The global electroplated PCB industry output value accounts for the proportion of the total output value of the electronic components industry has grown rapidly, and it is the largest industry in the electronic components subdivision industry, occupying a unique position, and the annual output value of electroplated PCB is $60 billion. The volume of electronic products is becoming thinner and shorter. Direct stacking of holes through blind holes is a design method to obtain high density interconnection. To do a good job of stacking holes, first of all, the flatness of the hole bottom should be done. There are several typical flat hole surface production methods, electroplating hole filling process is a representative one.
In addition to reducing the need for additional process development, the electroplating hole filling process is also compatible with existing process equipment, which is conducive to obtaining good reliability. Electroplating hole filling has the following advantages:
a. conducive to the design of Stacked holes and holes on the plate (Via.on.Pad);
b. improve electrical performance, contribute to high-frequency design;
c. help heat dissipation;
d. plug hole and electrical interconnection one step complete;
e. The blind hole is filled with electroplated copper, which has higher reliability and better electrical conductivity than conductive adhesive.
1. chemical influencing factors: inorganic chemical composition including copper (Cu:) ions, sulfuric acid and chloride.
a. Copper sulfate. Copper sulfate is the main source of copper ions in the bath. Copper ions in the plating solution maintain constant concentration through coulomb equilibrium between cathode and anode. Usually the anode material is the same as the coating material, where copper is both the anode and the ion source. Of course, the anode can also be used insoluble anode, Cu2+ using the way of dissolution outside the tank, such as the use of pure copper Angle, CuO powder, CuCO, etc. However, it should be noted that the method of supplementation outside the tank is easy to mix air bubbles, and Cu2 is in a critical state of supersaturation in the low current region, which is not easy to precipitate. It is worth noting that increasing copper ion concentration has a negative effect on through-hole dispersion.
b. sulfuric acid. Sulfuric acid is used to enhance the conductivity of the bath, and increasing the concentration of sulfuric acid can reduce the resistance of the bath and improve the efficiency of electroplating.
However, if the concentration of sulfuric acid increases in the process of filling the hole electroplating, the addition of copper ions in the hole filling will be affected, and the hole filling will be poor. Low sulfuric acid concentration system is generally used in hole filling electroplating in order to obtain better hole filling effect.
c. acid copper ratio. The traditional high acid and low copper (C+ : C: +=8-13) system is suitable for through hole plating, and the plating hole should be filled with low acid and high copper (C+ : Cz=3-10) plating solution system. This is because in order to obtain a good hole filling effect, the plating rate in the micro-through hole should be greater than the plating rate on the surface of the substrate, in this case, compared with the traditional electroplating solution of electroplating through the hole, the solution formula is changed from high acid and low copper to low acid and high copper, ensuring the supply of copper ions in the depression without worries.
d. chloride ion. The role of chloride ions is mainly to form a stable electron transfer bridge between copper ions and metal copper in the double electric layer.
In the process of electroplating, the chloride ions in the anode can help to dissolve the phosphorus copper ball uniformly and form a uniform anode film on the anode surface. The synergistic action between the cathode and the inhibitor makes the copper ion deposit stably, reduces the polarization, and makes the coating fine.
In addition, the conventional chloride ion analysis is carried out in the UV-visible spectrophotometer, and due to the strict requirements of the chloride ion concentration of the electroplating bath, and the blue color of the copper sulfate bath, the measurement of the spectrophotometer is very important, so the automatic potentiometric titration analysis should be considered.
2. organic additives
The use of organic additives can make the copper grain fine, improve the dispersion ability, and make the coating bright and smooth. There are three main types of additives in acidic copper plating solution: Carrier, Leveler and Brightener.
a. Transport agent. The carrier is a combination of polyalcohols of high polymers.
The carrier is adsorbed by the cathode surface, and acts together with chloride ions to inhibit the plating rate, reducing the difference between the high and low current regions (that is, increasing the polarization resistance), so that the plating copper can be uniformly and continuously deposited. The inhibitor also acts as a wetting agent, reducing the surface tension of the interface (reducing the contact Angle), allowing the bath to enter the hole more easily and increasing the mass transfer effect. In hole filling electroplating, the inhibitor can also uniformly deposit the copper layer.
b. leveling agent. Levelling agent is usually nitrogen-containing organic matter, the main function is to adsorb in the high current density area (convex area or corner), so that the plating speed there slows down but does not affect the plating of the low current density area (concave area), so as to smooth the surface, is a necessary additive in plating. Generally, high copper and low acid system will make the coating rough. The research shows that adding leveling agent can effectively improve the problem of poor coating.
c. brightener. Brighteners are usually sulfur-containing organic matter, and their main role in electroplating is to help copper ions accelerate the reduction at the cathode, while forming new copper-plated crystal nuclei (reducing the surface diffusion deposition energy), so that the copper layer structure becomes more detailed. Another role of brightener in hole filling plating is that if there is more brightener distribution ratio in the hole, it can help the copper plating in the blind hole to deposit rapidly. For the laser blind hole filling electroplating, all three additives are used, and the amount of leveling agent should be appropriately increased, so that in the higher current area on the board, the leveling agent and Cuz competition is formed, and the copper surface is fast and thick. On the contrary, the dimple with more brightener distribution in the micro-conductive hole has the opportunity to be plated faster, which is similar to the Demascene CopperPlating in the IC copper plating process.
