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 a non-conductive substrate through a series of chemical treatment methods, which are then thickened by subsequent plating methods. The specific thickness of the design is typically 1 mil (25.4 um) or thicker, and sometimes even directly deposited by chemical means to the copper thickness of the entire circuit. The chemical copper process ultimately completes the deposition of chemical copper through a series of necessary steps, each of which is important for the entire process.
Circuit board processing factory introduction The purpose of this section is not to elaborate the production process of the circuit board, but to highlight some points of the chemical copper deposition in the circuit board production and production. For readers who want to understand circuit board production and processing, it is recommended to refer to other articles, including some of the referenced bibliography listed later in this chapter.
The concept of plated through holes (metalized holes) includes at least one of the following two meanings or both:
1. Forming part of a component conductor line;
2. Forming an inter-layer interconnection or printed circuit;
A typical circuit board is etched (on a copper-clad substrate) on a non-conductive composite substrate (epoxy-fiberglass cloth substrate, phenolic paper substrate, polyester fiberglass board, etc.) or Electroless plating (applied on a copper clad substrate or a copper foil substrate) is manufactured and processed.
PI polyimide resin substrate: used for flexible plate (FPC) production, suitable for high temperature requirements;
Phenolic paper substrate: can be stamping and processing, NEMA grade, common such as: FR-2, XXX-PC;
Epoxy paper substrate: better mechanical properties than phenolic paperboard, NEMA grade, common as: CEM-1, FR-3;
Epoxy glass fiber board: with glass fiber cloth as reinforcement material, with excellent mechanical properties, NEMA grade, common 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;
Holes in non-conductive substrates can achieve better solderability in the interlayer interconnect or assembly, or both, after metallization is complete. The inner layer of the non-conductive substrate may have inner circuits. The circuit has been etched before the non-conductive substrate is laminated (pressed). This process is also called a multilayer board (MLB). In multilayer boards, metallized holes not only serve to connect the two outer layers, but also serve as inter-layer interconnects, adding holes designed to pass through non-conductive substrates. Hole concept).
At present, the rubbing and many circuit boards use laminate substrate blanking in the process characteristics. That is, the outer surface of the non-conductor substrate is copper foil that is pressed and bonded to a certain thickness of electrolytic method. The thickness of the copper foil is expressed in terms of the weight (in ounces) of copper foil per square foot. This representation translates into thickness as shown in Table 13.1: These methods are generally milled using a fine abrasive such as glass beads or alumina. Material. Nozzles are used in the wet slurry process. Some chemical raw materials are used to dissolve the polymer resin during the etch back and/or desmear process. Normal (eg epoxy systems), concentrated sulfuric acid , Chromic acid in aqueous solutions, etc. have been used. Either way, you need a good post-processing, otherwise it may result in the subsequent wet process of perforated chemical copper deposition is not so many problems.
The presence of hexavalent chromium in the pores causes many problems with the chemical copper coverage in the pores. It destroys the tin-palladium colloids through the oxidation mechanism and hinders the reduction of the chemical copper. Pore rupture is a common result of this obstruction. This situation can be solved by secondary activation, but the cost of rework or secondary activation is too high, especially in the automatic line, secondary activation process is not very mature.
After the chromic acid tank is treated, there is often a neutralization step. Generally, sodium hydrite is used to reduce hexavalent chromium to trivalent chromium. The temperature of the neutralizer sodium bisulfite solution is generally about 100F, and the temperature after neutralization is washed. Generally 120-150F, there may be clean sulfite, avoiding other baths in the process, interfere with activation.
Concentrated sulfuric acid method:
After bath treatment, there must be a very good water wash, preferably hot water, try to avoid strong alkaline solution when washing. It may form some sodium salt residues of epoxy resin sulfonate, this compound is difficult to It is cleaned and removed from the hole. The presence of it will create contamination within the hole, which may cause a lot of plating difficulties.
There are also several other chemical methods applied to the desmear/drilling and etch back process. Among these systems are the mixture of organic solvents (bulk/swollen resin) and potassium permanganate, which were previously used in the post-treatment of concentrated sulphuric acid treatment and now even directly replace the concentrated sulphuric acid/chromic acid method.
There is also the plasma method, which is still in the experimental application stage and is difficult to use for large-scale production, and the equipment investment is large.
Non-electrochemical copper process
The main purpose of the pre-processing steps:
1. To ensure the continuous integrity of the chemical copper deposition layer;
2. Ensure the binding force between the chemical copper and the substrate copper foil;
3. Ensure the bonding force between chemical copper and inner copper foil
4. Ensuring the Bonding Force Between the Chemical Copper Immersion Layer and the Non-Conductive Substrate The above is a brief description of the chemical copper/electroless copper pretreatment effect.
