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Understanding PCB manufacturing - Part I (Drilling)
We have so far covered, in our earlier articles, basic understanding of various types of PCBs, how to read technical capability documents, which PCB design softwares available in the market, how to audit PCB manufacturer and how you can make the most out of your money.
From this article and onwards, we will understand core PCB manufacturing processes one by one. You will learn basic concepts of each process in this series of articles. Once the whole cycle is complete, we will take you to the next series of articles in which we will discuss more about design and manufacturing specific problems and learn various techniques to avoid them.
We will begin our journey with drilling, a first step in the PCB manufacturing cycle.
Let’s Get The Basics Right
If you are using Alitum software for designing then we can accept Altium files without you generating gerber and NC data separately. For other softwares, we require you to generate gerber and NC data for manufacturing.
For NC data, you should provide following information to us:
- NC Drill file in ASCII format
- Tool file/chart, telling which tool is what size and how many drill holes of each tool is used
- Distinctive information about PTH and NPTH holes
Here is an example of a typical NC data:
We consider the hole sizes provided in your drill data as “FINISH” size. This means, we will have to choose actual drill tool size according to plating compensation.
Typical platting compensation for via and component hole is 0.1mm and 0.15mm respectively. This compensation may vary +/-0.05mm from design to design and/or manufacturer to manufacturer. If hole size is greater than 0.5mm then it is, normally, considered as component hole.
E.g. if via hole size in PCB design is 0.3mm, then we will consider 0.4mm drill tool for actual drilling. After plating, hole size will measure close to 0.3mm as specified in the PCB design originally.
It is very important for any PCB designer to understand the concept of plating compensation, which will help them to formulate correct design rules without being caught for design rule violation at the manufacturing stage. It is needless to mention how much cost and time involve, if designer has to change whole design with new rule.
We have explained the whole concept with following diagram. Note that the minimum annular ring criterion for QualiEco normally refers with respect to Production Holes and NOT Finish Holes.
Drill tools normally come in the step of 0.05mm up to drill size 4mm. Between 4mm and 6mm drill sizes, all tools come in the step of 1mm. Hole sizes required beyond 6mm are normally routed instead of drilled.
PTH and NPTH holes
PTH Hole, if –
- Connected holes are having copper pads on top and bottom side with their pad sizes larger than the corresponding hole size
NPTH Hole, if –
- Non-connected holes are found without any copper pad on top and bottom side
- Non-connected holes are having copper pads on top and bottom side with their pad sizes smaller than or equal to the corresponding hole size
- Connected holes are having copper pad only on one side (outer), no connection on any other layer (outer or inner) and no copper pad on the other side (outer)
If you provide NPTH and PTH tool list but do not follow above rule in your PCB design, a technical query will most likely be raised from our technical team.
If your PCB design has finish hole size defined as 1.2mm for example, it doesn’t mean the hole size will be measured exactly 1.2mm in final PCB. Hole size variation in finished PCB must fall within a pre-specified range, which is known as drill tolerance.
Drill tolerance varies from manufacturer to manufacturer. PTH drill tolerance acceptable by QualiEco is +/-0.075mm. If your requirement has strict higher or lower limit, you may inform us and we can adjust the tolerance on a particular hole or tool size.
If you request, following combinations are possible and acceptable by us:
NPTH drill tolerance is normally lower than PTH drill tolerance i.e. +/-0.05mm.
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