PCB Tech - Detailed explanation of the copper sinking process for PCB processing and proofing

Circuit board copper sinking process flow introduction and technical analysis The layer of copper is then thickened by subsequent electroplating methods to achieve the specified thickness of the design, generally 1 mil (25.4um) or thicker, and sometimes even directly chemically deposited to the copper thickness of the entire circuit. The chemical copper process is through a series of necessary steps to finally complete the deposition of chemical copper, each of which is very important to the entire process flow. The purpose of this chapter is not to describe the production process of the PCB, but to highlight some key points related to the chemical copper deposition in the production of the circuit board.

The concept of plated through holes (metallized holes) includes at least one or both of the following two meanings:

1. Form part of the component conductor line;
2. Form interlayer interconnection lines or printed lines; a general circuit board is etched on a non-conductor composite substrate (epoxy-glass fiber cloth substrate, phenolic paper substrate, polyester fiberglass board, etc.) (On a copper-clad substrate) or electroless plating (on a copper-clad substrate or a copper foil substrate). PI polyimide resin substrate: used for flexible board (FPC) production, suitable for high temperature requirements; phenolic paper substrate: can be stamped and processed, NEMA grade, common such as: FR-2, XXX-PC; epoxy paper substrate: relatively Phenolic paperboard has better mechanical properties, NEMA grade, common such as: CEM-1, FR-3; epoxy glass fiber board: glass fiber cloth is used as a reinforcing material inside, which has excellent mechanical properties, NEMA grade, common such as: FR-4, FR-5, G-10, G-11; non-woven glass fiber polyester substrate: suitable for some special purposes, NEMA grade, common such as: FR-6; chemical copper/immersion copper non-conductive substrate The upper holes can achieve better solderability in interlayer interconnection or assembly or both after the completion of metallization. There may be internal circuits inside the non-conductive substrate --- the circuit has been etched before the non-conductive substrate is laminated (pressed). The PCB board processed by this process is also called a multi-layer board (MLB). In the multilayer board, the metallized hole not only plays the role of connecting the two outer layers, but also plays the role of interconnection between the inner layers, adding a hole designed to pass through the non-conductive substrate (when there is no buried blindness in fashion) The concept of holes). Nowadays, raw-wiping and many circuit boards use laminated substrate blanking in terms of process characteristics, that is to say, the outside of the non-conductor base material is a copper foil made by pressing and pressing a certain thickness of electrolysis method. The thickness of copper foil is expressed by the weight of copper foil per square foot (ounces). This expression method is converted into thickness as shown in Table 13.1: These methods generally use fine abrasives such as glass beads or alumina for grinding. Materials. In the wet slurry process, nozzles are used to treat the holes. Some chemical raw materials are used to dissolve polymer resins in the etchback and/or desmear process. Usually (such as epoxy resin systems), concentrated sulfuric acid Chromic acid aqueous solution, etc. have all been used. No matter which method, it needs good post-treatment, otherwise it may cause many problems such as the subsequent wet process perforation chemical copper deposition. Chromic acid method: six holes in the hole The presence of valence chromium will cause many problems with the coverage of chemical copper in the pores. It will destroy the tin-palladium colloid through the oxidation mechanism and hinder the reduction reaction of chemical copper. Pore rupture is a common result caused by this obstacle. This situation can be It is solved by secondary activation, but the cost of rework or secondary activation is too high, especially in the automatic line, the secondary activation process is not very mature. After the chromic acid tank is treated, there is often a neutralization step. The valence chromium is reduced to trivalent chromium. The temperature of the neutralizer sodium bisulfite solution is generally about 100F, and the washing temperature after neutralization is generally 120-150F. The sulfite can be cleaned to avoid other things in the process. Bath liquid interferes with activation. Concentrated sulfuric acid method: After the bath solution is treated, there must be a very good washing, preferably hot water, try to avoid strong alkaline solutions during washing. Some sodium salt residues of epoxy resin sulfonate may be formed, which This compound is difficult to clean and remove from the hole. Its presence will cause contamination in the hole, which may cause a lot of electroplating difficulties. Other systems: There are also some other chemical methods used in de-smear/de-drilling and etchback processes. In these systems, including the application of a mixture of organic solvents (bulking/swelling resin) and potassium permanganate treatment, it was previously used in the post-treatment of concentrated sulfuric acid treatment, and now it even directly replaces the concentrated sulfuric acid method/chromic acid method. In addition, there is the plasma method, which is still in the experimental application stage, which is difficult to be used in large-scale production, and the equipment investment is relatively large.
The main purpose of the pre-treatment steps of the electrochemical-free copper process: 1. Ensure the continuous integrity of the electroless copper deposition layer; 2. 2. Ensure the bonding force between chemical copper and base copper foil; 3. Ensure the bonding force between the chemical copper and the inner copper foil 4. Ensuring the bonding force between the electroless copper layer and the non-conductive substrate The above is a brief description of the effect of the electroless copper/electroless copper pretreatment.

