Via Plug Types


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Pros and Cons of Various Via Plugging Options


Vias Not Covered  

Description
In this industry standard process, surface finish applied to barrel of the exposed via.

Pros
Via barrels covered with surface finish metal. This allows access to testing from both sides of the circuit board.
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Cons
During BGA rework, possible wicking of solder paste into via. Localized thermal energy causes LPI mask to lift between the ball and via capture pads. 

Comment
Most recommended if assembly allows.


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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 board. Reworkable as solder wicking is not a concern.  Standard industry process.
  button-print
Cons
Perceived mask height issue at assembly.  Industry max height of mask over copper has been reduced from to 0.002.  Requires additional mask application process, post surface finish application. Not recommended for OSP or Sn finish. 

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


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Primary LPI Solder Mask Tenting Vias  

Description
No apertures are supplied on mask data layer.  Mask covers via pads and tents hole.

Pros
One Stop Application.
  soldermask-tenting-vias
Cons
Via tenting precarious with LPI mask. Spray coating cannot ensure via tented on both sides. Squeegee coating limited by hole size, liquid mask surface tension, and board thickness. Vias not tented on both sides subject to chemical entrapment from surface finish pre-clean lines. All finishes subject to micro-etch, which when trapped in capped via will crystallize rapidly causing Cu sulfate crystals. With ENIG, Au and exposed Cu near cap can form galvanic cell and accelerate etch process. 

Comment
NOT ADVISED for long term reliability.  Many board designs are seen with vias tented via primary mask.   Probably lack Reliability Data.  Saturn procedure requires full via plugging even if not dictated by customer.


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Plugged Vias  

Description
Vias plugged with mask or other non-conductive media. LPI mask applied over plug.

Pros
100% of the required vias are tented.
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Cons
Increased process. Restricted size. No finish applied to via. Failure to control rate of rise in curing risks volatile remnants producing soldermask smearing in reflow.

Comment
HIGHLY RECOMENDED if via cannot be free of mask on both sides.


  




 
Active Pad  

Description
Vias plugged with a conductive or non-conductive media, planarized and plated over.

Pros
Reduces routing issues on external layers.  Minimizes inductance.
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Cons
Additional steps include dual plating, which negatively affects min feature size capable on external layers. NOT ADVISED in conjunction with PTFE substrates.

Comment
ADVISED for Via-in-Pad applications only.


  




 
Plating Vias Shut  

Description
Vias are specified to be plated shut with electrolytic copper.

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

Comment
NOT ADVISED.


  
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