Juding circuit technology PCB factory HDI circuit board hole plating process
The manufacturing of the HDI board/BUM board is made by coating or laminating a dielectric layer (or a resin-coated copper foil) on the surface of the "core board" and forming micro-guides and through-holes. These microvias are implemented by via metallization and electroplating of copper to achieve electrical interconnection between the layers. Today, Juding Circuit Technology wants to share the characteristics of the HDI board/BUM micro-blind buried hole and the plating.
The most fundamental feature of microvias formed in HDI/BUM boards with "core" is blind holes. Unlike the conventional pcb through-holes, the micro-vias do not penetrate the entire HDI/BUM board, but only the holes are exposed on one surface of the HDI/BUM board, and the bottom is a copper conductor. s surface. Such a hole is called a blind hole or a blind via. When metallizing and plating such blind holes are performed, the biggest problem is the entry and replacement of the plating solution.
For the through hole, if it is vertical hole plating, a hydraulic pressure difference can be generated between the two plates. This hydraulic difference will force the plating solution into the hole and drive away the gas in the hole to fill the hole. The existence of a small hole with a high thickness (high aspect ratio: the ratio of the thickness of the dielectric layer to the micro-via), the presence of this hydraulic difference is more important, followed by pore formation and electroplating, in the case of pore plating Part of the Cu2? ions in the bath in the well are consumed because the concentration of Cu2? in the bath is getting lower and lower. The efficiency of pore formation and electroplating is getting smaller and smaller, and the effect of the plating fluid in the through-hole and the uneven distribution of the current density are always so that the thickness of the plating at the center of the hole is always lower than the thickness of the plate surface.
In order to reduce the difference in the thickness of this coating, the most fundamental method is:
One is to increase the flux of the plating solution in the hole or the number of times the plating solution in the hole is exchanged per unit time; instead, it is difficult to increase the plating density in the hole, or the water is said to be impracticable, so that the plating in the hole is improved. The current density of the liquid is bound to increase the current density of the plate surface, which in turn causes a greater difference in the thickness of the plating layer in the center of the hole and the thickness between the plating layers on the plate surface;
The second is to reduce the plating density and the concentration of Cu2? ions in the plating solution, and at the same time increase the amount of plating solution in the pores, which can reduce the difference of Cu2? ion concentration in the plating solution between the surface of the plate and the hole. It is possible to improve the difference between the plating thickness of the plating and the thickness of the plating in the hole, but it is often at the expense of PCB productivity, which is undesirable.
The third is to use the pulse plating method, according to different high-thickness ratio micro-vias, using the corresponding pulse current for electroplating, can significantly improve the difference between the plating thickness of the PCB surface and the thickness of the coating in the hole, and even Achieve the same coating thickness. Are these measures applicable to the perforation of microvias in HDI/BUM boards?
As mentioned above, the micro-via holes of the HDI board/BUM board are performed in the blind holes in the hole-forming plating. When the hole depth of the blind hole is small or the aspect ratio is small, the above three plating measures have been practiced. Can get good results, but the depth of the blind hole is high or the aspect ratio is large, how is the reliability of the micro-via hole punching plating? Or how to control the depth or thickness ratio of blind holes?
When the aspect ratio of the HDI board/BUM board is not large, the horizontal hole type plating should be used for reliable electrical interconnection, while for the larger aspect ratio blind hole, the lower surface of the HDI/BUM board is blind. The via hole is difficult to get rid of the gas in the hole, and even the plating solution is difficult to enter the hole, let alone the problem of exchange of the plating solution in the hole unless the plate surface is periodically flipped. Obviously, the use of horizontal hole plated HDI/BUM plated blind vias (especially those with large aspect ratios, such as thickness to diameter ratio greater than 0.8) is not as good as vertical hole plating.
The above is the characteristics of the hole-forming and electroplating of the HDI board/BUM micro-blind hole shared by the PCB manufacturer as a PCB manufacturer, which is equivalent to complicated and high process requirements.