The following is a brief description of the typical pretreatment steps for non-electrochemical copper:
Degreasing purposes: 1. Remove grease and grease from copper foil and holes; 2. Remove copper foil and dirt from holes; 3. Helps to remove contamination and subsequent heat treatment from the copper foil surface; 4. Drilling holes for polymer resin drilling are simple to deal with; 5. The removal of bad drill holes to produce burr copper powder adsorbed in the hole; 6. Degreasing adjustment In some pretreatment lines, this is the first step in the treatment of composite substrates (including copper foils and non-conductive substrates). Degreasing agents are generally alkaline, and some are neutral and acidic. . It is mainly in some atypical deoiling processes; deoiling is a key bath in the pretreatment line. Dirt-stained areas may cause problems with chemical copper coverage due to insufficient activator adsorption (ie, the formation of microvoids and copper-free areas). The microcavities are covered or bridged by the subsequent electroplating of copper, but in the absence of any binding force between the copper layer and the non-conductive substrate of the base, the end result may cause the separation of the pore walls and the creation of blowholes. The internal plating stress caused by the electroplating layer deposited on the electroless copper layer and the entrapped moisture or gas in the substrate due to the subsequent thermal expansion (baking, spraying of tin, welding, etc.) tend to expand the electroplating layer Pulling off the non-conductive substrate of the hole wall may cause the hole wall to separate; the copper powder produced by the burr tip in the same hole is absorbed in the hole and will not be removed in the degreasing process and will be coated by the copper plating layer. Also in the absence of any binding force between the copper layer and the non-conductive substrate, this condition may eventually result in detachment of the hole wall.
No matter whether the above two results occur or not, there is no denying that the binding force at the place is obviously worsened and the thermal stress at the place is obviously higher, which may destroy the continuity of the plating layer, especially in the welding or wave soldering process. In the middle, the result was a blow hole. The blowhole phenomenon is actually caused by the vapor generated by the thermal expansion of the non-conductive substrate under the weak binding force! If our electroless copper is deposited on the substrate copper foil or on the contaminants on the inner layer 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 poorer, and the result of unfavorable bonding may result in: if the oil is spotted, it may cause blistering; if the dirt area is large, it may even cause the electroless copper to separate. ;
Important factors in the degreasing process:
1. How to choose the right degreasing agent - type of cleaning/degreasing agent
2. Degreasing agent working temperature
3. Degreasing agent concentration
4. Degreasing agent dipping time
5. In addition to mechanical stirring in the tank;
6. Degreasing agent cleaning effect;
7. Degreasing effect after washing;
In the above cleaning operation, temperature is a key factor to be concerned about. Many degreasing agents have a minimum temperature limit below which the cleaning and degreasing effect drops sharply!
Washing influence factors:
1. Washing temperature should be above 60F;
2. Stirring in the air
3. It is best to spray;
4. The entire wash has enough fresh water to replace it in time.
The washing after deoiling is in a sense just as important as the degreasing itself, and the degreasing agent left on the board surface and hole wall itself will become a pollutant on the circuit board, which in turn will contaminate other subsequent main processing solutions such as micro-etching and activation. The most typical water wash here is as follows:
a. Water temperature above 60F,
b. air stirring;
c. When the nozzle is equipped with a groove in the plate, fresh water is used to wash the plate;
Condition c is not used often, but ab two are required; The flow of clean water depends on the following factors: 1. Waste discharge volume (ml/hang); 2. Washing tank working plate load capacity; 3. Washing tank number (countercurrent rinse)
two. Charge adjustment or hole:
After the degreasing process, the charge adjustment process is used. Generally, in the production of some special plates and multilayers, the charge factor of the resin, after the process of removing the slag from the slag, needs to be adjusted in terms of charge; The important role is to "super-wet" the non-conductive substrate. In other words, the surface of the previously negatively charged resin is treated with an adjustment fluid to be denatured to have a slightly positively-charged active surface. In some cases, a uniformly continuous positively charged polar surface is provided so that subsequent activators can be effectively and sufficiently adsorbed on the pore walls. Sometimes the adjusted drug is 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 deoiling adjustment fluid, the industry The trend has combined the two into one, and the regulator is actually just some surfactants. Adjusted water washing is extremely important. Insufficient water washing can cause surfactants to remain on the copper surface of the board and contaminate subsequent micro-etching and activation fluids, which may affect the bonding force between the final copper and copper. As a result, the chemical copper and the substrate are reduced. The bond between copper. Here should pay attention to the temperature of the cleaning water and the effective cleaning of the water flow. The concentration of the adjusting agent should pay special attention, and the use of a high concentration of the adjusting agent should be avoided, and an appropriate amount of the adjusting agent will play a more significant role. III. Microetching The next step in the pretreatment of electroless copper deposition is microetching or microetching or a micro-roughening or roughening step. 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 substrate copper will be greatly reduced; the roughened surface can play a role:
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 base copper.
2. If some of the surfactants are not washed out during the water wash, the microetching agent can remove the surfactant on the substrate surface by etching away the copper base on the copper surface of the bottom substrate, but completely rely on the microetcher to remove the surface activity. The agent is not very realistic and effective, because the surface area of the residual copper surface of the surfactant is large, the chance of allowing the effect of the micro-etching agent is very small, and the copper surface of the residue of the large surface active agent is often not micro-etched;