Hi Victor, 

Terminology first: plug vs. fill

"PLUG", to us, means high-solids solder mask ink, which Is squeegeed into the target holes and then usually covered with solder mask. We use this on non-test point vias to minimize potential corrosion sites on the board. Can NOT be used for via-in pad. Acceptable finished hole size typically ranges from 0.25-0.4mm (usually multiple hole sizes on the same board must be within 50um of each other). 

PROs: 
- cheap (usually 2-5% cost adder) 
- plugged and covered vias are effectively removed as potential corrosion sites
- can also be used for light or acoustic seal 
- can be used on thermal pads to prevent solder wicking down the vias (with negligible impact on thermal transfer)

CONs: 
- shrinkage and voiding is inevitable
- ~60% barrel fill is typical (in our experience, as long as min 60% is achieved, there is no risk of acid trap in the hole like you might get with a "tented" via in which surface finish fluids flow in but don't flow out due to capillary forces; >=60%, the aspect ratio of the shallow cup facilitates complete fluid exchange) 



"FILL" means epoxy resin, often squeegeed in (sometimes under vacuum), then cured and planarized. Can be cap plated for via in pad. Typical FHS range is 0.2-0.45mm.

PROs
- highly reliable process with little to no voiding or shrinkage
- epoxies typically match the CTEs of the FR-4
- required for VIP and buried PTHs
- as with plugging, filled vias can be covered with SM to minimize corrosion sites, acoustic/light sealing and preventing solder wicking on thermal pads.

CONs
- more expensive (+10-15% cost usually)


Does that answer your question? 
Todd


-----------------------------------------
Todd MacFadden
Component Reliability Engineer

BOSE CORPORATION
Design Assurance Engineering
Tel: 508.766.6259

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Subject: [TN] Small PTH Plugging

Fellow TechNetters:

   What are the PROS and CONS on inadequate, partial, small PTH fill/plugging under BGA?

Victor,