VIA FINISH
Tented Via, Blind Via, Buried Via, Stacked Via & Via in Pad
Via Tenting and Via Plugging are additional treatments to the vias later in the production sequence to improve either thermal performance or PCB assembly yield. These additional process steps are most often intended to eliminate PCB assembly issues as solder shorting between a component pad to a via pad or solder migrating down through the barrel of a via which has been drilled into a component soldering land. These problems lead to costly troubleshooting and rework. But fortunately, it is possible to eliminate most issues by specifying an appropriate via treatment.
Via Tenting
This is the easiest and most economic via treatment, which means to cover the annular ring and via holes with solder mask ink in printed circuit board (PCB) manufacturing. No special steps are taken to ensure the hole opening remains closed, even if there are some vias are totally not closed because this is not guaranteed.
Why via tenting? The main purpose shouldn’t be to close the opening of a hole but rather, cover the annular ring to prevent exposure to the elements and reduce accidental shorting for contact with the circuit.
Via tenting process is so simple as it sounds. In this process, we simply remove the solder mask clearances from the via which want to be tented. It almost no extra cost added into whole PCB manufacturing processes. Smaller vai with diameter no more than 0.3mm (12mil) has the best chance of remaining closed.
If you are tenting some of the vias in your design, always make a fabrication note either in a readme.txt file or Gerber fab drawing so the PCB manufacturer knows you have intentionally removed the solder mask clearances from some of your vias. Otherwise the PCB manufacturer my put the order on an engineering hold to verify your intention.
Via Plugging
Via Plugging, also known as LPI (liquid photo-imageable) Mask Filled Via or Non-Conductive Filled Via, which means specific measures are taken to ensure the via is plugged and sealed with solder mask ink and the annular ring is covered. This PCB via finish requires all vias specified should be fully covered and sealed. Compared to Via tenting, it will increase the PCB manufacturing cost.
Check to know more about Factors That Increase PCB Cost
Via plugging is a common via treatment technique applied for the PCB designs with BGA, especially those BGA designs with Dog Bone layout method, where vias are commonly found in very close proximity to the BGA’s SMD pads. The concern is during assembly, solder paste will wick always from the intended pad and flow down the via creating poor or non-existent solder joints.
If you require non-conductive plugged vias, this must be specified in your fab notes or readme.txt. The PCB manufacture will need to know the quantity, sizes and locations of the vias to be plugged so they can be sure all the required holes are plugged.
Check to know about IPC-4762 Via Protection Type, Tented Vias, Plugged Vias, Filled Vias, Filled and Capped Vias
Via In Pad
Via In Pad, also known as Active Pad, which is becoming more and more common as BGA packages are becoming tighter. Rather than using the standard “Dog Bone” land pattern to transfer signal from the BGA footprint to a via that passes signal to other layers, vias can be drilled directly into the BGA footprint pads. This allows much simpler routing by soldering directly over the via. Make sure this process is called out in your fab notes. And via in pad process is more expensive than via plugging.
PCB manufacturer need to know the location, vias quantity and size of the holes that need to be Via in Pad. The following steps are taken in this process.
- Drill the via (thru hole or blind)
- Fill the via with non-conductive material. Conductive material can be used but it’s not recommended since conductive material expands much more quickly than the surrounding laminate when heated which can ause cracks and pad failure during the assembly process.
- The surface of the plugged via is then plated over with copper.
- The newly plated copper surface is flattened and smoothed (planarize) to be even with surrounding copper features.
- The final finish is applied.
- Now have a solderable surface mount pad that also passes signal to inner layers eliminating the need to rout the signal to a via on the surface layer.
Key Benefits of Via In Pad:
- Tighter BGA pitches
- Increased thermal dissipation
- Reduced layer count or PCB size, which ultimately may reduce cost
- Improved routing density
- Strengthening pad attachment
- Gives high frequency designs the shortest possible route to bypass capactors
- Overcomes high speed design issues and constrains such as low inductance
Conductive vs Non-Conductive Filled Vias
Conductive Fill: Generally, a conductive filled via will be used when heat or a large amount of current needs to be carried from one side of the board to another. These can be found under chips that will be giving off a lot of heat where overheating is a concern. The metallic nature of the fill will naturally wick heat away from the chip to the other side of the board in many ways like a radiator.
The main drawback to conductive filled vias is the difference in CTE (Coefficient of Thermal Expansion) between the metallic fill and surrounding laminate. Metal will heat and expand much more rapidly than the surrounding laminate and this may cause fractures between the pads and hole wall. Because of this, conductive filled vias are not recommended for Via In Pad processing where the purpose of the fill is the reinforce the stability of the copper pad that is plated over the hole.
Non-Conductive Fill: A common misconception is that a non-conductive fill will either not pass any or only a very weak electrical signal through the via. This is not true. The barrels of the vias will still be plated with copper the same as any other via on the board, the only difference is the empty air in barrel is replaced with the fill material. This is usually done to prevent solder or other contaminants from entering the via or provide structural support for a copper pad covering the open hole in the case of a Via In Pad. Non-Conductive fill is another term for mask plugged vias.
