Vias treatment in PCB manufacturing
If vias are not covered, they are exposed and the surface finish is applied to via barrel. This is standard process in printed circuit board fabrication.
The via barrels are covered with surface finish metal. This allows circuit board testing access available from both sides of the board.
It is possible for wicking of solder paste to get into the via. In the case of BGA rework, paste loss due to wicking into the via is a result of the localized thermal energy causing the LPI solder mask to lift on the short distance between the ball and via capture pads. This is not a concern at first pass assembly.
Solder Mask Tenting Vias
In tenting vias, no apertures are supplied on the mask data layer. The solder mask covers the via pads and tents hole. No surface finish is applied to the via barrel. For this process it is possible for entrapment to occur.
In the past, tenting of vias with Dry Film Soldermask was a standard offering. Due to the limitations on feature size resolution and high thickness for SMT applications of the Dry Film Masks, this process is not readily available. Assemblers may require plugged vias due to vacuum draw or to prevent paste wicking into vias.
This process is not advised for long term reliability. Many printed circuit board designs are seen with vias tented via primary mask. This could be a result of a lack of reliability data.
The pros to primary LPI solder mask tenting vias is it is only a one step application.
Via tenting cannot be guaranteed with LPI mask. There are three common methods of applying LPI mask. Curtain, Spray and Screen coating. Curtain and Spray coating cannot ensure that the via is tented both sides. Screen coatings ability to tent is limited by the hole size, surface tension of the liquid mask, and board thickness.
If a via is not tented on both sides, chemical entrapment from surface finish preclean lines is probable. All the finishes will be subject to a micro-etch process. The micro-etchant that gets trapped in the capped via will crystallize rapidly causing copper sulfate crystals. Over time, these crystals can cause long term reliability issues. In the case on ENIG finish, the gold and small area of exposed copper near the cap could form a galvanic cell, accelerating the etch process.
In the plugged vias process, vias are plugged with solder mask or some other non conductive media. LPI mask is then applied over plug. During the plugged vias process, no surface finish is applied to via barrel.
This process was developed as a modification from the LPI tent to guarantee that 100% of the vias are fully tented.
In plugged vias, 100% of the required vias are tented.
For plugged vias, additional process steps are required during fabrication. No surface finish is applied to the via and the via size is restrictive. The control of rate of rise during curing is critical to ensure 100% of volatiles are evacuated. Failure to control this can lead to soldermask smearing the surface during the assembly reflow process.
There are other treatment of Vias in PCB, we only introduce 3 of them this time. Normally, PCB manufacturers will choose Solder Mask Tenting Vias if not specified in Gerber file or manufacturing requirements, but few PCB manufacturers do just as the design of PCB, it means strict follow the design, so if your PCB design with Vias not tented but your requirements is tented, better tell PCB manufacturer when inquiry. For Plugged Via, it need more process then other 2 Options, price is slightly high than normal requirement of Vias.