Via Tenting - Plugging and Filling

VIA STRUCTURES, COVERING, PLUGGING, AND FILLING OPTIONS

Via hole plugging is a PCB manufacturing technique in which the via hole is filled with a solder mask or epoxy. This process partially or completely closes the via hole using conductive or non-conductive filling material. Filling via holes results in more reliable surface mounts, provides better assembly yields and improves the reliability of the PCB by decreasing the probability of trapped air or liquids in the via and thus the PCB Board.

TYPE I (TENTING)

The vias are covered with a dry film solder mask on one side (a) or both sides (b) – IPC-4761 Type I a/b.

Advantages	Disadvantages
Less expensive than the via plugging or filling options	Not a guaranteed process (uncontrolled), the surface across the via hole may or may not be covered (smaller hole size = better chance of coverage) Sealing from both sides is problematic: Air entrapment may lead to cracking. Chemical entrapment may lead to corrosion.

TYPE II (TENTING / COVERING)

The vias are covered with a dry film solder mask on one side (a) or both sides (b) and then printed over with a normal solder mask – IPC4761 Type II a/b.

Advantages	Disadvantages
Less expensive than the via plugging or filling options.	Type II is not supported anymore Not a guaranteed process Sealing from both sides is problematic: Air entrapment may lead to cracking Chemical entrapment may lead to corrosion

TYPE III (PLUGGING)

The vias are partially plugged with material (normally solder mask) on one (a) or both sides (b) – IPC4761 Type III a/b A 70% hole fill is accepted.

Advantages	Disadvantages
Less expensive than the via plugging or filling options.	Not a guaranteed process. Sealing from both sides is problematic: Air entrapment may lead to cracking. Chemical entrapment may lead to corrosion. Not suitable for a via in pad process. Could leave a void.

TYPE IV (PLUGGING)

The vias are partially plugged with material (normally solder mask) on one side (a) or both sides (b) and are printed over with a normal solder mask – IPC4761 Type IV a/b A 70% hole fill is accepted.

Advantages	Disadvantages
Less expensive than Type V, VI and VII.	Not a guaranteed process. Sealing from both sides is problematic: Air entrapment may lead to cracking Chemical entrapment may lead to corrosion Not suitable for a via in pad process

TYPE V (FULL VIA FILLING)

The via is filled with a non-conductive epoxy resin – IPC-4761 Type V.

Advantages	Disadvantages
90% to 100% resin filling. Co-planar surface. Prevents solder or any other contaminants from entering the via hole. Provides strength and structural support to active pads (Via in Pad process. Offers better stability and reliability of pad and via due to close matching of the CTE between the filling material and the surrounding laminate.	Higher cost than tenting or plugging.

TYPE VI (FULL VIA FILLING AND COVERING)

The via is filled with a non-conductive epoxy resin and is printed over with a normal solder mask – IPC-4761 Type VI a/b 90% – 100% hole fill is accepted.

Advantages	Disadvantages
90% to 100% resin filling. Co planar surface Prevents solder or any other contaminants from entering the via hole. Provides strength and structural support to active pads (Via in Pad process) Offers better stability and reliability of pad and via due to close matching of the CTE between the filling material and the surrounding laminate.	Higher cost than tenting or plugging.

TYPE VII (FULL VIA FILLING AND CAPPING)

The via is filled with a non-conductive epoxy resin and is plated over with copper on the top and bottom – IPC-4761 Type VII 90% – 100% hole fill is accepted.

Advantages	Disadvantages
90% to 100% resin filling. Co planar surface. Prevents solder or any other contaminants from entering the via hole. Provides strength and structural support to active pads (Via in Pad process). Offers better stability and reliability of pad and via due to close matching of the CTE between the filling	Higher cost than tenting or plugging.

VIA RESIN FILLING

Conductive Epoxy Resin Filling

For PCB designs that require the transfer of a high amount of heat or current from one side of the board to another, conductive via plugging is a handy solution. It can also be used to dissipate excessive heat generated underneath some components. The metallic nature of the fill will naturally wick heat away from the chip to the other side of the board in many ways like a radiator.

Advantages	Disadvantages
The heat sink or transfer where other conventional methods are impractical. Increased current carrying capacity.	High instability of copper pad and copper plating inside the via hole barrel. This occurs due to the difference in the CTE (coefficient of thermal expansion) value of conductive material and laminate surrounding it. When the PCB goes through thermal cycles, metal will heat and expand more rapidly than the surrounding laminate, which can cause a fracture between the pad and via hole barrel and lead to an open circuit. Thermal conductivity is not too high (compared to electroplated copper that has a thermal conductivity of more than 250W/mK) so it is possible to add a few more vias and avoid this process to more reliable non-conductive via plugging. More expensive than non-conductive via plugging. Not in high demand so minimal manufacturers can supply this type of filling.

Advantages

Disadvantages

The heat sink or transfer where other conventional methods are impractical.
Increased current carrying capacity.

High instability of copper pad and copper plating inside the via hole barrel. This occurs due to the difference in the CTE (coefficient of thermal expansion) value of conductive material and laminate surrounding it. When the PCB goes through thermal cycles, metal will heat and expand more rapidly than the surrounding laminate, which can cause a fracture between the pad and via hole barrel and lead to an open circuit.
Thermal conductivity is not too high (compared to electroplated copper that has a thermal conductivity of more than 250W/mK) so it is possible to add a few more vias and avoid this process to more reliable non-conductive via plugging.
More expensive than non-conductive via plugging.
Not in high demand so minimal manufacturers can supply this type of filling.

Non-Conductive Epoxy Resin Filling

This is the most common and popular method of via plugging, especially for a via In Pad process. The barrel of the via hole is filled with non-conductive material. The via will still be plated as normal before non-conductive material is plugged inside. It means via will work as normal as in any other standard PCB.

Advantages
Prevents solder or any other contaminants from entering the via hole. Provides strength and structural support to active pads (Via in Pad process). Offers better stability and reliability of pad and via due to close match of CTE between filling material and surrounding laminate when comparing the same with conductive material.

THE VIA HOLE FILLING PROCESS (EPOXY RESIN)

Conductive Epoxy Resin Filling

If conductive resin filling and copper plating is implemented on the PCB, then the PCB cost increment is significant. The reasons for this increase are the numerous additional processes that need to be applied to achieve the filled and plated vias.

Numerous processes need to take place to achieve via filling and plating (plugged and plated), these include:

Drilling of only the vias that need filling
Cleaning: plasma, brushing, deburring
Black Hole treatment (preparation for plating)
Apply dry resist (photo imageable)
Imaging of ONLY the via holes (UV exposure)
Via hole galvanization (PTH)
Striping of dry resist
Brushing if needed
Baking to ensure that the via holes are completely dry
Via plugging with resin
Baking to cure the resin
Brushing (surface cleaning)
Copper plating over via

As can be seen, the process itself is extensive with the addition of approximately 8 steps during manufacture.

Via Tenting – Plugging and Filling