PCB manufacturing tolerances and its impact on fabrication and assembly process!
It is imperative that when you offer the documentation for PCB manufacturing, all the requisite details are provided. Very often there are missing details that, if provided, can go a long way in quoting the right cost as well as manufacturing PCBs that match up to your bespoke requirements. Robust data needs to include tolerances as they will ensure there are no defaults in manufacturing as well as no production delays. Let us look at these tolerances in some detail:
Important PCB Tolerances
1. PCB Component Tolerances
When it comes to components, tolerance refers to the acceptable variant to be able to ensure proper functioning. Tolerance in this case also refers to the difference in between the lower & upper dimensions of the component parameters.
The overall functionality of the PCB is dependent on how the components perform with each other and therefore it is important to know the limit of each component. Component tolerance modeling is thus a crucial step that ensures reliability testing of the board to determine if the board components succeed in performing as a unit. In fact component tolerance modeling allows the quantification of how individual components relate to the output. The more accurate the tolerance, the more reliable the final product will be. Component tolerance modeling, therefore is an important function of the design as well as the manufacturing process.
What is important to remember is also that since each component brings its own component limit, what needs to be considered is the tolerance stack-up or what is the additive effect of various component tolerances. The tolerance stack-up estimates the effects of the accrued variations & is defined on an engineering design.
2. PCB Material Tolerances
When it comes to PCB laminate materials, it is important to provide specific Tg ratings. Additionally providing a stack-up with specific dielectrics is important. What you also need to provide so that the right material can be selected is the overall printed circuit board thickness. What is needed here is a certain material tolerance since it will be impossible to meet the exact details. If your requirement is for a tolerance less than 10 percent, you might need a change in the core as well as a tighter process control. In turn this will lead to added costs. Materials & cores therefore need to be correctly chosen at the prototype stage as if this is done at the production stage, it can prove to be far more costly.
3. PCB Drill Tolerances
What is also needed when creating a fabrication document is the need to look at PCB manufacturing tolerances to avoid PCB fabrication errors.
For example, plated and non-plated holes need to come with different tolerances as drills cater to specific sizes with strict tolerance. It is important to remember that there are many factors that affect the size of the hole. Some of these include:
- Plating baths
- Time in & out of the tanks
- Additional plating
- Finished Plating
The need for tolerances is therefore a given. However it is important to remember that not all drills are exact, so some amount of tolerance is required. Where tighter tolerances are required, it is imperative that they should be noted clearly.
In fact, larger holes, plated or non-plated above the size of 0.200”, will require a tolerance of +\-0.005”.
4. PCB Routing Tolerances
When it comes to routing, tolerances are required right from slotting tools to routing cutoffs. Typically, a tolerance of +\-0.005” is the default standard on the size or cut.
Other standard PCB tolerance that you should know
Following is the analysis of outer layer (width annular ring, min annular ring, NP to copper, KO to copper, etc.):
|Outer copper thickness||1OZ||2OZ||3OZ||4OZ|
|Normal Width between pads||0.12MM||0.15MM||0.2MM||0.25MM|
|Normal Width between pad and trace||0.12MM||0.15MM||0.2MM||0.25MM|
Normal Width between pad or tarce and copper area
|Normal VIA pad single side width||0.15mm||0.16mm||0.17mm||0.18mm|
|Normal PTH pad single side width||0.2mm||0.21mm||0.22mm||0.23mm|
|Normal Width between NP holes and copper||0.2mm||0.2mm||0.2mm||0.2mm|
Normal Width between profile and copper
|Normal width between via and copper||0.2mm||0.2mm||0.2mm||0.2mm|
|Max width between pads||0.1MM||0.13MM||0.18MM||0.23MM|
|Max width between pad and trace||0.1MM||0.13MM||0.18MM||0.23MM|
|Max width between pad or trace and copper area||0.18MM||0.18MM||0.18MM||0.18MM|
|Min width between NP holes and copper||0.15mm||0.15mm||0.15mm||0.15mm|
|Min Width between profile and copper||0.25mm||0.25mm||0.25mm||0.25mm|
|Min width between V-Cutline and copper||0.35mm||0.35mm||0.35mm||0.35mm|
|Min width between via and copper||0.18mm||0.18mm||0.18mm||0.18mm|
To sum up
Tolerance modeling is a wonderful way to achieve a balance between performance & cost as otherwise the less the tolerance, the more the cost of production. Tolerance modeling therefore is the key aspect that reduces manufacturing cost while ensuring that the quality is maintained.
Of course, in case you do not specify the details in the manufacturing document, the contract manufacturer will refer to the IPC standards & performance specifications as a guide. However detailed information on aspects such as surface finish copper weights, material, hole sizes and more can go a long way in ensuring that the final product matches your bespoke needs.
One of the top PCB manufacturing firms, Mer-Mar Electronics offers complete packages for PCB assembly. Customers appreciate our reliable quality and quick turnaround, seeing us as a one-stop shop for all their PCB manufacturing needs.
Please do not hesitate to contact our team if you have any questions about our PCB assembly and PCB manufacturing service, or if you require immediate assistance. Send us an email at email@example.com or give us a call at (760) 244-6149 with any questions you may have.