The following is a brief description of the typical pretreatment steps of electrochemical-free copper: 1. The purpose of degreasing and degreasing: 1． Remove the oil and grease in the copper foil and the hole; 2. Remove the dirt in the copper foil and the hole; 3. 3. It is helpful to remove pollution from the surface of copper foil and follow-up heat treatment; 4． Simple treatment of the polymer resin drilling dirt produced by the drilling; 5. Remove the burr copper powder adsorbed in the hole by the bad drilling; 6. Degreasing adjustment In some pre-treatment lines, this is the first step in processing composite substrates (including copper foil and non-conductive substrates). Degreasing agents are generally alkaline, but some neutral and acidic raw materials are also used . Mainly in some atypical oil removal processes; oil removal is a key tank liquid in the pretreatment line. The area contaminated by dirt will cause the problem of chemical copper coverage (that is, the generation of microvoids and copper-free areas) due to insufficient activator adsorption. The microvoids will be covered or bridged by subsequent electroplating copper, but as far as there is no bonding force between the electrical copper layer and the non-conductive substrate of the base, the final result may cause the hole wall to detach and blow holes. The internal coating stress generated by the electroplating layer deposited on the chemical copper layer and the moisture or gas wrapped by the coating in the substrate due to subsequent heating (baking, tin spraying, welding, etc.) Pulling away from the non-conductive substrate of the hole wall may cause the hole wall to detach; the copper powder produced by the burr in the same hole is adsorbed in the hole and is not removed during the degreasing process, and it will also be covered by the electroplated copper layer Also, as far as there is no bonding force between the copper layer and the non-conductive substrate, this situation may eventually result in the separation of the hole wall. Regardless of whether the above two results occur or not, there is one thing that is undeniable. The bonding force there is significantly worse and the thermal stress there is significantly increased, which may damage the continuity of the electroplating layer, especially in the welding or wave soldering process. As a result, blow holes are produced. The blow-hole phenomenon is actually caused by the steam generated from the non-conductive substrate under the fragile coating layer due to thermal expansion! If our electroless copper is deposited on the dirt of the base copper foil or on the contaminants on the inner copper foil ring of the multilayer board, the bonding force between the electroless copper and the base copper will be better than that of the cleaned copper. The bonding force between the foils is much different, and the results of poor bonding may occur: if the oil stain is spot-like, it may cause blistering; if the stain area is large, it may even cause the non-electric copper to peel off. ;

Important factors in the degreasing process:
1. How to choose the right degreaser-type of cleaning/degreaser
2. Working temperature of degreaser
3. Concentration of degreaser
4. Dipping time of degreaser
5. Mechanical stirring in the degreasing tank;
6. The cleaning point where the cleaning effect of degreaser decreases;
7. Water washing effect after degreasing; in the above cleaning operation, temperature is a key factor worthy of attention. Many degreasing agents have a minimum temperature lower limit, and the cleaning and degreasing effect drops sharply below this temperature!

Influencing factors of 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 for the entire washing to be replaced in time. The water washing after the degreasing tank is as important as the degreasing itself in a sense. The degreasing agent remaining on the board surface and the hole wall itself will become a pollutant on the circuit board, and then pollute other subsequent main treatment solutions such as micro-etching and activation.
Generally, the most typical washing at this place 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 surface of the plate during water washing; condition c is not used frequently, but two ab are necessary;
The flow rate of the cleaning water depends on the following factors:
1. The amount of waste liquid carried out (ml/hanging);
2. The load capacity of the working plate in the washing tank;
3. The number of washing tanks (countercurrent rinsing)

Charge adjustment or hole adjustment: The typical charge adjustment process is used after degreasing. Generally, in the production of some special plates and multilayer boards, because of the charge factor of the resin itself, after the process of de-smear and etch, it is necessary to pay attention to the charge. Make adjustment treatment; the important function of adjustment is to "super infiltrate" the non-conductive substrate, in other words, it is to denature the original weakly negatively charged resin surface to a weakly positively charged active surface after being treated with a conditioning solution . In some cases, a uniform and continuous positively charged polar surface is provided, which can ensure that the subsequent activator can be effectively and fully adsorbed on the pore wall. Sometimes the adjusted chemicals will be added to the degreasing agent, so it is also called the degreasing adjustment fluid. Although the separate degreasing fluid and the adjustment fluid will be better than the combined degreasing adjustment fluid, but the industry The trend has combined the two into one, and the modifier is actually just some surfactants. The adjusted washing with water is extremely important. Insufficient washing will cause the surfactant to remain on the copper surface, contaminate the subsequent microetching and activation solution, which may affect the bonding force between the final copper and copper, resulting in lower chemical copper and the substrate The bonding force between copper. Attention should be paid to the temperature of the cleaning water and the effective cleaning water flow. 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. 3. The next step of the pre-treatment of micro-etching electroless copper deposition is the step of micro-etching or micro-etching or micro-roughening or roughening. The purpose of this step is to provide a micro-rough active copper surface structure for subsequent electroless copper deposition . If there is no microetching step, the bonding force between the chemical copper and the base copper will be greatly reduced; the roughened surface can play the following roles: 1. The surface area of the copper foil is greatly increased, and the surface energy is also greatly increased, providing a large contact area between the chemical copper and the substrate copper; 2. If some surfactants are not cleaned off during water washing, the microetching agent can remove the surface active agent from the surface of the substrate by etching off the copper base on the copper surface of the bottom substrate, but it is completely dependent on the microetching agent to take out the surface activity The agent is not realistic and effective, because when the surface area of the residual copper surface of the surfactant is large, the chance of allowing the effect of the microetching agent is small, and the copper surface where a large area of the surface active agent remains is often not microetched.