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An analysis of the reasons why the hole of PCB is bronze free in the electroless plating copper

January 16, 2018

PCB Image:An analysis of the reasons why the hole of PCB is bronze free in the electroless plating copper

PCB employs different resin system and substrate of different materials. Differences in the resin system will lead to significant distinctions in both the activation effect of treating electroless plating copper and the process of treating electroless plating copper. In particular, there are great differences in terms of CEM compound substrate and high-frequency silver substrate, special methods need to be adopted in the treatment of chemical electroless plating copper as the normal way of treating the chemical electroless plating copper tends not to achieve desired results.


An analysis of the reasons why the hole of PCB Prototype is bronze free in the electroless plating copper

The pre-treatment of the substrate


Some substrate can easily get damp and poor solidification occurred in parts of the resin in the process of being laminated into substrate, which may contribute to poor quality of drilling holes for the resin in itself is not strong enough, more drilling sewage, or heavy digging of resin in the wall of holes. As a result, baking is a necessary step in the board cutting. In addition, after the multi-layer boards are laminated, poor solidification of resin may occur in the substrate areas where PP is semi solidified and the effect of both drilling holes and smear removal & activation of electroless plating copper will be directly affected.


The poor performance of drilling holes mainly exhibited in the following aspects: the resin dust in the holes are abundant, the hole walls rough, the rags in the portholes abundant, rags also abundant inside the holes, nailhead of cooper foil in the internal layer, uneven in the length of fracture surface in the glass fiber zone, which can all serve as hidden trouble for the quality of the chemical copper.


For cleaning the boards, in addition to the mechanical way of removing pollutants from the surface of substrate and clearing away rags in the portholes or removing all blurs, to conduct facial cleaning also plays an important role in cleaning the dust inside the holes, which is especially important to the treatment of double-sided board without the processing of smear removal.


What should be noted is that it is not true that smear removal can help get rid of the smear and dust inside the holes as is commonly believed. Under most circumstances, the effect of smear removal technique in treating dust is very limited as the dust will form small smear micelle in the tank solution, which makes it difficult to handle the tank solution. The smear micelle attached to the walls of holes may easily become solidified inside the holes or drop off from the hole walls in the following manufacturing process, which may result in the phenomenon of dotted bronze free inside the holes. Therefore, it is necessary to conduct some mechanical ways of cleaning the board and high-pressure cleaning, especially in following the development trend and under the circumstances when small-hole board and high-aspect ratio board is becoming more and more common, and it has become a trend to remove dust inside the holes with ultrasonic waves.


Proper technique of gear removal can greatly increase the binding force of holes and the reliability of connectivity of inside layers. However, problems may also occur due to poor coordination between gear removal technique and the relevant tank solution. If gear removal is not sufficient, potential quality hazard may also appear: micro pores inside the walls of the hole, poor cohesion in the inside layer, separation of hole walls, hole blowing, etc.. On the contrary, over gear removal may also bring about bad effect: protrusion of glass fiber inside the holes, roughness inside the holes, interception of the glass fiber, copper infiltration, fracture or discontinuity of copper with holes or crease in the coating plate due to separation of black coppers inside the layer and break of the inside wedge holes. Besides, the problem of how to coordinate the tank solution in the process of gear removal well is also a matter of importance.


Insufficiency in the bulking or swelling may contribute to insufficiency of gear removal while over bulking or swelling can better help to remove the bulking resin but bring about activation of poor electroless plating copper during the process, or the resin may drop off in the following process or separation of hole walls may appear even if the electroless plating copper can happen. For the tank of smear removal, adoption of new tanks and high activation may lead to excessive smear removal in the single-function resin, double-function resin and parts of the triple-function resin due to low degree of connectivity; protrusion of glass fiber in the hole walls, difficulty in the activation of glass fiber and worse connectivity between the chemical copper and resin. After the electroless plating copper, the coating layers may accumulate on the uneven bases, which may multiply the force of chemical copper. If serious enough, chemical copper can be clearly seen to drop off from the hole walls piece by piece after the electroless plating copper and cause no copper existing in the holes.