Electroless copper immersion 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, and then thicken it to a specific designed thickness through subsequent electroplating methods. It is generally 1mil (25.4um) or thicker, sometimes even directly chemically deposited to the copper thickness of the entire circuit. The chemical copper process goes through a series of necessary steps to finally complete the deposition of chemical copper. Each step is very important to the entire process.
The concept of plated through holes (metallized holes) includes at least one or both of the following two meanings:
1.Make up part of the component conductor circuit; 2. Form an interlayer interconnection circuit or printed circuit;
Usually, the circuit board is made by etching (on a copper-clad substrate) or chemical plating (on a copper-clad substrate) on a piece of non-conductive composite substrate (epoxy resin-glass fiber cloth substrate, phenolic paper substrate, polyester fiberglass board, etc.) or on a copper-clad substrate).
PI polyimide resin substrate: used in flexible board (FPC) production, suitable for high temperature requirements;
Phenolic paper base material: can be stamped and processed, NEMA grade, common ones such as: FR-2, XXX-PC;
Epoxy paper base material: mechanical properties are better than phenolic cardboard, NEMA grade, common ones such as: CEM-1, FR-3;
Epoxy resin fiberglass board: uses fiberglass cloth as reinforcing material, has excellent mechanical properties, NEMA grade, common ones such as: FR-4, FR-5, G-10, G-11;
Non-woven fiberglass polyester substrate: suitable for some special purposes, NEMA grade, common such as: FR-6;
Chemical copper/immersed copper
Holes in non-conductive substrates allow for better solderability in interlayer interconnects or assembly or both after metallization is complete. Non-conductive substrates may have internal circuitry on the inside – the circuitry has been etched before the non-conductive substrate is laminated (pressed). Boards processed by this process are also called multilayer boards (MLB). In multilayer boards, metallized holes not only serve to connect two outer layers of circuitry, but also serve as interconnections between 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 materials and many circuit boards use laminated substrate blanks in terms of process characteristics. That is to say, the outside of the non-conductive substrate is pressed with a certain thickness of electrolytic copper foil. The thickness of copper foil is expressed in weight (ounces) of copper foil per square foot. These methods generally use fine-grained abrasives such as glass beads or alumina abrasives. The holes are prepared using nozzles during the wet slurry process. Some chemicals are used to dissolve polymer resins during etchback and/or decontamination processes. Common (such as epoxy resin systems) concentrated sulfuric acid, chromic acid aqueous solutions, etc. have been used. No matter which method is used, good post-processing is required, otherwise it may lead to many problems such as chemical copper deposition in subsequent wet perforations.
Chromic acid method:
The presence of hexavalent chromium in the pores can 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. Porosity breakdown 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 yet.
After chromic acid bath 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 around 100F, and the washing temperature after neutralization is generally between 120-150F. The sulfite can be cleaned to avoid bringing in other bath solutions during the processing and interfering with activation.
Concentrated sulfuric acid method:
After bath treatment, there must be a very good wash, preferably hot water, and try to avoid strong alkaline solutions when washing. Some sodium salt residue of epoxy resin sulfonates may form and this compound is difficult to remove from the pores by cleaning. Its presence can cause contamination within the hole and may cause many plating difficulties.
Other systems:
There are other chemistries used in desmear/drill removal and etch back processes. In these systems, including the application of organic solvent mixtures (leafing/swelling resins) and potassium permanganate treatment, which were previously used in the post-treatment of concentrated sulfuric acid treatment, now even directly replace the concentrated sulfuric acid method/chromic acid method.
There is also the plasma method, which is still in the experimental application stage, is difficult to produce on a large scale, and requires a large investment in equipment.
Electroless copper process
The main purpose of the preprocessing step:
- Ensure the continuous integrity of the electroless copper layer;
- Ensure the bonding force between chemical copper and base copper foil;
- Ensure the bonding force between chemical copper and inner copper foil
- Ensure the bonding force between the electroless copper layer and the non-conductive substrate. The above is a brief explanation of the effect of electroless copper plating/electroless copper plating pretreatment.
The following is a brief introduction to the typical pretreatment steps for electroless copper:
1.Degreasing
The purpose of degreasing: 1. Remove oil and grease from the copper foil and the holes; 2. Remove dirt from the copper foil and the holes; 3. Help remove surface contamination of the copper foil and subsequent heat treatment; 4. Aggregate the drilled holes. Simple treatment of resin drilling dirt; 5. Remove burrs and copper powder adsorbed in the holes caused by bad drilling; 6. Oil removal adjustment in some pre-processing lines, which is for composite substrates (including copper foil and non-conductive substrates) ) In the first step of treatment, degreasers are generally alkaline, but some neutral and acidic materials are also used.
