Electroless 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 treatments, which is then thickened to a specified thickness by a subsequent electroplating method. Usually 1mil (25.4um) or thicker, sometimes even directly through chemical methods to deposit the copper thickness of the entire circuit. The chemical copper process is a series of necessary steps to finally complete the deposition of chemical copper, and each step is very important to the entire process flow.
The concept of plated through holes (metallized holes) includes at least one or both of the following two meanings: a. Forming part of the conductor circuit of the component; b. Forming an interlayer interconnection circuit or printed circuit;
Usually, circuit boards are made by etching (on copper clad substrates) or electroless plating (on copper clad substrates) on a piece of non-conductive composite substrates (epoxy-fiberglass cloth substrates, phenolic paper substrates, polyester fiberglass substrates, etc.) or copper clad substrate) method of production and processing.
1. PI polyimide resin substrate: used for flexible board (FPC) production, suitable for high temperature requirements;
2. Phenolic paper base material: can be stamped, NEMA grade, common such as: FR-2, XXX-PC;
3. Epoxy paper substrate: mechanical properties are better than phenolic cardboard, NEMA grade, common such as: CEM-1, FR-3;
4. Epoxy glass fiber board: glass fiber cloth is used as reinforcement material, which has excellent mechanical properties, NEMA grade, common such as: FR-4, FR-5, G-10, G-11;
5. Non-woven glass fiber polyester base material: suitable for some special purposes, NEMA grade, common such as: FR-6;
Electroless Copper/Immersion Copper: Holes on non-conductive substrates allow for better solderability in interlayer interconnects or assembly or both after metallization is complete. Non-conductive substrates may have internal circuits inside them - circuits that have been etched before laminating (pressing) the non-conductive substrate. Boards processed by this process are also known as multi-layer boards (MLBs). In a multilayer board, the metallized holes not only serve to connect the two outer layers, but also serve as the interconnection between the inner layers. If you add a hole designed to go through a non-conductive substrate (when there is no concept of buried blind vias).
Today's raw wipes and many circuit boards use laminate substrate blanking in terms of process characteristics, that is to say, the outside of the non-conductor substrate is a copper foil made by electrolysis with a certain thickness by pressing. The thickness of copper foil is expressed in ounces of copper foil per square foot. These methods generally use fine-grained abrasives, such as glass beads or alumina abrasives. The holes are treated with a nozzle spray during the wet slurry process. Some chemical feedstocks are used to dissolve polymer resins in etch back and/or decontamination processes. Common (such as epoxy resin systems) concentrated sulfuric acid, aqueous solutions of chromic acid, etc. have been used. Regardless of the method, good post-processing is required, otherwise it may lead to many problems such as subsequent wet perforation electroless copper deposition.
Chromic acid method: The presence of hexavalent chromium in the hole can cause many problems with the coverage of chemical copper in the hole. It will destroy the tin-palladium colloid through oxidation mechanism and hinder the reduction reaction of chemical copper. Pore fracture 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 for automatic lines, and the secondary activation process is not very mature.
After chromic acid tank treatment, there is often a neutralization step. Typically, sodium bisulfite is used to reduce hexavalent chromium to trivalent chromium. The temperature of the neutralizing agent sodium bisulfite solution is generally about 100F, and the washing temperature after neutralization is generally 120-150F, which can clean the sulfite so as not to be brought into other baths during processing and interfere with activation.
Concentrated sulfuric acid method: After the bath is treated, there must be a very good washing, preferably hot water, and try to avoid strong alkaline solutions when washing. Some sodium salt residues of epoxy sulfonate may be formed, a compound that is difficult to wash away from the pores. Its presence can cause contamination in the hole and can cause a lot of plating difficulties.
Other Systems: There are several other chemical methods used in desmear/drill and etch back processes. In these systems, the application of organic solvent mixtures (bulking/swollen resins) and potassium permanganate treatment, formerly used in the post-treatment of concentrated sulfuric acid treatment, is now even directly replacing the concentrated sulfuric acid process/chromic acid process. In addition, there is the plasma method, which is still in the stage of experimental application, which is difficult to produce on a large scale and requires a large investment in equipment.
Non-electrochemical copper process: the main purpose of the pretreatment step: a. to ensure the continuous integrity of the electroless copper layer; b. to ensure the bonding force between the chemical copper and the base copper foil; c. to ensure the combination of the chemical copper and the inner layer copper foil force; d. to ensure the bonding force between the electroless copper layer and the non-conductive substrate. The above is a brief description of the pretreatment effect of electroless copper plating/electroless copper plating. The following is a brief introduction to the typical pretreatment steps of electroless copper:
The purpose of degreasing: a. Remove the oil and grease in the copper foil and the hole; b. Remove the dirt in the copper foil and the hole; c. Help to remove the surface contamination of the copper foil and subsequent heat treatment; d. Aggregate the drilling E. Remove the burr copper powder adsorbed in the hole generated by the bad drilling; f. Adjust the oil removal in some pre-treatment lines, which is the most suitable for composite substrates (including copper foil and non-conductive substrates). ) The first step of the treatment, the degreaser is generally alkaline, and some neutral and acidic materials are also used.
