Drilling and Drill File Tech Tips
Essentially they are the same thing and many engineers use the terms interchangeably. Technically, the term "via" should be used when the hole is used to pass signal to another layer of your board and not for mounting a thru-hole component. A thru-hole usually refers to a hole that will have a component lead inserted into it and soldered in place.
Thru-hole Via – This is a standard hole that is drilled all the way through your PCB.
Blind Via – A via that is visible on an outer layer but does not pass all the way through the board to the other side. On a 4 layer board, a blind via might be drilled only from L1-L2.
Buried Via - A via that is invisible from the surface of the board. It is completely internal to the PCB. In a 4 Layer board, it would connect L2-L3.
Microvia – This is a very small hole that is laser drilled, is between 4-6mils (0.10mm – 0.15mm) in diameter and is usually either blind or buried.
A separate drill file needs to be supplied for each set of drills that needs to be blind or buried. For instance, if you have a 6 layer design with a blind and buried construction per the picture, we would need 4 separate drill files.
At PCB Universe, we follow IPC Class 2 standards.
PTH (plated thru holes) will be +/- 3mil (+/- 0.0762 mm)
NPTH (non-plated thru holes) will be +/- 2mil (+/- 0.0508 mm)
There are 2 types of drilling equipment, mechanical and laser. Any finished hole that is 7 mils (.178mm) or larger will be drilled with a standard mechanical drill bit. Anything smaller will likely require the use of a laser drill. Min finished size for a laser drill is currently 4mils (.1mm). Mechanical bits simply aren't strong enough to drill through the laminate without breaking or deflecting (bending) when the bit has a very small diameter.
Be aware that these minimum hole sizes are assuming standard .062" (1.6mm) board thickness. If you require a thicker board, then the minimum hole sizes will change depending on the thickness required. Contact your sales rep if you have concerns about min hole sizes.
12mil (0.3mm) and larger holes have no extra cost.
11mil-7mil (0.28mm-0.18mm) holes will carry a small premium.
6mil-4mil (0.15mm-0.10mm) holes are considered microvias and will typically need to be laser drilled and would need to be custom quoted.
We have a wide range of drill bits so we can drill any size hole from 4mil (0.1mm) and up. Holes that are .250" (6.35mm) and larger will actually be routed with the CNC router. Unlike some PCB shops, we don't restrict you to a few 'standard' hole sizes.
Since more than 95% of designs have less than 10 holes per square inch, we opted to streamline the quote process and not ask for the total number of drills. If your design has more than 10 drill hits per square inch, please check with us before placing your order since this may require additional cost.
Your Gerber files are used to print the images needed on your PCB. The copper patterns, solder mask reliefs, silkscreen, board outline and even fabrication notes should all be in 274X Gerber format. (We can also accept 274D Gerber or ODB++ formats).
The file that is used to program the CNC drilling equipment is not a Gerber file. It needs to be in Excellon CNC Drill File format. This is an industry standard format and all PCB layout software packages should be capable of creating this file. Some software packages automatically include this file during Gerber output and others need to be specifically generated as a separate step when creating your data. Contact your software support if you have questions regarding your software.
If your CNC drill file is created correctly, your drill sizes should be included in the drill file. You may also opt to include a drill map and chart in a fab print. This not required but can be helpful to ensure the CNC drill file matches the drill map. Errors such as missing drill hits and incorrect drill sizes can be caught before manufacturing begins.
The fab print can be submitted in a Gerber layer and/or other common file type such as PDF, DOCX, TXT, etc. Including the fab print in a Gerber layer is highly recommended since the drill map and drill file can be compared much more easily than if it was only in a PDF. Putting the fab print in Gerber format also guarantees that your notes will be seen when your board is tooled for production. Here is a common layout for a drill map and chart.
Specify the size you'd like the holes to be when you receive the board. This is called the Finished Hole Size. With plated thru-holes (PTH), you don't need to account for plating in the barrel of the hole. We'll automatically compensate for the reduction of the hole caused by plating and use a slightly larger drill than you specify as your finished size. For non-plated thru-holes (NPTH), we will use the closest drill size available since we don't have to compensate for the copper plating.
If the via is only used to pass signal to another layer, the hole can be quite small since a component lead doesn't need to physically fit into the hole. If spacing requirements are not a concern, use 12mil (0.3mm) holes or larger since they do not add cost to your design.
If your PTH is used as a component hole, make sure your specified size will be large enough for the component lead to fit inside the hole. Usually component manufacturers will state a recommended finished hole size on their datasheets, consult this first. If this isn't specifically stated in the datasheet, a good rule of thumb is to make the hole at least 10mils (.25mm) larger than the diameter of the lead that's going into the hole. The tolerance on a plated thru-hole (PTH) is +/- 3mils (+/- .0762mm). Keep in mind the lead on the component will also have a tolerance. If you have a perfect storm where your part is on the high end of its tolerance and the hole is on the low end, you'll need that extra room to fit your part into the hole.