PCB News - PCB electroplating hole filling process

As the volume of electronic products becomes thinner and shorter, the PCB design method of high density interconnection is to directly stack holes through blind holes. To do a good job of stacking holes, first of all, the hole bottom flatness. There are several typical methods of making flat hole surface, among which electroplating hole filling process is a representative one. In addition to reducing the need for additional process development, the electroplating hole filling process is compatible with existing process equipment and is conducive to good reliability.

Electroplating hole filling has the following advantages:

(1) is conducive to the design of Stacked and plate hole (via.on.pad);

(2) Improved electrical performance, conducive to high-frequency design;

(3) conducive to heat dissipation;

(4) Plug holes and electrical interconnection are completed in one step;

(5) The blind hole is filled with electroplated copper, which has higher reliability and better conductivity than conductive adhesive.

Physical influence parameter

The physical parameters to be studied are: anode type, gap between anode and cathode, current density, agitation, temperature, rectifier and waveform, etc.

(1) Type of anode. When it comes to anodes, there are only soluble and insoluble anodes. Soluble anodes are usually phosphorous

copper spheres, which can easily produce anode slime, contaminate bath and affect bath performance. Insoluble anodes, also known as inert anodes, typically consist of titanium mesh coated with a mixture of tantalum and zirconium oxides. Insoluble anode, good stability, no need for anode maintenance, no anode mud generation, pulse or DC plating is suitable;However, the consumption of additives is large.

(2) Distance between anode and cathode. The spacing between cathode and anode is very important in electroplating hole filling process, and the design of different types of equipment is not the same. However, it is important to point out that no design should violate Farrah's law.

(3) stir. There are many kinds of stirring, including mechanical swaying, electric vibration, air vibration, air stirring, Eductor and so on.

For electroplating hole filling, generally tend to increase the jet design on the basis of the configuration of traditional copper cylinder. However, whether it is the bottom jet or the side jet, how to layout the jet tube and the air mixing tube in the cylinder; The rate of emission per hour; Distance between jet tube and cathode; If side jet is used, is the jet in front or behind the anode? If the bottom jet is used, whether it will cause uneven stirring, the plating solution stirring is weak and strong; The number, spacing and Angle of the jet on the jet pipe are the factors that have to be considered in the design of the copper cylinder, and a large number of tests should be carried out.

In addition, the ideal way is that each jet pipe is connected to the flow meter, so as to achieve the purpose of monitoring flow. Temperature control is also important because the solution is easy to heat up due to the large emission rate.

(4) Current density and temperature. Low current density and low temperature can reduce the deposition rate of copper on the surface, while providing sufficient Cu2 and brightener into the hole. Under these conditions, the capacity of hole filling is enhanced, but the electroplating efficiency is also reduced.

(5) rectifier. Rectifier is an important part of electroplating process. At present, the research of electroplating hole filling is mostly limited to the whole plate plating, if the graphic electroplating hole filling is considered, the cathode area will become very small. At this time, high requirements for the output of the rectifier are put forward.

The selection of rectifier output should depend on the product line and the size of the hole. The thinner the line, the smaller the hole, the higher the requirement for rectifier. Rectifier with output less than 5% should be selected. Selecting too high a rectifier will increase the investment of equipment.Rectifier output cable wiring, first of all, the rectifier should be placed on the edge of the plating tank as far as possible, so as to reduce the length of the output cable, reduce the pulse current rise time. The specification of the rectifier output cable should be selected to ensure that the line voltage drop of the output cable is less than 0.6V when the output current is 80%. The required cable cross-sectional area is usually calculated as the carrying capacity of 2.5 A/mm:. If the cable cross-sectional area is too small or the cable length is too long or the line voltage drop is too large, the transmission current cannot reach the current value required for production.

For plated grooves larger than 1.6 m, the use of bilateral input mode should be considered, and the length of bilateral cables should be equal. In this way, the bilateral current error can be controlled within a certain range. A rectifier shall be connected to both sides of each feibar in the plating tank so that the current on both sides of the piece can be adjusted separately.

(6) waveform. At present, from the Angle of waveform, there are two kinds of electroplating holes: pulse electroplating and dc electroplating. Both of these electroplating methods have been studied. Dc plating hole filling using traditional rectifier, easy to operate, but if the plate is thick, powerless. Pulse plating hole filling using PPR rectifier, many steps, but for thick plate processing ability.

Substrate effects:

The influence of substrate on electroplating hole filling can not be ignored, generally there are dielectric layer material, hole shape, thickness to diameter ratio, chemical copper coating and other factors.

(1) Medium layer material. The medium layer material has an effect on the hole filling. Compared with glass fiber reinforced materials, non-glass reinforced materials are easier to fill holes. It is important to note that the chemical copper is adversely affected by fiberglass protrusions in pores. In this case, the difficulty of electroplating hole filling lies in improving the adhesion of chemical coating seed layer, rather than the hole filling process itself.

In fact, electroplating holes on glass fiber reinforced substrates has been used in practical production.

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

(3) Chemical copper plating layer. The thickness and uniformity of chemical copper coating and the placement time after chemical copper plating all affect the performance of hole filling. Chemical copper too thin or uneven thickness, its hole filling effect is poor. In general, chemical copper thickness is recommended. Holes are filled at 0.3pm. In addition, the oxidation of chemical copper also has a negative effect on the effect of hole filling.