Process introduction and technical points analysis of copper deposition process for evi circuit boards Chemical copper is widely used in the production and processing of printed circuit boards with through holes. Its main purpose is to deposit a layer of copper on the non-conductive substrate through a series of chemical treatment methods, and then thicken it to the designed specific thickness by subsequent electroplating methods. Generally, it is 1mil(25.4um) or thicker, and sometimes it is even directly deposited to the entire circuit copper thickness by chemical methods. Chemical copper process is the final deposition of chemical copper through a series of necessary steps, each of which is very important for the whole process. The purpose of this chapter introduced by circuit board manufacturers is not to detail the manufacturing process of circuit boards, but to point out some key points about chemical copper deposition in circuit board manufacturing. The concept of plated through hole (metallized hole) includes at least one or both of the following two meanings:
1. forming a part of the component conductor circuit;
2. Forming interconnection lines or printed lines between layers; Generally, a circuit board is produced and processed on a non-conductive composite substrate (epoxy resin-glass fiber cloth substrate, phenolic paper substrate, polyester glass fiber board, etc.) by etching (on copper-clad substrate) or electroless plating (on copper-clad substrate or copper-clad substrate).PI polyimide resin substrate: used for flexible board (FPC) production, suitable for high temperature requirements; Phenolic substrate: it can be stamped, NEMA grade, such as FR-2,XXX-PC; Epoxy substrate: better mechanical properties than phenolic paperboard, NEMA grade, such as CEM-1,FR-3; Epoxy glass fiber board: reinforced by glass fiber cloth, it has excellent mechanical properties, NEMA grade,Common examples: FR-4,FR-5,G-10,G-11; Non-woven glass fiber polyester substrate: suitable for some special applications, NEMA grade, such as FR-6; After metallization, the holes on the chemical copper/copper-deposited non-conductive substrate can achieve better solderability in interlayer interconnection or assembly, or both. There may be an inner circuit inside the non-conductive substrate-the circuit has been etched before the non-conductive substrate is laminated (pressed). The board processed by this process is also called MLB. In multilayer boards, metallized holes not only serve to connect the two outer circuits, but also serve to interconnect the inner layers. If holes designed to pass through non-conductive substrates are added (there was no concept of buried blind holes at that time). At present, the raw brush and many circuit boards adopt laminated substrate blanking in the process characteristics, that is to say, a certain thickness of electrolytic copper foil is laminated on the outside of the non-conductive substrate. The thickness of copper foil is expressed by the weight of copper foil per square foot (ounces). This expression method is converted into the thickness as shown in Table 13.1: These methods generally use fine abrasive such as glass beads or alumina abrasive materials. In the wet slurry process, nozzles are used to spray holes. Some chemical raw materials are used to dissolve polymer resins in the etching back and/or deslagging process. Usually (such as epoxy resin system), concentrated sulfuric acid, Chromic acid aqueous solution has been used before. No matter which method, it needs good post-treatment, otherwise it may cause many problems such as poor deposition of chemical copper in subsequent wet process perforation. Chromic acid method: the existence of hexavalent chromium in the hole will cause many problems in the coverage of chemical copper in the hole. It will destroy Sn-Pd colloid through oxidation mechanism and hinder the reduction reaction of chemical copper. Hole breakage is a common result of this obstruction. This situation can be solved by secondary activation. However, the cost of rework or secondary activation is too high, especially in the automatic line, and the secondary activation process is not very mature. After chromic acid bath treatment, there is often a neutralization step, and generally sodium bisulfite is used to reduce hexavalent chromium to trivalent chromium. The temperature of neutralizing agent sodium bisulfite solution is generally around 100F, and the washing temperature after neutralization is generally 120-150 F. Sulfite can be cleaned to avoid other bath liquid brought into the process and interfering with activation. Concentrated sulfuric acid method: the bath solution should be washed with very good water, preferably hot water. Try to avoid strong alkaline solution during washing. Some sodium salt residues of epoxy resin sulfonate may be formed, which is difficult to clean and remove from the hole. Its existence will cause pollution in the hole, which may cause a lot of electroplating difficulties. Other systems: There are also some other chemical methods used in the process of removing glue residue/drilling dirt and etching back. These systems, including the mixture of organic solvents (bulking/swelling resin) and potassium permanganate treatment, were used in the post-treatment of concentrated sulfuric acid treatment before, and now even directly replace concentrated sulfuric acid/chromic acid method. In addition, the plasma method is still in the experimental application stage, which is difficult to be used in large-scale production, and the equipment investment is large.
The main purposes of pretreatment steps of electroless copper-free process are as follows: 1. Ensure the continuity and integrity of electroless copper deposition; 2. Ensure the binding force between chemical copper and base copper foil; 3. Ensure the binding force between chemical copper and inner copper foil. 4. Ensure the binding force between chemical copper deposit and non-conductive substrate. The above is a brief description of the pretreatment effect of chemical copper/electroless copper.
