Resin Plugging and Via in Pad: Process and Risks

When a ball grid array is dense enough that the escapes cannot fit between the pads, the via has to move into the pad. That decision fixes the routing but changes the fabrication process, because an open hole under a solder joint will draw the paste away from the joint during reflow.

Why the Via Moves Into the Pad

A conventional fanout places a via between the balls and routes to another layer from there, which costs one channel of routing between adjacent pads. On a fine pitch device with a large ball count, that channel is exactly what the design does not have.

Drilling in the pad and dropping straight to an inner layer frees the channel. The routing works, but the hole now sits under a joint, and unless it is filled and covered with copper, the solder has somewhere to go other than the pad.

What Happens Without Plugging

Solder paste printed over an open via wicks into the barrel during reflow, so the joint is starved and the surface of the pad becomes a crater. On the front side the result is an open or a weak joint, and on the back side the alloy can appear where it was never intended to be.

The defect is often intermittent rather than obvious. A joint with half the solder it needs will pass a continuity test and fail later, which is why the process is controlled rather than left to the assembly house to manage.

The Resin Plugging Sequence

The plugged vias are processed before the rest of the drilling. The holes are drilled, plated so that the barrel is conductive, filled with a resin paste and baked, and then ground back until the surface is flat and the resin is level with the surrounding copper.

Because the cured resin contains no copper, a further plating step covers the filled area and turns it back into a pad. Only after that is the panel processed in the normal way, with the remaining holes drilled and the outer layers patterned.

Planarity and Surface Flatness

The quality of the pad after plugging depends on planarity. If the resin is left proud of the copper, the ball sits on a bump; if it is ground below the surface, the pad becomes a shallow dish. Either case changes the volume of paste that ends up in the joint.

Grinding has to remove the excess without thinning the copper around the pad, which is one of the reasons the process is specified by the fabricator rather than by the designer. The tolerance is expressed as a flatness across the plugged area, not as a simple depth.

Bubbles and Moisture

A plug that contains a bubble is a long term risk. Air expands and contracts with temperature, and the bubble collects moisture during storage and during the assembly process, so a panel that is not baked before reflow can split at that point.

Bubbles also distort the plug as it cures. The expanding air pushes the resin out of the hole, leaving one side low and the other high, which is visible on the pad and is a reason to reject the panel rather than to continue.

What Air Does at Reflow

The main cause of a split in service is moisture rather than air alone. Resin, laminate and the surface finish all absorb water from the atmosphere, and when the board reaches reflow temperature that water turns to steam and expands faster than the material can relax.

A board that has been baked before assembly, or one that has just left the fabricator with no time to absorb moisture, is far less likely to split. Baking is therefore part of the assembly procedure for any board with plugged or filled vias, not an optional step.

Screening the Panels

Inspection before shipment is what separates a controlled process from a hopeful one. A filled via can be checked by cross section on a coupon, and the surface of a plugged pad can be examined optically for depressions, discolouration or resin left above the copper.

X-ray inspection of the assembled joint complements the panel check, because it shows whether the paste stayed on the pad. Together the two measurements tell the story: the fabricator confirms the plug, and the assembly house confirms the joint.

Design Rules for Via in Pad

The hole should be as small as the drill allows, because a smaller barrel is easier to fill and easier to plate over. A tented or covered via that is not plugged is acceptable for a non critical net, but not under a joint that has to carry current.

The pad should be large enough to leave a ring of copper around the filled hole, and the paste aperture should be adjusted for the fact that some paste will be lost into any residual depression. The design rules for via in pad and for a plated through via are not the same, and treating them as one is a common mistake.

Cost and Where It Is Justified

Resin plugging adds process steps, so it adds cost and lead time. It is justified where the routing density cannot be solved another way, where the thermal path needs the via under the pad, and where the joint reliability of a fine pitch device depends on the pad being flat.

Where the density allows, a conventional fanout with a via beside the pad is cheaper and easier to inspect. The cost of plugging should be compared against the cost of an extra pair of layers or a larger board before the choice is made.

Alternatives to Consider

A microvia formed by laser is one alternative, because the hole is small enough to be filled during plating. A blind or buried via that ends below the surface is another, since the pad remains solid copper and no filling is needed.

The choice among these options depends on the stackup and on the number of transitions the signal can tolerate. The plating approach is described under via filling for HDI, and void control at the joint is covered under solder void prevention.

Plugging Tolerance and Verification

The fabricator controls the plug by the depth of the grind and by the flatness of the finished pad, and both are measured on a coupon rather than on the product. A sample is cross sectioned to show the resin fill, the plating over it and the copper thickness at the pad.

Where a panel is intended for a fine pitch device, the flatness across a group of pads matters more than the value at any single pad, because the paste volume is shared by the stencil opening. A group that is level overall will print consistently even if one pad is slightly low.

FAQ

Can an open via in a pad be used if the paste volume is increased? No. The paste still wicks into the barrel. The hole has to be filled, or the joint has to be moved off the pad.

Does a plugged via need baking before assembly? Yes. The filled resin and the laminate absorb moisture, and baking before reflow is the control that prevents a split at the plug.

How is a plugged via verified? By cross section on a coupon from the same panel, and by optical inspection of the pad surface for depressions or resin left above the copper.