PCB Plugging: What It Is, Why It Matters, and How It’s Done

The via hole is used to connect the circuits to each other. The development of the electronics industry has also promoted the development of PCBs, and also put forward higher requirements for the production process of printed circuit boards and surface mounting technology. The via hole plugging process came into being, and it should meet the following requirements:

(i) It is sufficient for the via hole to contain copper, and solder mask may or may not be used. (ii) There must be tin-lead in the via hole, and a certain thickness requirement (4 microns) is required. Solder mask ink must not enter the hole, causing tin beads to be hidden in the hole. (iii) The via hole must be plugged with solder mask ink, must not be light-transmissive, and must not contain tin rings, tin beads, or other requirements for flatness.

As electronic products develop towards “light, thin, short and small”, PCBs also develop towards high density and high difficulty. Therefore, a large number of SMT and BGA PCBs appear. When customers place components, they require plugging holes, which has five main functions:

(1) Prevent tin from penetrating the component surface from the via hole during wave soldering of the PCB and causing a short circuit; especially when we put the via on the BGA pad, we must first plug the hole and then gold-plate it to facilitate BGA soldering.

(ii) Avoid flux residue in the vias;

(iii) After the surface mounting and component assembly of the electronics factory is completed, the PCB must be vacuumed to form a negative pressure on the test machine before it is completed: (iv) Prevent the surface solder paste from flowing into the hole and causing cold soldering, affecting the mounting; (v) Prevent the solder balls from popping out during wave soldering and causing a short circuit.

Plug holes are divided into resin plug holes and electroplating plug holes.

Resin plugging: Using solvent-free ink to plug holes can not only make up for the problem that ordinary inks are difficult to plug, but also reduce the “cracks” caused by the ink being heated. It is generally used for holes with larger aspect ratios.

Benefits of resin plugging:

1. The use of resin plugging for via plugging on multi-layer BGA can reduce the distance between holes and solve the problem of wires and wiring;
2. The buried hole of the inner layer HDI can balance the contradiction between the thickness control of the laminated dielectric layer and the design of the inner layer HDI buried hole filling;
3. Through holes with thicker board thickness can improve product reliability;
4. The process of using resin to plug holes in PCB is often because of BGA parts. Because traditional BGA may make VIA between PAD and PAD to route to the back, but if the BGA is too dense and the VIA cannot go out, you can drill holes directly from the PAD to make vias to other layers for routing, and then fill the holes with resin and plate copper to make PAD, which is commonly known as the VIP process (via in pad). If you only make vias on the PAD without using resin to plug the holes, it is easy to cause tin leakage, resulting in short circuits on the back and empty solder joints on the front.

The process of PCB resin plugging includes drilling, electroplating, plugging, baking, and grinding. After drilling, the hole is plated through, then plugged with resin and baked, and finally ground flat. Because the resin after grinding does not contain copper, it is necessary to plate another layer of copper to turn it into a PAD. These processes are done before the original PCB drilling process, that is, the holes to be plugged are processed first, and then other holes are drilled according to the original normal process.

If the hole is not plugged properly and there are bubbles in the hole, the board may burst when it passes through the tin furnace because the bubbles easily absorb moisture. However, if there are bubbles in the hole during the plugging process, the bubbles will squeeze out the resin during baking, causing one side to be concave and the other side to be convex. At this time, defective products can be detected, and a board with bubbles will not necessarily burst, because the main cause of bursting is moisture. Therefore, if the board has just left the factory or has been baked when it is loaded, it generally will not cause bursting.

Electroplating hole filling: Currently, the characteristics of additives are used to control the growth rate of copper in each part to fill the hole. It is mainly used in continuous multi-layer stacking hole production (blind hole process) or high current design.

Advantages of electroplating hole filling:

1. It is conducive to the design of overlapping holes and holes on the plate;

2. Improve electrical performance and help high-frequency design;

3. Helps to dissipate heat;

4. Plug holes and electrical interconnection are completed in one step;

5. Filling the blind hole with electroplated copper has higher reliability and better conductivity than conductive glue.

So, how is the PCB circuit board via plugging process implemented?

1. Hot air leveling and plugging process

The process flow is: board surface solder mask → HAL → plugging → curing. The non-plugging process is used for production. After hot air leveling, an aluminum screen or ink blocking screen is used to complete the plugging of all the vias. The plugging ink can be photosensitive ink or thermosetting ink. This process flow can ensure that the vias do not drop oil after hot air leveling, but it is easy to cause the plugging ink to contaminate the board surface and make it uneven, which is easy to cause cold solder joints during mounting.

2. Hot air leveling before plugging process

1.Use aluminum sheet to plug holes, solidify, grind the board and then transfer the pattern

This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen, and plug the hole; the plugging ink can also be thermosetting ink, which has high hardness and good bonding with the hole wall. The process flow is: pre-treatment → plugging → grinding → pattern transfer → etching → board surface solder mask

This method can ensure that the vias are plugged and leveled with hot air, and there will be no quality problems such as oil explosion and oil loss at the hole edges. However, this process requires one-time thickening of the copper, so it has very high requirements for copper plating of the entire board.

2. Use aluminum sheet to plug the hole and then directly print the solder mask on the board

This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen, and install it on a screen printer to plug the holes. After the plugging is completed, it should be parked for no more than 30 minutes, and the solder mask is directly printed on the board surface with a 36T screen. The process flow is: pretreatment – plugging – screen printing – pre-baking – exposure – development – curing.

This process can ensure that the via cover is well-oiled and the via plug is flat. After hot air leveling, it can ensure that the via is not tinned and there are no tin beads in the hole. However, it is easy to cause the ink in the hole to reach the pad after curing, resulting in poor solderability.

3. Aluminum sheet plugging, development, pre-curing, grinding and then solder masking on the board

Use a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen, and install it on a shift screen printer for plugging; the plugged holes must be full and protruding on both sides, and then after curing, the plate is ground for board surface treatment. The process flow is: pre-treatment – plugging – pre-baking – development – pre-curing – board surface solder mask.

This process uses plug curing to ensure that the vias do not lose oil or explode after HAL. However, after HAL, it is difficult to completely solve the problems of tin beads in vias and tin on conductive holes.

4. Solder mask and plugging are completed at the same time

This method uses a 36T (43T) screen, installed on a screen printer, and uses a pad or nail bed to plug all the vias while completing the board surface. The process flow is: pre-treatment – screen printing – pre-baking – exposure – development – curing.

This process is time-saving and has high equipment utilization. It can ensure that the vias do not lose oil and the through holes do not get tinned after hot air leveling. However, since silk screen printing is used for plugging the holes, a large amount of air is stored in the vias. During curing, the air expands and breaks through the solder mask, causing voids and unevenness.