Basic factors affecting PCB manufacturing process
The global electroplated PCB industry's output value accounts for a rapid increase in the total output value of the electronic component industry. It is the industry with the largest proportion in the electronic component subdivision industry and holds a unique position. The annual output value of electroplated PCB is 60 billion US dollars. Electronic products are becoming thinner and thinner, and direct stacking of vias on blind vias is a design method for obtaining high-density interconnects. To make a stacked hole, the flatness of the bottom of the hole should be made first. There are several typical methods for making a flat hole surface, and the plating and hole filling process is a representative one of them.
In addition to reducing the need for additional process development, the electroplating hole filling process is also compatible with existing process equipment, which is beneficial to obtain good reliability.
Plating hole filling has the following advantages:
(1) It is beneficial to design stacked holes (Via. On. Pad);
(2) Improve electrical performance and help high frequency design;
(3) helps to dissipate heat;
(4) Plug hole 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.
1 Chemical influencing factors
1.1 Inorganic chemical composition
Inorganic chemical composition includes copper (Cu :) ions, sulfuric acid and chloride.
(1) Copper sulfate. Copper sulfate is the main source of copper ions in the plating solution. The copper ions in the plating solution pass the Coulomb balance between the cathode and the anode, maintaining the constant concentration. Usually the anode material and the coating material are the same, where copper is both the anode and the ion source. Of course, the anode can also use an insoluble anode. Cu2 + is dissolved and added outside the tank, such as pure copper angle, CuO powder, CuCO, etc. However, it should be noted that, by using the method of adding outside the tank, air bubbles are easily mixed, and Cu2 is in a supersaturated critical state in the low current region, which is not easy to precipitate. It is worth noting that increasing the concentration of copper ions has a negative effect on the dispersion ability of the vias.
(2) Sulfuric acid. Sulfuric acid is used to enhance the conductivity of the plating solution. Increasing the concentration of sulfuric acid can reduce the resistance of the bath and improve the efficiency of electroplating.
However, if the sulfuric acid concentration increases during the hole filling and plating process, it will affect the copper ion replenishment of the hole filling, which will result in poor hole filling. Low hole sulfuric acid system is usually used in hole filling and plating, in order to obtain better hole filling effect.
(3) Copper-acid ratio. The traditional high-acid and low-copper (C +: C: + = 8-13) system is suitable for through-hole electroplating, and low-acid and high-copper should be used for plated hole filling.
(C +: Cz = 3-10) bath system. This is because in order to obtain a good hole filling effect, the plating rate in the micro-vias should be greater than the plating rate on the substrate surface. In this case, compared with the traditional plating solution for plating through-holes, the solution formulation is made of high acid The change from low copper to low acid and high copper guarantees no worries about the supply of copper ions in the depression.
(4) Chloride. The role of chloride ions is to make copper ions and metal copper form a stable electron transfer bridge between electric double layers.
During the electroplating process, chloride ions in the anode can help uniformly dissolve and attack the phosphor copper balls, forming a uniform anode film on the anode surface. Synergistic interaction with the inhibitor at the cathode allows stable deposition of copper ions, reduces polarization, and makes the coating fine.
In addition, the conventional chloride ion analysis is carried out in a UV-visible spectrophotometer. Due to the strict requirements for chloride ion concentration in the plating and filling solution, the copper sulfate plating solution is blue, which affects the measurement of the spectrophotometer. Very, so you should consider using automatic potentiometric titration analysis.
2. Organic additives
The use of organic additives can refine the copper particles of the coating, improve the dispersion ability, and make the coating bright and level. There are three main types of additives in acid copper plating baths: Carrier, Leveler, and Brightener.
(1) Carrier. The carrier is a polymer-based polyol compound.
The rhenium carrier is adsorbed on the surface of the cathode and works with chloride ions to suppress the plating rate, reduce the difference between the high and low current regions (that is, increase the polarization resistance), and allow the electroplated copper to be uniformly and continuously deposited. The inhibitor can also act as a wetting agent, reducing the surface tension of the interface (reducing the contact angle), making it easier for the plating solution to enter the holes and increasing the mass transfer effect. In hole filling plating, the inhibitor can also be uniformly deposited on the copper layer.
(2) Leveling agent. The leveling agent is usually a nitrogen-containing organic substance, and its main function is to adsorb in the high current density area (the raised area or the corner), so that the plating speed there is slowed down without affecting the plating in the low current density area (the depressed area). This leveling surface is an essential additive in electroplating. In general, high-copper and low-acid systems are used in electroplating to fill holes to make the coating rough. Studies have shown that the addition of a leveling agent can effectively improve the problem of poor plating.
