One of my favorite childhood memories was digging holes at the beach: while most kids got a plastic trowel and bucket, my dad insisted on bringing an adult-sized shovel to dig out holes on the beach for me to stand in. He would check the tide times before we left the house (this was the pre-smartphone era, after all) so he could safely dig our holes as close to the water as possible without the chance of water rushing in. While I’ve since given up my beach exploits, I’ve discovered the simple joy of digging holes while gardening as a reasonable stand-in.
Vias are some – but not all – of the holes drilled into a PCB, although they are often the most populous type. Whether a hole is a net interconnect or provides a recess and surface for pin soldering, the fabrication sees them as all the same. Poorly planned hole sizes can cause problems during board manufacture. A quality contract manufacturer (CM) can sidestep fabrication issues by adhering to standard PCB via sizes.
Standard Via Sizes According to the Board Aspect Ratio |
||
|---|---|---|
| >10:1 | 6:1~10:1 | <6:1 |
| Above this aspect ratio, the reliability of the via suffers tremendously, and a poor service life undoes any improvement to the layout. Instead, consider microvias – this will add to fabrication (and carries separate reliability concerns), but better meets HDI demands. | The “Goldilocks” zone via sizing, although the design will typically settle closer to the higher end to improve layout space. Fabricators should be able to provide this level of sizing for typical board thicknesses of .063” (63 mils) / 1.6 mm. | This range is more readily producible by fabricators with fewer production issues. However, only the most sparse assemblies can afford the additional space requirement of the vias. However, microvias will occupy this aspect ratio with a catch: a depth limit of .010” (10 mils) / .25 mm. |
How A CM Categorizes Drilled Holes
Plated through-holes (PTH) are the most commonly drilled hole types outside BGAs and other HDI-conducive elements. With dense assemblies, signals must pass from one side of the board to the other, and the most efficient path is a direct line through the z-axis. Although a via is technically a plated through-hole, there is a notable distinction between the two terms:
- A PTH bestows mechanical support and integration of through-hole components to the board. While the signal-carrying capabilities of the PTH are welcome and necessary, they are a secondary benefit to the assembly.
- A via is exclusively for signal transmission; there is no ability to support pins mechanically. While this may seem like a noticeable drawback, the upshot is that vias can be significantly smaller than PTHs, making them a superior option for saving space during routing.
A good rule of thumb is that miniaturization greatly favors surface mount technology (SMT) devices and a via over PTHs; this implementation will depend on the component parameters and packaging options. Also, via diameters are not required to differ within a design, as the optimal size depends on the thickness of the board. Alternatively, PTHs are sized according to pin (as indicated in the manufacturer’s datasheet). Another drilling option is non-PTH – drilled holes with no conductive capabilities – that provide additional mechanical support to connector legs, press-fits, guide pins, and occasionally fasteners. There are fewer design constraints on these purely mechanical features.
Problems With PTHs That Can Affect Assembly
A lack of design considerations for vias and PTHs results in unproducible features or poor quality. Board designers and manufacturers will want to confer early to shape design rules and optimize DFM outcomes:
- Holes too small for the intended lead can lead to bends or the inability to install the component. An uninstallable part interrupts assembly, while bent leads can cause soldering and intermittent electrical problems.
- Plating smaller holes can be more difficult, leading to inconsistent hole sizes; it is much easier to maintain consistent plating on larger holes.
- However, larger holes have problems, like loose parts requiring soldering care. These problems are usually easier to fix and often resolved with manual assembly processes.
- Larger vias used on large ground pads under surface mount components, such as BGAs, may cause the solder to wick through and compromise the connection.
- Large plated through-holes used for grounding hardware, such as screws or posts, must have enough clearance on all layers to accommodate the larger drill tolerances; otherwise, these holes may end up shorting to other layers if the drills exceed the expected tolerance.
Designers must also specify holes of the same size with more extreme precision and registration as the “first” to be drilled. Then, those same-sized holes with greater tolerances require listing as the “seconds” to be drilled. This sequence ensures that tighter-tolerance holes are drilled when the bit is freshest and is without mechanical wear.
Standard Via Sizes to Avoid Assembly Problems
Via sizes will vary according to the design needs and the board dimensions. The minimum diameter of the via must adhere to the aspect ratio, which is a ratio of the hole depth (the thickness of a board for a through-hole) and the hole diameter. Aspect ratios higher than 10:1 generally incur production problems due to the plating process that makes these vias much less reliable and more susceptible to thermal expansion failure. Beyond the aspect ratio, there are additional PTH size considerations:
- Smaller vias – These sizes support smaller-pitch/higher-density components, such as 0.5mm BGAs. However, they are more sensitive to drill and plating errors, which can result in scrapped boards. Only use these smaller sizes when necessary–it is much better for manufacturing to use an eight-mil hole instead of a five-mil hole if it only requires minor layout modifications.
- Larger vias – These sizes are much easier for the fabricator to work with, but tolerance and clearance must ensure no shorts to other layers. It is also important to consider thermal sinking so larger PTHs don’t pull the solder away from ground pads that require an acceptable solder joint.
- Blind and buried vias – These are essential for high-density, high-speed, noise-sensitive designs, but their advantage comes with a price. These vias require increased fabrication costs and time to process the vias first before laminating the layers of the board together.
- Many same-size vias – While the via size doesn’t have to vary, too many same-size holes require the fabricator to stop the drilling process partway through, replace the drill bit, re-register the board, and then continue the operation.
- Plating – PTH must specify the correct size for the drill needed for the hole, considering the plating; usually, the artwork will indicate the “finished” hole size. Differences between PTH and non-PTH finished hole sizes may lead to pin insertion issues.
Your CM Has Experience Sizing Up PCBA Designs
Incorporating standard PCB via sizes will optimize layout space without endangering yield, making it a critical component of DFM. Bare board drilling will occur at the fabricator, and the more complex or stringent the design requirements are, the more specialized the fabrication vendor needs to be. A contract manufacturer that fully understands the process and has solid business relationships with multiple fabricators can decide upon and furnish the best fabricator fit.
At VSE, our engineers are committed to building electronics for our customers. Giving your design a full review lets us choose from the list of cultivated fabrication vendors to find the perfect fit. Alongside these manufacturing partners, we’ve been building for quality and reliability in numerous life-saving and life-changing industries.