The following is a brief description of the typical pretreatment steps of electroless copper: 1. The purpose of degreasing: 1. Remove oil stains and grease from copper foil and holes; 2. Remove the copper foil and dirt in the hole; 3. It helps to remove pollution from the surface of copper foil and subsequent heat treatment; 4. Simple treatment of polymer resin drilling dirt generated by drilling; 5. Remove the burr copper powder adsorbed in the hole caused by bad drilling; 6. Oil removal adjustment In some pretreatment lines, this is the first step to treat composite substrates (including copper foil and non-conductive substrates). Generally, the oil remover is alkaline, and some neutral and acidic raw materials are also used. Mainly in some atypical degreasing processes; It is a key bath solution in the pretreatment line of oil removal. The place contaminated by dirt will cause the problem of chemical copper coverage (that is, the generation of micro-voids and copper-free areas) due to insufficient adsorption of activator. Micro-voids will be covered or bridged by the subsequent copper plating, but there is no binding force between the copper plating layer and the non-conductive substrate of the base, and the final result may cause hole wall separation and blow holes. The internal plating stress generated by the plating layer deposited on the chemical copper layer and the steam expansion force generated by the moisture or gas wrapped by the plating layer in the substrate due to subsequent heating (baking, spraying tin, welding, etc.) tend to pull the plating layer away from the non-conductive substrate of the hole wall, which may cause the hole wall to detach; Similarly, the copper powder generated by burr in the hole is adsorbed in the hole, and if it is not removed in the degreasing process, it will also be covered by the electroplated copper layer. Similarly, there is no binding force between the copper layer and the non-conductive substrate, and this situation may eventually lead to the separation of the hole wall. No matter whether the above two results happen or not, it is undeniable that the bonding force at this place is obviously deteriorated and the thermal stress at this place is obviously increased, which may destroy the continuity of the electroplated layer, especially in the process of welding or wave welding, resulting in blowholes. Actually, the blowhole phenomenon is caused by the steam emitted from the non-conductive substrate under the coating with weak bonding force due to thermal expansion! If our electroless copper is deposited on the dirt of the base copper foil or the pollutants on the inner copper foil ring of the multilayer board, the bonding force between the electroless copper and the base copper will be much worse than that between the cleaned copper foils, and the poor bonding may result in: if the oil stain is spotty, it may cause blistering. If the dirt area is large, it may even cause the phenomenon of separation of electroless copper;
Important factors in degreasing process:
1. How to choose the appropriate degreaser-the type of cleaning/degreaser?
2. Working temperature of degreaser
3. Concentration of degreaser
4. Immersion time of degreaser
5. Mechanical stirring in the oil removal tank;
6. Cleaning points where the cleaning effect of degreaser decreases;
7. Washing effect after degreasing; In the above cleaning operation, temperature is a key factor worthy of attention. Many degreasers have a minimum temperature limit, below which the cleaning and degreasing effect drops sharply!
Influencing factors of water washing:
1. The washing temperature should be above 60F;
2. air stirring;
3. It is best to have a spray;
4. There is enough fresh water in the whole washing process to replace it in time. In a sense, the water washing after oil tank removal is as important as the oil removal itself. The residual oil remover on the board surface and the hole wall will also become the pollutants on the circuit board, and then pollute other subsequent main treatment solutions such as micro-etching and activation.
Generally, the most typical water washing here is as follows:
A. the water temperature is above 60F,
B. air stirring;
C, when the nozzle is equipped in the tank, fresh water is used to wash the plate surface when the plate is washed; C is not often used, but ab is necessary;
The water flow rate of washing water depends on the following factors:
1. the carrying amount of waste liquid (ml/ hanger);
2. Load of working plate in washing tank;
3. The number of washing tanks (counter-current rinsing) II. Charge adjustment or whole hole: typically, charge adjustment process is adopted after degreasing. Generally, in the production of some special plates and multi-layer plates, due to the charge factor of the resin itself, it is necessary to adjust the charge after the process of debonding and pitting; The important role of adjustment is to “super-infiltrate” the non-conductive substrate. In other words, the original resin surface with weak negative charge is denatured into an active surface with weak positive charge after being treated by adjustment solution. In some cases, a uniform and continuous positive polarity surface is provided, which can ensure that the subsequent activator can be effectively and fully adsorbed on the pore wall. Sometimes the adjusted drugs will be added to the degreasing agent, so it will also be called degreasing adjustment solution. Although the single degreasing solution and adjustment solution will have better effect than the combined degreasing adjustment solution, the trend of the industry has combined the two into one, and the adjustment agent is actually just some surfactants. The adjusted water washing is very important. Inadequate water washing will leave the surfactant on the copper surface of the plate, pollute the subsequent micro-etching and activation solution, which may affect the binding force between the final copper and copper, and reduce the binding force between the chemical copper and the base copper. Here, attention should be paid to the temperature of cleaning water and the flow rate of effective cleaning water. Special attention should be paid to the concentration of the regulator, and the use of too high a concentration of regulator should be avoided. A proper amount of regulator will play a more obvious role instead. 3. The next step of pretreatment of micro-etching electroless copper deposition is micro-etching, micro-etching or micro-coarsening or coarsening. The purpose of this step is to provide a micro-rough active copper surface structure for the subsequent electroless copper deposition. If there is no micro-etching step, the binding force between chemical copper and base copper will be greatly reduced; The roughened surface can play the following roles: 1. The surface area and surface energy of the copper foil are greatly increased, providing more contact areas between chemical copper and base copper; 2. If some surfactants are not washed away during washing, the micro-etching agent can remove the surfactants on the surface of the base material by etching away the copper groups on the copper surface of the base material, but it is not practical and effective to take out the surfactants completely by the micro-etching agent, because when the area of the residual copper surface of the surfactant is large, the chances of allowing the micro-etching agent to act are very small, and the copper surface with large surfactant residues is often not etched.