Mainly in some atypical degreasing processes; degreasing is a key bath fluid in the pretreatment line. Contamination-contaminated areas can cause electroless copper problems (ie, the creation of microvoids and copper-free areas) due to insufficient activator adsorption. Microvoids can be covered or bridged by subsequent copper electroplating, but where there is no bond between the electroplated copper layer and the non-conductive substrate at the base, the end result can lead to pore wall detachment and porosity.
The internal coating stress generated by the electroplating layer deposited on the chemical copper layer and the vapor expansion force generated by the subsequent heating (baking, tin spraying, welding, etc.) Pulling away from the non-conductive substrate of the hole wall may cause the hole wall to fall off. Similarly, the copper powder generated by the burr in the hole is adsorbed in the hole and is not removed during the degreasing process, and will also be covered by the electroplated copper layer. In the absence of any bonding force between the copper layer and the non-conductive substrate, this condition may eventually result in detachment of the hole walls.
Regardless of whether the above two results occur or not, one thing is undeniable. The bond strength here is significantly worse and the thermal stress here is significantly higher, which can disrupt the continuity of the plating, especially during soldering or wave soldering. As a result, blow holes are produced. The hole blowing phenomenon is actually caused by the thermal expansion of the steam generated from the non-conductive substrate under the bonding force coating! If our electroless copper is deposited on the contamination of the base copper foil or the contamination on the inner copper foil ring of the multilayer board, the bonding force between the electroless copper and the base copper will also be stronger than that of the well cleaned copper foil The bonding force between them is very poor, and the result of poor bonding may occur: if the oil stain is in the form of dots, it may cause foaming. If the dirt area is large, it may even cause the electroless copper to detach.
Important factors in the degreasing process: a. How to choose a suitable degreaser - the type of cleaning/degreaser; b. The working temperature of the degreaser; c. The concentration of the degreaser; d. The immersion time of the degreaser ; e. Mechanical stirring in the degreasing tank; f. The cleaning point where the cleaning effect of the degreasing agent is reduced; g. The water washing effect after degreasing;
In the above cleaning operations, temperature is a key factor to be concerned about. Many degreasers have a minimum temperature limit. Below this temperature the cleaning and degreasing effect drops sharply! Influencing factors of water washing: a. The washing temperature should be above 60F; b. Air stirring; c. It is best to have spray; d. There is enough clean water in the entire washing process to be replaced in time;
The water wash after the degreasing tank is in a sense as important as the degreasing itself. The degreaser remaining on the board surface and hole wall itself will also become a contaminant on the circuit board, which in turn contaminates other subsequent main processing solutions such as micro-etching and activation. Generally speaking, the most typical water washing in this place is as follows: a. The water temperature is higher than 60F; b. Air stirring; c. When the nozzle is equipped in the tank, the surface of the board is washed with clean water when washing; Condition c is not commonly used, but ab two Item is necessary; the water flow of cleaning water depends on the following factors: a. The amount of waste liquid carried out (ml/hang); b. The load of the working plate in the washing tank; c. The number of washing tanks (countercurrent rinsing);
Charge Adjustment or Whole Well: Use the typical charge adjustment process after degreasing. Generally, in the production of some special boards and multi-layer boards, due to the charge factor of the resin itself, after decontamination, etching and other processes, the charge needs to be adjusted; the important role of adjustment is to "super-penetrate" the non-conductive substrate. In other words, the resin surface with a weak negative charge is denatured into an active surface with a weak positive charge 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 walls. Sometimes the adjusted chemicals are added to the degreasing agent, so it is also called degreasing adjustment liquid. While separate degreasing and conditioning fluids will work better than combined degreasing conditioning fluids, industry trends have brought the two into one, and modifiers are really just surfactants. The adjusted water wash is extremely important.
Insufficient washing will cause the surfactant to remain on the copper surface and contaminate the subsequent micro-etching and activation solutions, which may affect the final binding force of copper and copper, resulting in a decrease in the binding force between chemical copper and substrate copper. Attention should be paid to the temperature of the cleaning water and the effective flow rate of the cleaning water. Special attention should be paid to the concentration of the modifier and the use of too high concentrations of the modifier should be avoided. The right amount of regulator will play a more obvious role.
Microetching: The next step in the pretreatment of electroless copper deposition is the microetching or microetching or microroughening or roughening step. The purpose of this step is to provide a micro-rough active copper surface structure for subsequent electroless copper plating. Without the micro-etching step, the bond between the chemical copper and the substrate copper is greatly reduced; a rough surface can do the following:
a. The surface area of the copper foil is greatly increased, and the surface energy is also greatly increased, providing more contact area between the chemical copper and the substrate copper;
b. If some surfactants are not removed during washing, the microetchant can remove the surfactant on the surface of the substrate by etching away the copper base on the copper surface of the underlying substrate, but it is completely removed by microetching. Micro-etching surfactants are unrealistic and effective, because when the surface area of the copper surface remaining by the surfactant is large, the chance for micro-etching agents to act is very small, and it is often impossible to micro-etch copper with large-area surface activity. surface of residues.
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