is the jet flow per hour; what is the distance between the jet tube and the cathode; if the side jet is used, the jet is at the anode Front or back; if the bottom jet is used, will it cause uneven mixing, and the plating solution will be stirred up weakly and strong down; the number, spacing, and angle of the jets on the jet tube are all factors that have to be considered when designing the copper cylinder. A lot of experimentation is required.

In addition, the most ideal way is to connect each jet tube to a flow meter, so as to achieve the purpose of monitoring the flow rate. Because the jet flow is large, the solution is easy to generate heat, so temperature control is also very important.

(4) Current density and temperature. Low current density and low temperature can reduce the surface copper deposition rate, while providing enough Cu2 and brightener into the hole. Under this condition, the hole filling ability is enhanced, but at the same time the plating efficiency is reduced.

(5) Rectifier. The rectifier is an important link in the electroplating process. At present, the research on electroplating hole filling is mostly limited to full plate electroplating. If the pattern electroplating hole filling is considered, the area of the cathode will become very small. At this time, very high requirements are put forward for the output accuracy of the rectifier.

The output accuracy of the rectifier should be selected according to the product line and the size of the via. The thinner the lines and the smaller the holes, the higher the accuracy requirements of the rectifier. Generally, a rectifier with an output accuracy of less than 5% should be selected. The high precision of the selected rectifier will increase equipment investment. For the output cable wiring of the rectifier, first place the rectifier on the side of the plating tank as much as possible, so that the length of the output cable can be reduced and the pulse current rise time can be reduced. The selection of the rectifier output cable specifications should satisfy that the line voltage drop of the output cable is within 0.6V when the maximum output current is 80%. The required cable cross-sectional area is usually calculated according to the current-carrying capacity of 2.5A/mm:. If the cross-sectional area of the cable is too small or the cable length is too long, and the line voltage drop is too large, the transmission current will not reach the current value required for production.

For plating tanks with a groove width greater than 1.6m, the double-sided power supply method should be considered, and the length of the double-sided cables should be equal. In this way, it can be ensured that the bilateral current error is controlled within a certain range. A rectifier should be connected to each side of each flybar of the plating tank, so that the current on the two sides of the piece can be adjusted separately.

(6) Waveform. At present, from the perspective of waveforms, there are two types of electroplating hole filling: pulse electroplating and DC electroplating. Both electroplating and filling methods have been studied. The direct current electroplating hole filling adopts the traditional rectifier, which is easy to operate, but if the plate is thicker, there is nothing that can be done. Pulse electroplating hole filling uses PPR rectifier, which has many operation steps, but it has strong processing ability for thicker in-process boards.

The influence of the substrate

The influence of the substrate on the electroplated hole filling is also not to be ignored. Generally, there are factors such as dielectric layer material, hole shape, thickness-to-diameter ratio, and chemical copper plating.

(1) Material of the dielectric layer. The material of the dielectric layer has an effect on the hole filling. Compared with glass fiber reinforced materials, non-glass reinforced materials are easier to fill holes. It is worth noting that the glass fiber protrusions in the hole have an adverse effect on chemical copper. In this case, the difficulty of electroplating the hole filling is to improve the adhesion of the seed layer of the electroless plating layer, rather than the hole filling process itself.

In fact, electroplating and filling holes on glass fiber reinforced substrates have been used in actual production.

(2) Thickness to diameter ratio. At present, both manufacturers and developers attach great importance to the filling technology for holes of different shapes and sizes. The hole-filling ability is greatly affected by the hole thickness-to-diameter ratio. Relatively speaking, DC systems are used more commercially. In production, the size range of the hole will be narrower, generally 80pm~120Bm in diameter, 40Bm~8OBm in depth, and the ratio of thickness to diameter should not exceed 1:1.

(3) Electroless copper plating layer. The thickness and uniformity of the electroless copper plating layer and the placement time after electroless copper plating all affect the hole filling performance. The electroless copper is too thin or uneven in thickness, and its hole filling effect is poor. Generally, it is recommended to fill the hole when the thickness of the chemical copper is> 0.3pm. In addition, the oxidation of chemical copper also has a negative impact on the hole filling effect.

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