Analysis of HDI PCB Electroplating Filling

Why do PCBs need plugged holes?

• Plugging holes can prevent solder from penetrating through the drilled hole during wave soldering, causing short circuit and the ball of solder to pop out, resulting in a short circuit in the PCB.
• When there are blind vias on BGA pads, plugging the holes is necessary before the gold plating process in order to facilitate BGA soldering.
• Plugged holes can prevent flux residue from remaining inside the through holes and maintain surface smoothness.
• It prevents surface solder paste from flowing into the hole, causing false soldering and affecting assembly.

What are the plugged hole techniques for PCB?

Plugged hole processes are varied and lengthy, and difficult to control. Currently, common plugged hole processes include resin plugging and electroplating filling. Resin plugging involves copper plating the holes first, then filling them with epoxy resin, and finally, copper plating the surface. The effect is that the holes can be opened and the surface is smooth without affecting soldering. Electroplating filling involves filling the holes directly with electroplating without any gaps, which is beneficial for the soldering process, but the process requires high technical ability. Currently, the blind hole electroplating filling for HDI printed circuit boards is usually accomplished through horizontal electroplating and continuous vertical electroplating filling, and then subtractive copper plating. This method is complex, time-consuming, and wastes electroplating liquid.

The global electroplated PCB industry has rapidly grown to become the largest segment of the electronic component industry, accounting for a unique position and a production value of $60 billion per year. The demands for slim and compact electronic devices have continuously compressed the board size and have led to the development of multi-layer, fine-line, and micro-hole printed circuit board designs.

In order to not affect the strength and electrical performance of printed circuit boards, blind holes have become a trend in PCB processing. Direct stacking on blind holes is a design method for obtaining high-density interconnections. To produce stacked holes, the first step is ensuring flatness of the hole bottom. Electroplating filling is a representative method for producing flat hole surfaces.

Electroplating filling not only reduces the need for additional process development but is also compatible with current process equipment, and promotes good reliability.

Advantages of electroplating filling:

• Favorable for designing stacked holes and Via on Pad, which increases the board's density and enables more I/O foot packages to be applied.
• Improves electrical performance, facilitates high-frequency design, improves connection reliability, increases operating frequency, and avoids electromagnetic interference.
• Facilitates heat dissipation.
• Plugging holes and electrical interconnection are completed in one step, avoiding defects caused by resin or conductive adhesive filling, and also avoiding CTE differences caused by other material filling.
• Blind holes are filled with electroplated copper, avoiding surface depression, and conducive to the design and production of finer lines. The copper column inside the hole after electroplating filling has better conductivity than conductive resin/adhesive and can improve heat dissipation of the board.