When I was a child, building tunnels was an important part of my day. What started out with simple holes dug in the ground with my friends later on became tunnels large enough to create fortresses, traps, or whatever else came to mind. At that point in our lives, the need for thrill-seeking often trumped common sense, and there was more than one occasion in which the inevitable tunnel cave-in buried someone that had to be pulled out. The lesson learned here was that an incorrectly dug tunnel was eventually going to fail, hopefully without the miner still inside. Years later, I am dealing with yet another tunnel problem, but this time it is in the assembly of a printed circuit board, where a failed hole is a whole different story.
Vias are the “tunnels” on PCBs. You may be surprised to find out that your contract manufacturer will typically think of a via as any hole in the board. Whether a hole is serving as a net interconnect or being used to solder the lead of a thru-hole component, it is still a via to your CM. If vias aren’t planned out correctly, they can cause problems during the manufacturing of your board. Let’s take a look then at some of the common problems that can happen during PCB manufacturing with vias, and how you and your CM can avoid these problems by following good PCB via size guidelines.
How Your Contract Manufacturer Categorizes PCB Vias
For fabrication, any hole in a printed circuit board can be considered a via. This includes non-plated thru-holes and plated thru-holes for parts, as well as an assortment of interconnect vias. These interconnect vias include thru-hole, blind, buried, and micro-vias.
A thru-hole via within a surface mount pad is often considered to be a different type of via because it usually requires unique design rules within the CAD tools for use. For fabrication purposes, however, these are still considered a regular thru-hole filled via.
As far as PCB assembly goes, the major via consideration are those thru-holes that will be used for installing parts. These include regular thru-hole components that will be soldered into plated thru-holes, such as connectors, resistors, and capacitors. Also in this category are parts that are installed into holes that are not soldered. These would include connector legs, press-fit parts, guide pins, screws, and other mounting hardware.
Problems With Vias That Can Affect Assembly
When via holes for thru-hole components aren’t fabricated correctly, there can be a lot of problems during assembly:
- Holes that are too small for the component lead intended for that via could cause bent leads or the inability to install the component. A part that can’t be installed can ruin an assembly, while bent leads may cause soldering problems and intermittent electrical problems. All of this may result in the PCB having to be refabricated.
- The plating process on smaller holes can be more difficult to work with, which can lead to inconsistent hole sizes. Once again, you may run into component insertion or solderability problems. It is much easier to maintain consistent plating on larger holes.
- Larger holes also can cause problems, such as loose parts that require care in soldering, but these problems are usually easier to fix and can often be resolved with manual assembly processes.
- Larger vias used on large ground pads under surface mount components, such as BGAs, may cause solder to wick through and compromise the connection. This can impact the electrical, mechanical, and thermal characteristics of the part, resulting in its failure.
- Large plated thru-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.
It is also important that holes of the same size with the more extreme precision and registration be specified as the “first” to be drilled. Then, those same sized holes with greater tolerances should be specified as “seconds” to be drilled. If this sequence isn’t correct, the holes with the tighter tolerances may end up being drilled after the drill bit has been worn down, resulting in miss-drilled holes. Making sure to specify the correct drill tolerances for priority drill sequencing can help you to avoid these problems and ensure a successful assembly.
PCB Via Size Guidelines to Avoid Assembly Problems
Some via hole size considerations to remember when setting up the vias in your PCB design include:
- Smaller vias: These vias are important in the support of smaller-pitch/higher-density components, such as 0.5mm BGAs, but they are more sensitive to drill and plating errors, which can cause PCB failures. Only use these smaller sizes when necessary. It is much better for manufacturing to use an 8 mil hole instead of a 5 mil hole if it will only take minor layout modifications to do so.
- Larger vias: These are much easier for the fabricator to work with, but the correct tolerances and clearances must be used to ensure there aren’t any shorts to other layers. It is also important to consider where the placement of larger vias will be so that they don’t pull the solder away from ground pads that need a good connection to their component.
- 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.
- Too many vias of the same size: Try to vary the drill sizes so that you don’t have too many holes that are the same size. For those boards with a high density of the same drill size, the drill bit may not be able to keep its integrity for the entire drill operation. This requires the fabricator to stop the drilling process partway through, replace the drill bit, re-register the board, and then continue the operation.
- Plating: Your holes will be plated with copper for connectivity unless you specify the hole as being unplated. Make sure that you specify the correct size for the drill needed for the hole taking into account the plating. Usually, this is done by specifying the “finished” hole size. If the finished (plated) hole size isn’t accounted for, you may end up not being able to insert the component leads into their holes.
Not only will keeping these via considerations in mind help you to create a better design, but these are also part of the list of design issues that a quality CM will have their engineering staff looking for. By including the best via design practices initially in your PCB, there will be less design cleanup needed to get your board through assembly.
How Your CM Can Help
The drilling of your printed circuit board will take place at the fabricator, and it is essential that they do a good job to eliminate any potential assembly problems due to via size. The more complex or stringent the via requirements are in your design, the more specialized the fabrication vendor needs to be. To ensure your PCB is fabricated correctly, you need a contract manufacturer that fully understands the process and has solid business relationships with multiple fabricators. This way, they can choose the best fit for the board technology that you are having built.
This all starts with working with a CM that understands the specific requirements of your design. That is why, at VSE, we take the time to give your design a full review beforehand so that we know up front what you will need. This enables us to choose from the list of our fabrication vendors that we have been cultivating over many years the best possible fit for your board. This way, when we get your board back in-house, there won’t be any problems with assembly due to via size problems.