PCB News - Detailed explanation of printed circuit board via clogging solution

Printed circuit boards via hole must be plugged. After a lot of practice, the traditional aluminum plug hole process has been changed. Via hole plays the role of interconnection and conduction of lines. The development of the electronics industry also promotes the development of printed circuit boards, and also puts forward higher requirements on the PCB manufacturing process and surface mount technology. Via hole plugging technology came into being, and should meet the following requirements at the same time:
1. There is copper in the via hole, and the solder mask can be plugged or not plugged;
2. There must be tin and lead in the via hole, with a certain thickness requirement (4 microns), and no solder mask ink should enter the hole, causing tin beads to be hidden in the hole;
3. The through holes must have solder mask ink plug holes, opaque, and must not have tin rings, tin beads, and flatness requirements. With the development of electronic products in the direction of "light, thin, short, and small", printed circuit boards have also developed to high density and high difficulty. Therefore, a large number of SMT and BGA printed circuit boards have appeared, and customers require plugging when mounting components, mainly Five functions:

(1) Prevent short circuit caused by tin passing through the component surface from the via hole when the printed circuit board is wave soldered; especially when we put the via hole on the BGA pad, we must first make the plug hole and then gold-plated to facilitate the BGA soldering.
(2) Avoid flux residue in the via hole;
(3) After the surface mounting of the electronics factory and the assembly of the components are completed, the printed circuit board must be vacuumed to form a negative pressure on the testing machine to complete:
(4) Prevent surface solder paste from flowing into the hole, causing false soldering and affecting placement;
(5) Prevent the tin balls from popping up during wave soldering, causing short circuits.

Realization of Conductive Hole Plugging Process
For surface mount boards, especially the mounting of BGA and IC, the via hole plug must be flat, convex and concave plus or minus 1mil, and there must be no red tin on the edge of the via hole; the via hole hides the tin ball, in order to reach the customer The process of plugging via holes can be described as diverse. The process flow is particularly long and the process is difficult to control. The oil is often dropped during the hot air leveling and the green oil solder resistance test; problems such as oil explosion after solidification occur. Now according to the actual conditions of production, we summarize the various plugging processes of printed circuit board, and make some comparisons and explanations in the process and advantages and disadvantages: The working principle of hot air leveling is to use hot air to remove excess solder on the printed circuit board surface and holes, and the remaining Solder uniformly covers the pads, non-resistive solder lines and surface packaging points, which is one of the surface treatment methods of printed circuit boards.

1. Hole plugging process after hot air leveling
The process flow is: board surface solder mask-HAL-plug hole-curing. The non-plugging process is adopted for production. After the hot air is leveled, the aluminum sheet screen or the ink blocking screen is used to complete the via hole plugging required by the customer for all the fortresses. The plugging ink can be photosensitive ink or thermosetting ink. Under the condition that the color of the wet film is consistent, the plugging ink uses the same ink as the board surface. This process can ensure that the through holes will not lose oil after the hot air is leveled, but it is easy to cause the plug hole ink to contaminate the board surface and uneven. Customers are prone to false soldering (especially in BGA) during mounting. So many customers do not accept this method.

2. Hot air leveling and plug hole technology
2.1 Use aluminum sheet to plug the hole, solidify, and polish the board to transfer the graphics
This technological process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged to make a screen, and plug the hole to ensure that the via hole is full. The plug hole ink can also be used with thermosetting ink, and its characteristics must be high in hardness., The shrinkage of the resin is small, and the bonding force with the hole wall is good. The process flow is: pretreatment - plug hole - grinding plate - pattern transfer - etching - surface solder mask. This method can ensure that the via hole plug hole is flat, and hot air leveling will not cause quality problems such as oil explosion and oil drop on the edge of the hole. However, this process requires thickening copper to make the copper thickness of the hole wall meet the customer's standard. The requirements for copper plating on the entire plate are very high, and there are also high requirements for the performance of the plate grinding machine, to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and not polluted. Many printed circuit board factories do not have a thick copper process, and the performance of the equipment does not meet the requirements, resulting in not much use of this process in printed circuit board factories.

2.2 After plugging the hole with aluminum sheet, directly screen-print the board surface solder mask
This process uses a CNC drilling machine to drill out the aluminum sheet that needs to be plugged to make a screen, install it on the screen printing machine to plug the hole, and park it for no more than 30 minutes after completing the plugging. Use 36T screen to directly screen the surface of the board. The process flow is: pretreatment-plug hole-silk screen-prebaking-exposure-development-curing. This process can ensure that the via hole is well covered with oil, the plug hole is flat, and the wet film color is consistent. After the hot air is leveled, it can ensure that the via hole is not tinned and the tin bead is not hidden in the hole, but it is easy to cause the ink in the hole after curing. The pads cause poor solderability; after the hot air is leveled, the edges of the vias bubbling and oil is removed. It is difficult to use this process to control production, and it is necessary for process engineers to use special processes and parameters to ensure the quality of the plug holes.

2.3 The aluminum sheet is plugged into holes, developed, precured, and polished, and then solder resist is performed on the surface of the board.
Use a CNC drilling machine to drill out the aluminum sheet that requires plugging holes to make a screen, install it on the shift screen printing machine for plugging holes. The plugging holes must be full and protruding on both sides. The process flow is: pretreatment-plug hole-prebaking-development-precuring-board surface solder mask. Because this process uses plug hole curing to ensure that the through hole after HAL does not drop or explode, but after HAL, It is difficult to completely solve the problem of tin beads hidden in via holes and tin on via holes, so many customers do not accept them.

2.4 The board surface solder mask and plug hole are completed at the same time.
This method uses a 36T (43T) screen, installed on the screen printing machine, using a backing plate or a bed of nails, and when completing the board surface, all the through holes are plugged. The process flow is: pretreatment-silk screen-Prebaking-exposure-development-curing. The process time is short and the equipment utilization rate is high. It can ensure that the via holes will not lose oil after the hot air leveling, and the via holes will not be tinned. However, due to the use of silk screen for plugging the holes, there is a large amount of air in the via holes., The air expands and breaks through the solder mask, resulting in cavities and unevenness. There will be a small amount of through holes hidden in the hot air leveling in printed circuit boards.