tenting-pcb-vias

Tenting PCB vias

Tenting vias for PCB designs provides added protection from damage and aids soldering during PCB assembly.

Keep reading to learn why this process is important, what its risks are, and how to reduce those as best as possible.

What is via tenting?

Tenting a via during PCB manufacture refers to the protecting of a via with soldermask to enclose or cover over the drilled hole.

A via that isn’t tented is open to the elements and this gives a greater risk of corrosion and possible short circuits.

Why is tenting important?

The aim of tenting PCB vias is to protect the electrical integrity of the PCB. Tenting PCB vias gives extra protection against the external elements, which will increase the lifespan and quality of a product.

Soldermask that is used in via tenting will protect any copper feature (tracks or vias) from a number of potential issues.

These include:

  • Damage
  • Oxidation
  • Solder bridging
  • Stopping solder wicking down the hole if placed too close to a component pad to avoid dry solder joints
  • Product failures. This is particularly true around BGAs where re-work is very difficult, even impossible in some cases

What is the process of tenting PCB vias?

The Institute of Printed Circuits (IPC) describes via tenting with the four designations as seen in the table below:

 
IPC 4761 Designation
Description
Type 1-a Tented Via
Covered with dry film solder mask on one side
Type 1-b Tented Via
Covered with dry film solder mask on both sides
Type II-a Tented and Covered Via
Covered with dry film solder mask and additional covering of LPI solder mask on one side
Type II-b Tented and Covered Via
Covered with dry film solder mask with an additional covering of LPI solder mask on both sides

The process of tenting PCB vias is as follows:

  1. Remove mask clearances from the required vias.
  2. Fill drilled via with non-conductive material
  3. Plate the plugged via
  4. Smooth the plated vias for conformity with PCB
  5. Apply desired solder mask finish

Soldermask is a liquid called Liquid Photo Imageable. It’s applied by a screen printing process and is UV curable.

The decision of where the solder mask will be applied to the PCB is established during the custom PCB design process. 

When you’re designing your PCB, make sure that you either know exactly what you’re doing, or use an expert specialist service to do it for you. This is because if the PCB design process isn’t done correctly, PCB manufacturing issues can arise.

Even if the design has a 100% check from the CAD tool for electrical shorts and open circuits, that doesn’t mean that it can be manufactured in any way you want. Expertise at the manufacturing stage is sometimes overlooked, whereby a perfectly good design can be manufactured with a topology that is unable to support the product’s electrical requirements.

What are the risks with via tenting?

Since the soldermask is a liquid, its aim is to “bridge” the gap over the via hole. If the hole is too big by being over 0.3mm, holes in the soldermask will start to appear.

This can allow substances to flow into the hole, get trapped and cause failures during manufacture.

How can you mitigate the risks of tenting PCB vias?

There are solutions that help to mitigate the risks of tenting PCB vias. These are:

  • Plugging and capping via holes.

Via holes can be plugged and capped. The only disadvantage with this is that it does increase the cost of the PCB due to the extra processes involved during PCB manufacture.

  • Partially cover the via pad with solder mask.

For vias that are greater than 0.3mm in size, you can partially cover the via pad with soldermask by leaving the hole (plus 50um) exposed so no soldermask flows into the hole. This still prevents solder wicking into the hole from a SMD pad that a via is connected to.

 

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