Via Plug Types

Pros and Cons
Soldermask design, with respect to the vias, dramatically affects reliability and manufacturability of PCBs.

This document discusses the pros and cons of the various options:



Vias Not Covered
vias-not-covered

Description
Vias are exposed.  Surface finish is applied to via barrel.

Pros
Via barrels are covered with surface finish metal.  Test access is available from both sides of card.

Consnot-covered-cons
Possible wicking of solder paste 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 mask to lift on the short distance between the ball and via capture pads.  This is not a concern at first pass assembly.

Comment
Most recommended if assembly allows.



Button Print
button-print
Description
Vias tented on one side by a secondary solder mask application.  Surface finish is applied to via barrel prior to button print.

Pros
Via barrels are covered with surface finish metal.  Test access is available from one side of card.  Reworkable as solder wicking is not a concern.  Standard industry process.

Cons
Perceived mask height issue at assembly.  Industry max height of mask over copper has been reduced over the years from 0.004" down to 0.002.  Requires an additional mask application process, post surface finish application. Not recommended for OSP or Tin finish.
button-print-cons

Comment
Not recommended as this can affect final surface finish plating.



Primary LPI Solder Mask Tenting Vias

Description

No apertures are supplied on mask data layer.  Mask covers via pads and tents hole.  No surface finish is applied to via barrel.  Possible entrapment.
soldermask-tending-vias

Pros
One step application

Cons
Via tenting cannot be guaranteed with LPI mask.  There are two methods of applying LPI mask at Saturn, spray coating and squeegee coating.  Spray coating cannot ensure that the via is tented both sides.  Squeegee coating's ability to tent is limited by the hole size, surface tension of the liquid mask, and board thickness.  If a via is not tented both sides, chemical entrapment from surface finish pre-clean 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 of ENIG finish, the gold and small area of the exposed copper near the cap could form a galvanic cell, accelerating the etch process.

Comment
NOT ADVISED for long term reliability.  Many board designs are seen with vias tented via primary mask.  This could be a result of a lack of reliability data.  Saturn procedure requires full via plugging even if not dictated by customer.



Plugged Viaplugged-via

Description
Vias are plugged with mask or other non conductive media.  LPI mask is applied over plug.  No surface finish is applied to via barrel.

Pros
100% of the required vias are tented

Cons
Additional process steps are required.  No surface finish is applied to the via.  Via size restrictive.  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 during the assembly reflow.

Comment
Highly recommended if vias cannot be free of mask on both sides.



Active Pad

Descriptionactive-pad
Vias are plugged with a conductive or non-conductive media, planarized and plated over.

Pros
Reduces routing issues on external layers.  Minimizes inductance.

Cons
Multiple additional process steps are required, in addition to dual plating processes.  The extra plating process has a negative effect on the minimum feature size capable on the external layers.  This process is not advised in conjunction with PTFE substrates.

Comment
ADVISED for via in pad applications only.



Plating Vias Shut

Descriptionplating-vias-shut
Vias are specified to be plated shut with electrolytic copper.

Pros
If feasible, a copper filled via increased thermal conductivity of the via.

Cons
Reliability.  Due to the standard plating chemistries deposition in high throw areas at a greater rate than low throw areas (via barrel), the possibility of plugging the surface of the via prior to the center is extremely high.  In this event, plating chemistries will be trapped in the via, and detrimentally effect the long term reliability.  The surface copper thickness will also be exceptionally high if a via could be reliably plated shut.  This process is not advised unless using appropriate copper plating chemistries that are formulated specifically for plating shut through vias.

Commentvias-shut-cons
NOT ADVISED