(3) Brightener. Brighteners are usually sulfur-containing organic compounds. The main role in electroplating is to help copper ions accelerate the reduction at the cathode, while forming new copper-plated nuclei (reducing the surface diffusion deposition energy), making the copper layer structure more detailed. Another role of brighteners in hole-filling electroplating is that if there are more brightener distribution ratios in the holes, it can help the rapid deposition of electroplated copper in blind hole holes. For the filling and plating of laser blind holes, all three additives are used, and the amount of leveling agent must be appropriately increased, so that the higher current region on the board surface, the leveling agent competes with Cuz, preventing
The copper surface is long and thick. In contrast, the depressions where the brightener is distributed in the micro-vias have the opportunity to plate faster. This concept and method is quite similar to the Demascene CopperPlating process of the IC copper plating process.
3. Physical influence parameters
The physical parameters that need to be studied are: anode type, cathode-anode spacing, current density, agitation, temperature, rectifier and waveform.
(1) Anode type. When it comes to anode types, it is nothing more than soluble anodes and insoluble anodes. Soluble anodes are usually phosphorous copper balls, which are prone to produce anode mud, pollute the plating solution, and affect the performance of the plating solution. Insoluble anodes, also known as inert anodes, are generally composed of a titanium mesh coated with a mixed oxide of tantalum and zirconium. Insoluble anode, good stability, no anode maintenance, no anode slime, pulse or DC plating are applicable; however, the consumption of additives is large.
(2) cathode and anode spacing. The design of the gap between the cathode and anode in the plating hole filling process is very important, and the design of different types of equipment is different. However, it should be pointed out that no matter how designed, Faraday's first law should not be violated.
(3) Stir. There are many types of agitation, including mechanical swing, electrical vibration, air vibration, air agitation, and Eductor.
For plating and hole filling, it is generally inclined to increase the jet design based on the configuration of traditional copper cylinders. However, whether it is the bottom jet or the side jet, how is the jet tube and air mixing tube arranged in the cylinder; what is the jet flow rate per hour; what is the distance between the jet tube and the cathode; if the side jet is used, the jet is on the anode The front or the back; if the bottom jet is used, will it cause uneven stirring and the plating solution will be stirred up and down? The number, spacing and angle of jets on the jet tube have to be considered in the design of the copper cylinder, and also A lot of experimentation is required.
In addition, the most ideal way is to connect each jet tube to the flow meter, so as to achieve the purpose of monitoring the flow. Because the jet volume is large and the solution easily generates heat, temperature control is also 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 pores. Under these conditions, the ability to fill holes is enhanced, but at the same time the plating efficiency is reduced.
(5) Rectifier. Rectifiers are an important part of the plating process. At present, the research on plated hole filling is mostly limited to full plate plating. If the pattern plated hole filling is considered, the cathode area will become very small. At this time, high requirements are imposed on 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 within 5% should be selected. Choosing a high accuracy rectifier will increase the investment in the equipment. The output cable of 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 rise time of the pulse current. The rectifier output cable specifications should be selected so that the line voltage drop of the output cable is within 0.6V at 80% of the maximum output current. Usually the required cable cross-sectional area is calculated according to the ampacity of 2.5A / mm :. The cross-sectional area of the cable is too small, the cable length is too long, and the line voltage drop is too large, which will cause the transmission current to fall below the current value required for production.
For plating tanks with a slot width greater than 1.6m, the method of bilateral power supply should be considered, and the length of the bilateral cables should be equal. In this way, the bilateral current error can be controlled within a certain range. A rectifier should be connected to each side of each flying bar of the plating tank, so that the current on the two sides of the part can be adjusted separately.
(6) waveform. At present, from the perspective of waveforms, there are two types of pulse filling and DC plating. Both of these plating hole filling methods have been studied. DC plating uses a traditional rectifier to fill holes, which is easy to operate, but if the board is thicker, it will be powerless. PPR rectifiers are used in pulse plating to fill holes, and there are many operating steps, but it has strong processing ability for thicker boards.
4. Influence of substrate
The influence of the substrate on the filling of the plating holes can not be ignored. Generally, there are factors such as the dielectric layer material, hole shape, thickness-diameter ratio, and chemical copper plating.
(1) Dielectric layer material. The material of the dielectric layer has an influence 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 pores have an adverse effect on chemical copper. In this case, the difficulty in filling holes by electroplating is to improve the adhesion of the seed layer of the electroless plating layer, rather than the hole filling process itself.
In fact, plated hole filling on glass fiber reinforced substrates has been used in actual production.
(2) Thickness to diameter ratio. At present, hole filling technology for holes of different shapes and sizes is highly valued by both manufacturers and developers. Hole filling ability is greatly affected by the hole thickness-diameter ratio. Relatively speaking, DC systems are more commercially used. In production, the size range of holes will be narrower. Generally, the diameter is 80 pm～120Bm, the hole depth is 40Bm ～ 8OBm, and the thickness-diameter ratio does not exceed 1: 1.
(3) Electroless copper plating. The thickness and uniformity of the electroless copper plating and the time after the 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. In general, it is recommended to fill holes when the thickness of chemical copper is> 0.3pm. In addition, the oxidation of chemical copper has a negative impact on the effect of hole filling.