Via-filling Plating in PCB Manufacturing Process
The global electroplating PCB industry’s output value accounts for a rapid increase in the proportion of the total output value of the electronic components industry. It is the industry with the largest proportion of the electronic component segmentation industry, and has a unique position. The annual output value of electroplated PCB is 60 billion US dollars. The volume of electronic products is becoming thinner and lighter, and direct stacking on blind vias is a design method for obtaining high-density interconnects. To make a stack of holes, you should first do a good job at the bottom of the hole. There are several methods for fabricating a typical flat hole surface, and a plating hole filling process is one of them.
In addition to reducing the need for additional process development, the electroplating and hole-filling process is also compatible with current process equipment, which is beneficial for good reliability.
Electroplating and filling holes have the following advantages:
(1) It is advantageous to design stacked and on-disk holes (Via.on.Pad);
(2) Improve electrical performance and contribute to high frequency design;
(3) Helps to dissipate heat;
(4) The plug hole and the 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 paste.
Physical influence parameter:
The physical parameters that need to be studied are: anode type, anode-anode spacing, current density, agitation, temperature, rectifier, and waveform.
(1)Type of anode. When it comes to anode type, it is nothing more than a soluble anode and an insoluble anode. The soluble anode is usually a phosphorus-containing copper ball, which easily produces anode mud, contaminates the plating solution, and affects the performance of the plating solution. Insoluble anodes, also known as inert anodes, are typically composed of a titanium mesh coated with a mixed oxide of cerium and zirconium. Insoluble anode, good stability, no anode maintenance, no anode mud generation, pulse or DC plating are applicable; however, the additive consumption is large.
(2)The anode-anode spacing. The design of the spacing between the cathode and the anode in the electroplating process is very important, and the design of different types of equipment is not the same. However, it should be pointed out that no matter how the design is, it should not violate the first law of Farah.
(3)Stirring. There are many types of mixing, such as mechanical rocking, electric shock, gas vibration, air agitation, and jet (Eductor).
For electroplating and filling holes, it is generally preferred to increase the jet design in the configuration of conventional copper cylinders. However, whether it is the bottom jet or the side jet, how is the jet pipe and the air agitating pipe arranged in the cylinder; what is the jet flow per hour; how much the jet pipe is spaced from the cathode; if the side jet is used, the jet is at the anode Whether the front or the back; if the bottom jet is used, will it cause uneven mixing, the plating solution is weak and strong; the number, spacing and angle of the jet on the jet tube are factors that must be considered in the design of the copper cylinder, and also A lot of trials are going on.
In addition, the most ideal way is to connect each flow tube to the flowmeter to achieve the purpose of monitoring the flow. Temperature control is also important because the flow rate is large and the solution is prone to heat.
(4)Current density and temperature. Low current density and low temperature can reduce the deposition rate of surface copper while providing sufficient Cu2 and brightener into the pores. Under these conditions, the ability to fill holes is enhanced, but at the same time the plating efficiency is also reduced.
(5) Rectifier. The rectifier is an important part of the electroplating process. At present, the research on electroplating and filling holes is mostly limited to full-plate electroplating. If the pattern plating hole is considered, the cathode area will become small. At this time, high requirements are placed on the output accuracy of the rectifier.
The output accuracy of the rectifier should be chosen according to the line of the product and the size of the via. The finer the lines and the smaller the holes, the higher the accuracy requirements for the rectifier. Generally, a rectifier with an output accuracy of 5% or less should be selected. Exceeding the accuracy of the selected rectifier will increase the investment in the equipment. For the output cable of the rectifier, the rectifier should be placed on the side of the plating tank as much as possible. This can reduce the length of the output cable and reduce the pulse current rise time. The rectifier output cable specification should be chosen to meet the line drop of the output cable within 0.6V at 80% of the maximum output current. The required cable cross-sectional area is usually calculated at a current carrying capacity of 2.5 A/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 current will not reach the current value required for production.
For plating tanks with a groove width greater than 1.6 m, bilateral input should be considered and the length of the bilateral cables should be equal. In this way, it is guaranteed that the bilateral current error is controlled within a certain range. A rectifier should be connected to each side of each fly-bar of the plating tank so that the currents on both sides of the part can be adjusted separately.
(6)At present, from the perspective of waveforms, there are two types of electroplating holes: pulse plating and DC plating. These two methods of electroplating and filling have been studied. The DC electroplating hole is made of a conventional rectifier, which is easy to operate, but if the plate is thick, it is powerless. The pulse plating filling hole adopts the PPR rectifier, and the operation steps are many, but the processing ability for the thicker plate is strong.
The influence of the substrate on the electroplating and filling holes is also negligible. Generally, there are factors such as dielectric layer material, hole shape, aspect ratio, and chemical copper plating.
(1) Dielectric layer material. The dielectric layer material has an effect on the filling of holes. Non-glass reinforced materials are easier to fill than holes in glass reinforced materials. It is worth noting that the glass fiber protrusions in the holes have an adverse effect on the chemical copper. In this case, the difficulty in electroplating is to increase the adhesion of the electroless plating seed 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, the hole-filling technology for different shapes and different sizes of holes is highly valued by both manufacturers and developers. The hole filling ability is greatly affected by the hole thickness ratio. Relatively speaking, DC systems are more widely used commercially. In production, the size of the hole will be narrower, generally 80pm ~ 120Bm diameter, hole depth 40Bm ~ 8OBm, thickness to diameter ratio does not exceed 1:1.
(3) Electroless copper plating. The thickness and uniformity of the chemical copper plating and the placement time after electroless copper plating all affect the hole filling performance. Chemical copper is too thin or uneven in thickness, and its hole filling effect is poor. Generally, it is recommended to fill holes when the thickness of the chemical copper is >0.3 pm. In addition, the oxidation of chemical copper also has a negative effect on the hole filling effect.