3. Physical impact parameters
The physical parameters that need to be studied are: anode type, anode and cathode distance, current density, agitation, temperature, rectifier and waveform.
a. Type of anode. When it comes to anode types, there are nothing more than soluble and insoluble anodes. Soluble anodes are usually phosphorus-containing copper balls, which are easy 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 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 are suitable; However, the consumption of additives is large.
b. anode and cathode distance. The design of the distance between cathode and anode is very important in the electroplating process, 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 designed, it should not violate Faraday's law.
c. Stir. There are many types of agitation, including mechanical shaking, electric vibration, air vibration, air agitation, and jet (Eductor).
For electroplated hole filling, it is generally preferred to add jet design based on the configuration of traditional copper cylinders. However, whether it is the bottom jet or the side jet, how to arrange the jet tube and the air mixing tube in the cylinder; What is the flow rate per hour; The distance between the jet tube and the cathode; If the side jet is used, whether the jet is in front of or behind the anode; If the bottom jet is used, whether it will cause uneven mixing, and the plating solution is weak on the top and strong on the bottom; The number, spacing and Angle of the jet on the jet pipe are all factors that have to be considered in the design of the copper cylinder, and a lot of tests should be carried out.
In addition, the most ideal way is to connect each jet pipe to the flow meter, so as to achieve the purpose of monitoring the flow rate. Due to the large flow rate, the solution is easy to heat, so temperature control is also important.
d. current density and temperature. Low current density and low temperature can reduce the deposition rate of copper on the surface while providing enough Cu2 and brightener into the hole. Under this condition, the hole filling capacity is enhanced, but the plating efficiency is also reduced.
e. rectifier. Rectifier is an important part of electroplating process. At present, the research on electroplating hole filling is mostly limited to full plate electroplating. If the graphic electroplating hole filling is taken into account, the cathode area will become very small. At this time, high requirements are put forward for the output accuracy of the rectifier.
The selection of the output accuracy of the rectifier should be determined according to the size of the product line and through the hole. The finer the lines and the smaller the holes, the higher the accuracy requirements of the rectifier should be. Generally, a rectifier with an output accuracy of less than 5% should be selected. The high accuracy of the rectifier selected will increase the investment of the equipment. The output cable of the rectifier should first be placed on the side of the plating tank as far as possible, which can reduce the length of the output cable and reduce the rise time of the pulse current. The selection of rectifier output cable specifications should meet the line voltage drop of the output cable within 0.6V at 80% of the maximum output current. The required cable cross-sectional area is usually calculated according to the current carrying capacity of 2.5 A/mm:. 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, which will cause the transmission current to fail to reach the current value required for production.
For the plating bath with a groove wider than 1.6m, the method of double-sided power supply should be considered, and the length of the double-sided cable should be equal. In this way, the bilateral current error can be controlled within a certain range. Each side of the flying bar of the plating tank should be connected with a rectifier, so that the current of the two sides of the piece can be adjusted separately.
f. waveform. At present, from the waveform point of view, there are two kinds of electroplating and DC electroplating. These two electroplating hole filling methods have been studied. Dc electroplating hole filling using the traditional rectifier, easy to operate, but if the board is thicker, it can do nothing. The pulse electroplating hole filling uses PPR rectifier, which has many operation steps, but has strong processing ability for thicker in-process plates.
4. The influence of substrate
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.
a. dielectric layer material. Dielectric layer material has influence on 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 protrusion in the hole has an adverse effect on the chemical copper. In this case, the difficulty of electroplating hole filling is to improve the adhesion of the seed layer of the chemical coating, rather than the hole filling process itself.
In fact, electroplating holes on glass fiber reinforced substrates has been used in actual production.
b. thickness to diameter ratio. At present, for different shapes, different sizes of hole filling technology, both manufacturers and developers attach great importance to it. Hole filling capacity is greatly affected by hole thickness to diameter ratio. Relatively speaking, DC systems are used more in business. In the production, the size range of the hole will be narrower, the general diameter of 80pm ~ 120Bm, the hole depth of 40Bm~8OBm, the thickness to diameter ratio does not exceed 1:1.
c. electroless copper plating layer. The thickness, uniformity and placement time of chemical copper plating affect the hole filling performance. Chemical copper is too thin or uneven thickness, its hole filling effect is poor. In general, it is recommended to fill holes when the chemical copper thickness is > 0.3 pm. In addition, the oxidation of chemical copper also has a negative effect on the filling effect.
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