Contrary to popular belief and although not very obvious, most printed circuit boards are actually full of holes. Designers and manufacturers require holes in PCBs for many purposes, with some of them necessary for mounting mechanical components, others for fixing the board in its enclosure, while still others necessary while mounting the board of machines while assembling them. However, a greater majority of the holes on a circuit board are necessary for mounting through-hole components and providing interlayer connectivity for signals. According to PCB Trace Technologies Inc, plated through holes have an additional function, that of thermal management. They conduct heat generated by components to suitable layers on the PCB, which then disperses the heat through heat sinks.
It is necessary for designers to understand about plated through holes in a PCB, as its successful layout relies on their proper placement and use. Incorrect placement and configuration of plated through holes can be detrimental to a circuit board while manufacturing it, degrading its performance.
For understanding the process of plated through holes, it is necessary to understand the basic steps of fabricating a multilayer PCB:
- In a circuit board, the inner layer pairs begin as a core material with copper foils on both sides.
- The fabricator creates traces and pads by exposing the image of the circuitry onto photoresist material they have applied on the copper foils. Once the exposed photoresist hardens, it leaves behind a protective pattern of circuitry, while the fabricator removes the soft unexposed part chemically.
- The fabricator next etches the board chemically to remove all the unprotected copper on it. After completing the etching process, they strip off the protective photoresist to expose the circuitry on the copper.
- Using either laser or mechanical drilling, the fabricator then creates blind, buried, and microvias on the board. Next, they stack layer pairs with a layer of prepreg between them, thereby creating the composite circuit board.
- On the exterior surface of the board, fabricators add thin layers of copper foil. They them subject the entire stackup to heat and pressure. This allows the resin in the prepreg to melt and bond the layers together.
- The fabricator then drills through holes in the PCB, which extend through the entire board thickness. Some of these holes are for mounting mechanical parts, some for processing, while the majority are for component soldering and vias. They may plate some holes, while leaving the others without plating.
- Finally, the fabricator creates the circuitry on the external layers of the board and plates it along with the through holes they have drilled. After finishing the plating process, they cover the exposed copper and through holes with a layer of tin, thereby protecting them.
- Next, the fabricator strips off the hardened photoresist covering the copper circuitry on the board. They chemically etch any exposed copper not covered by tin. Then they also remove the tin plating.
- This completes the primary fabrication of the board. To protect the exposed copper circuitry, the fabricator may add surface finish. They may also add solder mask, and silkscreen.
On a bare circuit board, it is possible to see many different-sized holes that represent vias and through holes for mounting component pins. Even when complete after plating, various holes are visible as those drilled through the board during fabrication.
The fabricator starts the plating process only after compositing the various layers with heat and pressure, and after drilling all the through holes. The various steps the fabricator takes for plating the through holes are:
- Cleaning the holes of any residue left over from the drilling process. The debris may include contaminants including burrs on the hole edges, and residual resin inside the holes. The cleaning requires the use of chemical agents and abrasive processes.
- The fabricator coats the board’s surface and the through holes with a thin layer of copper. This deposition of electroless copper provides a base in the holes for anchoring the regular copper plating the fabricator will do later.
- The fabricator may also micro-etch the interior of the holes for further enhancing the base copper for better anchoring of copper plating later.
- After removing the exposed and hardened photoresist, the fabricator proceeds to electroplate the exposed copper circuitry on the board along with the drilled holes. This increases the copper weight on the board.
The electroplating process deposits additional copper on the circuit board. The fabricator connects the board as the cathode, or negative electrode, and immerses it in a chemical bath with a copper plate as the anode or positive electrode. On passing current through the bath, copper ions will travel to the negative electrode and deposit on the external layers of the PCB including the inner sides of the through holes. To ensure a uniform application of copper on the board, it is necessary to control the electroplating process tightly. After completing the electroplating process, the fabricator may apply a protective tin coating on the board.
Although the use of Surface Mount Components or SMCs on PCBs does not require any type of holes to mount them, some boards still use through hole components for various reasons:
A PCB needs connectors and switches for interaction. Using through hole components adds to the mechanical strength of these parts.
Large SMC parts for power applications may be difficult to solder on account of the greater metal area necessary for heating. On the other hand, using through hole power components is easier, as their through hole mounting offers them better thermal and mechanical stability.
A plated through pin connection offers a much better conductor of heat. High temperature parts with bolt on design can dissipate additional heat through the ground plane of the PCB.
Designers must be aware of design guidelines for plated through holes, as many boards still use them extensively. These are:
Most through hole components or THCs require soldering to the board using the wave soldering technique. This is necessary as the soldering is effective on the bottom layer only, opposite to the top layer which mounts the component. The board essentially passes over a molten wave of solder that wells up to fill up through holes and any lead of components inside them. The wave soldering process does not access the top layer of the board that primarily holds the SMCs.
Therefore, if the board has mixed components, it requires two soldering processes—wave soldering for anchoring through hole components, and reflow soldering for anchoring surface mount components.
Wave soldering techniques require a greater amount of spacing between through hole components, to avoid creation of unwanted solder bridges between them. Large through hole components can sometimes act as shadows for smaller SMCs near them, preventing them from being adequately soldered. Through hole components often require manual rework, for which they need adequate room, and hence greater spacing around them.
Whether using SMCs or THCs, the designer must use the footprint dimensions as recommended by the manufacturer. This ensures a proper assembly for the board. While some THCs are flexible enough to adjust to different footprints, most cannot do so, and require properly spaced holes.
As component leads of THCs must enter the holes in the board, it is imperative to use recommended hole sizes for specific lead diameters. If the hole diameter is too big, molten solder may not cover it entirely, resulting in a poor solder joint. On the other hand, it may be difficult to insert the THC in a hole smaller than that recommended by its manufacturer.
Manufacturing a printed circuit board with plated through holes that will not have any problems starts with understanding the fabrication process and using the proper footprints. PCB Trace Technologies Inc recommends using professional PCB CAD software packages complete with libraries of component footprints. Most such packages come with built-in comprehensive constraint managers that allow designers to configure their layout with proper rules, thereby ensuring error-free layouts.