There are different and conflicting opinions when it comes to best practices for covering vias (tenting, filling or capping).
This is due to information that lingers in the cloud (internet) once it has been posted.
Here are my recommendations for reliability based on best known current practices.
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Tented Vias (Not Recommended)
Why? Trapped etchant chemicals can corrode the vias over time.
"This type of via can be subject to the "micro-etch" process. This is caused by a small amount of residual etchant trapped inside a tented via. This material will crystallize rapidly, creating copper sulfate crystals. Over time, these crystals can cause long term reliability issues. In the case of a electroless nickel immersion gold (ENIG) finish, the gold and small area of exposed copper near the tent could form a galvanic cell, accelerating the etch process"
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf see page 18.
Plugged Vias (Not Recommended)
Why? Trapped etchant chemicals can corrode the vias over time.
Same issue as the one sided tented via.
"Since it is only partially filled, chemical entrapment is a major concern."
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf
Filled Vias (Highly Recommended)
Why ? Higher Reliability.
"Filled vias are 100% filled, usually with a non-conductive material. This process uses additional process steps. This is done to ensure 100% of the vias are covered.
There are several variations of the filled via.
A filled and covered via has a secondary covering of material (liquid or dry film solder mask) applied over the via. It may be applied from either one side or both sides."
Expensive $$.
Via in Pad VIP (Recommended)
"This is the ultimate via and is commonly called Via-In-Pad (VIP). Vias are filled with a conductive or non-conductive media, planarized and then plated over. This process allows the use of via capture pads as SMD pads.
VIP has become very common in BGA PCB designs to reduce routing issues and lower inductance associated with high speed connections. However, it does drive up the overall PCB cost. If at all possible, consider other alternatives and use VIP only as a last resort."
VIP has become very common in BGA PCB designs to reduce routing issues and lower inductance associated with high speed connections. However, it does drive up the overall PCB cost. If at all possible, consider other alternatives and use VIP only as a last resort."
Source: TI - http://www.ti.com/lit/an/sprabb3/sprabb3.pdf
Cons: Most expensive $$$.
Soldermask Relieved (Recommended)
Soldermask relieved vias have the soldermask pulled back from the drilled hole. Typically the mask open is drill + 5mils. Note finished plating in via barrel (ENIG).
Open vias allows for rinsing and cleaning of the vias to remove any trapped enchants.
Soldermask Relieved (Recommended)
Soldermask relieved vias have the soldermask pulled back from the drilled hole. Typically the mask open is drill + 5mils. Note finished plating in via barrel (ENIG).
Open vias allows for rinsing and cleaning of the vias to remove any trapped enchants.
Via Encroachment (Recommended)
Leave the via open and relief the soldermask at via barrels.
Pros: Low Cost, Good Heat Sink
Cons: Solder wicking, paste volume needs to be compensated. Solder bumps can appear on bottom side if paste volume is not properly compensated.
Fab Note:
VIA IN PAD: SOLDER MASK ENCROACHED VIAS (DRILL SIZE 8.1 MILS)
References:
Texas Instruments:
Eurocircuits Printed circuits blog
Advanced Circuits
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