Mainly in some atypical oil removal processes; oil removal is a key bath liquid in the pre-treatment line. Areas contaminated by dirt can cause chemical copper coating problems (i.e., the creation of microvoids and copper-free areas) due to insufficient activator adsorption. The micro-voids will be covered or bridged by subsequent electroplated copper, but since there is no bonding force between the electroplated copper layer and the base non-conductive substrate, the final result may lead to hole wall detachment and pores.
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 in the plating layer due to subsequent heating (baking, tin spraying, welding, etc.) tend to push the plating layer from The non-conductive base material on the hole wall is pulled away, which may cause the hole wall to fall off; similarly, the copper powder produced by the burrs in the hole is adsorbed in the hole and is not removed during the oil removal process, and will also be covered by the electroplated copper layer. Similarly, this There is no bonding force between the copper layer and the non-conductive substrate
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 in the plating layer due to subsequent heating (baking, tin spraying, welding, etc.) tend to push the plating layer from The non-conductive base material on the hole wall is pulled away, which may cause the hole wall to fall off; similarly, the copper powder produced by the burrs in the hole is adsorbed in the hole and is not removed during the oil removal process, and will also be covered by the electroplated copper layer. Similarly, this Without any bonding force between the copper layer and the non-conductive substrate, this situation may eventually lead to hole wall detachment.
Regardless of whether the above two outcomes occur, one thing is undeniable. The bonding strength here is significantly worse and the thermal stress is significantly higher here, which may destroy the continuity of the plating, especially during soldering or wave soldering. The result is blow holes. The hole blowing phenomenon is actually caused by the thermal expansion of steam generated from the non-conductive substrate under the bonded coating! If our electroless copper is deposited on the dirt on 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 stronger than that on the well-cleaned copper foil. The bonding force is very different, and the result of poor bonding may be: if the oil stain is dotted, it may cause blistering. If the dirt area is large, it may even cause the electroless copper to detach.
Important factors in the degreasing process:
- How to choose a suitable degreaser – types of cleaning/degreasers
- Working temperature of degreaser
- Concentration of degreaser
- Soaking time of degreaser
- Mechanical stirring in the oil removal tank;
- Cleaning points where the cleaning effect of degreaser is reduced;
- Washing effect after oil removal;
In the above cleaning operations, temperature is a key factor worthy of attention. Many degreasers have a minimum temperature limit. Below this temperature, the cleaning and degreasing effect drops sharply!
Factors affecting washing:
- The washing temperature should be above 60F;
- Air stirring;
- It is best to have a spray;
- There is enough clean water during the entire washing process that can be replaced in time.
This post-greasing bath wash is, in a sense, as important as the degreasing itself. The residual degreaser on the board surface and hole wall itself will also become a contaminant on the circuit board, thereby contaminating other subsequent main processing solutions such as micro-etching and activation. Generally speaking, the most typical washes in this area are as follows:
1.The water temperature is higher than 60F. b. Air agitation is C. When a nozzle is equipped in the tank, the surface of the plate will be rinsed with clean water during water washing;
Condition c is not commonly used, but two items ab are necessary; the flow rate of the cleaning water depends on the following factors: 1. The amount of waste liquid taken out (ml/hang); 2. The load of the working plate in the water washing tank;
3. The water washing tank Number of pieces (countercurrent rinsing)
2.Charge adjustment or whole hole:
Use typical charge conditioning procedures after degreasing. Generally, in the production of some special plates and multi-layer boards, due to the charge factor of the resin itself, the charge needs to be adjusted after decontamination, etching and other processes; the important role of adjustment is to “super-penetrate” the non-conductive substrate. In other words, the originally weakly negatively charged resin surface is denatured into a weakly positively charged active surface after being treated with the conditioning solution. In some cases, a uniform and continuous positively charged polar surface is provided, which can ensure that subsequent activators 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 adjusting fluid. Although separate degreasing and conditioning fluids will work better than a combined degreasing and conditioning fluid, industry trends have combined the two into one, and modifiers are really just surfactants. The adjusted water wash is extremely important. Insufficient washing will cause surfactant to remain on the copper surface, contaminating subsequent micro-etching and activation solutions, which may affect the final bonding force between copper and copper, resulting in a reduction in the bonding 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 regulator and avoid using regulators with too high a concentration. An appropriate amount of regulator will have a more obvious effect.
3.Micro corrosion
The next step in pre-processing for electroless copper deposition is a microetching or microetching or microroughening or roughening step. The purpose of this step is to provide a slightly rough active copper surface structure for subsequent electroless copper plating. Without the micro-etching step, the bonding force between chemical copper and substrate copper will be greatly reduced; the rough surface can play the following roles:
- The surface area of copper foil is greatly increased, and the surface energy is also greatly increased, providing more contact area between chemical copper and base copper;
- If some surfactants are not removed during water washing, microetching can remove the surfactant on the surface of the substrate by etching away the copper base of 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 left by the surfactant is large, the chance of micro-etching agent acting is very small, and it is often impossible to micro-etch large-area surface activity on copper. agent residue on the surface.