To provide some context to the reason behind me writing this article, allow me to first relate part of a conversation I had with one of my clients.
Client: “All vias to have no tenting please.”
Pam: “Excuse me [surprised, as this was my first such request]. Please explain?”
Client: “We have found this the most effective way to capture solder balls.”
Pam: “But surely the PCBA (printed circuit board assembly) companies you are using have this under control?”
Client: :)
Now, I realised this would depend on assembly class and the PCBA company in question, and who am I to argue with my client about what company they utilise? After pondering the issue I decided that, although it would no doubt prove to be subjective, this article might nonetheless serve to clarify some of the considerations surrounding this topic.
For the sake of thoroughness, let me define some of the terms I will use throughout the rest of this article:
• A through-hole via is generally a small plated hole with a copper annular ring (AR), with a hole size and pad (AR) size determined by the end user and PCB supplier. It can be found on a printed circuit board (PCB) to route a track from one side of the PCB to the other side, or to an internal layer on a multilayer PCB.
• A blind via is one that is only visible from one side of the PCB.
• A buried via is not visible and is only used on a PCB with four layers or more (depending on layer stacking).
• Tenting is when solder mask covers the entire via, AR and hole. Conversely, an exposed or non-tented via is one that does not have solder mask covering it.
In this article I will consider vias of the through-hole variety only.
Full tenting
Full tenting occurs when the solder mask covers the annular ring and the hole, on both the top and bottom sides of the board.
Advantages of this approach include the fact that via placement can be at the minimum clearance (pad to pad) without interfering with another via or component pad. Placement can be under surface mount components, without fear of the component lifting during the solder process. Because the minimum sized hole is drilled to ensure the hole remains closed, the holes are typically small enough to allow fluid movement but block solder flow, leaving the tent functional. Full tenting also prevents excess solder in via holes, prevents processing chemical exposure and reduces accidental shorting.
The primary disadvantage of this technique is that, when both ends of a via are covered, there is the danger of trapped liquid evaporating and bursting the via open during any heating process.
Partial tenting
This is where the solder mask covers the annular ring while the hole is left exposed on both top and bottom sides.
As with full tenting, partial tenting benefits from the fact that via placement can be at the minimum pad-to-pad clearance without interfering with another via or component pad, and the minimal hole size is typically small enough to allow fluid movement but block solder flow. In particular, partial tenting is good for high-current traces, and for its heat dissipative properties.
I do not believe there to be any particular disadvantages to this technique.
Single sided tenting
Tenting can also be performed on only the component side of the PCB, with solder mask covering the annular ring and the hole. This allows placement to be under surface mount components, without fear of the component lifting, and also serves to reduce the likelihood of accidental shorting. Once again, the
minimum sized hole is drilled to ensure the hole remains closed, and the holes are typically small enough to allow fluid movement but block solder flow, leaving the partial tenting functional.
One disadvantage of single sided tenting is that it opens the door for ‘blow out’ to occur. It may also allow small amounts of fluids (from surface finish processes or PCB cleaning solutions) to remain in the barrel of the hole. These residues may be corrosive to the copper plating over time.
Exposed via
Finally, it is possible to have neither the annular ring nor the hole covered in solder mask. This technique is good for high-current traces and offers good heat dissipative properties, as well as sealing the via barrel during wave soldering.
There are several disadvantages to leaving vias exposed. Firstly, they will tarnish and eventually corrode, depending upon the environment. They also run the risk of being shorted together by contact with other surfaces. Wave soldering can cause molten solder to blow out of the holes, leading to damaged vias, solder splashes, solder balls etc. on the board, as well as later corrosion.
Using an exposed via for testing can lead to a damaged via and a break in the net. Additionally, finish processes or PCB cleaning solutions can remain in the barrel of the hole, and some of these residues may be corrosive to the copper plating over time.
Conclusion
In conclusion, then, it is advisable to have selected vias tented and strategically placed throughout the board, and to tent the annular ring but not the hole. It may prove beneficial to have vias exposed for prototypes and then tented for production. A combination of approaches can also be considered, namely to tent the via on the component side, while only tenting the annular ring on the opposite side.
Of course, your decision should also be based on your PCB supplier and whether you are using a dry film solder mask versus a liquid photo-imageable (LPI) solder mask.
Happy tenting…or not!
For more information contact Pam Landman, IJ-CCAL, +27 (0)82 894 4145, pam@ij-ccal.co.za, www.ij-ccal.co.za
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