1. With the development of electronic products towards “thin, light, and short” direction, PCBs (printed circuit boards) are also developing towards high density and high difficulty. Therefore, a large number of PCBs using SMT (surface mount technology) and BGA (ball grid array) technology have emerged, and customers require the use of through holes when installing components. After extensive practice, the traditional aluminum through-hole process has been improved to use white grids to complete PCB surface solder mask and through-hole treatment, achieving stable production and reliable quality.

2. Through holes play an important role in the interconnection and conductivity of circuits. The development of the electronics industry not only promotes the development of PCB technology, but also puts forward higher requirements for the production process and surface mount technology of printed circuit boards. Therefore, the through-hole filling process has emerged and must meet the following requirements:

(a) The through-hole with copper layer should be able to meet the requirements of welding resistance, but there should be no blockage phenomenon;

(b) Tin lead alloy must be coated in the through-hole with a certain thickness requirement (4 microns), and no solder ink should enter the hole to avoid the formation of hidden tin beads;

(c) The through-hole must be filled with solder ink, and after filling, it should be opaque, with solder rings and beads, and meet the flatness requirements.

1. As electronic products evolve towards “light, thin, short, and small,” PCBs are also advancing to higher density and complexity. This has led to the prevalence of SMT and BGA PCBs, where customers demand plug holes for mounting components, which serve five main functions:

2. To prevent solder from bridging through the component surface, which could cause short circuits during PCB wave soldering; particularly important when placing holes on BGA pads, where plug holes must be created before gold-plating to facilitate BGA soldering.

3. To avoid flux residue in the conductive holes.

4. After surface mounting and component assembly, the PCB should be vacuumized to create negative pressure on the testing machine.

5. To prevent solder paste from flowing into the holes, causing false soldering and impacting the mounting.

6. To prevent solder beads from surfacing during wave soldering, which could lead to short circuits.

7. Conductive hole plug hole process realization:

8. For surface-mount boards, especially with BGA and IC mounting, conductive hole plug holes must be smooth, with convex and concave deviations of ±1 mil, and no red solder on the edge of the conductive hole. Hidden solder beads in conductive holes require varied processes to meet customer requirements. The process is lengthy, difficult to control, and often leads to issues like oil explosions after curing. Here, we summarize various PCB plug hole processes, comparing their methods, advantages, and disadvantages:

9. Note: The hot air leveling principle involves using hot air to remove excess solder from the surface and holes of the PCB, ensuring that the remaining solder evenly covers the pad and open solder lines. It is one of the surface treatment methods for printed circuit boards.

10. Plug hole process after hot air leveling:

11. The process flow is: surface blocking – HAL – plug hole – curing. The non-plug hole process is used for production. After hot air leveling, an aluminum screen or ink screen is used to complete the plug holes as required by customers. The plug hole ink can be sensitive or thermosetting ink, with the same ink board preferred to ensure consistency. This process prevents oil drop after hot air leveling but may cause ink contamination and unevenness, leading to virtual soldering issues, particularly with BGA. Many customers reject this method.

12. Hot air leveling before plug hole process:

13. Graphic transfer is carried out after plug hole, curing, and grinding of aluminum sheet:

14. This process uses a CNC drilling machine to drill the aluminum sheet for plug holes and screen creation. It ensures full plug hole coverage with either sensitive or thermosetting ink. The process flow is: pretreatment – plug hole – grinding – graphic transfer – etching – surface resistance welding. This method guarantees smooth conductive holes without oil post-hot air leveling but requires thickening copper and high equipment performance, which many PCB plants lack, making it less commonly used.

15. Block welding of screen printing board surface directly after aluminum sheet plug hole:

16. This process uses a CNC drilling machine to create plug holes in the aluminum sheet, made into a screen, and then installed on the screen printing machine for plug hole application. After completion, the plug holes should not be left for more than 30 minutes. The process flow is: pretreatment – plug hole – screen printing – pre-baking – exposure – development – curing. This method ensures good coverage of conductive holes, consistent wet film color, and prevents hidden solder beads. However, it can result in poor solderability due to ink pad issues and potential solder bead formation post-hot air leveling.

17. Aluminum plug hole, development, pre-curing, grinding plate surface welding:

18. Using a CNC drilling machine, drill the aluminum sheet for plug holes, create a screen, and install it on the screen printing machine for plug hole application. After curing, grinding treatment is performed. The process flow is: pretreatment – plug hole – pre-drying – developing – pre-curing – surface welding. This process prevents oil drops and bursts through holes post-HAL but may not fully address hidden solder beads, making it unacceptable to some customers.

19. Plate surface resistance welding and plug hole simultaneously:

20. This method uses a 36T (43T) screen, installed on the screen printing machine, with a pad or nail bed. It completes all through-hole plug holes during PCB processing. The process flow is: pretreatment – screen printing – pre-drying – exposure – development – curing. It is efficient and ensures oil-free holes after hot air leveling. However, screen printing can trap air in the holes, leading to uneven curing and small amounts of tin after hot air leveling, which poses challenges for production control and quality assurance.