PCB News - The secret of PCB via clogging?

The secret of PCB via clogging?

The secret of PCB via clogging? Let me take you to find out, when you use the PCB circuit board, will you encounter problems caused by the blockage of the PCB circuit board via holes? Have you tried to solve it for many times to no avail! Then the editor will go with you to explore the secrets of PCB via clogging! There are also related solutions for

Via hole is also called via hole. In order to meet customer requirements, the via holes must be plugged in the PCB process. Through practice, it is found that in the process of plugging,

If the traditional aluminum sheet plugging process is changed, and the white mesh is used to complete the board surface solder mask and plugging, the PCB production can be stable and the quality is reliable. The development of the electronics industry while promoting

The development of PCB also puts forward higher requirements for printed board manufacturing process and surface mount technology. Via hole plugging technology came into being, and the following requirements should be met 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-lead in the through 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", PCBs have also developed to high density and high difficulty. Therefore, a large number of SMT and BGA PCBs have appeared, and customers require plugging when mounting components, mainly including Five functions:

(1) Prevent the tin from passing through the component surface through the through hole to cause a short circuit when the PCB is wave soldered; especially when the via is placed on the BGA pad, the plug hole must be made first, and then gold-plated, which is convenient for BGA soldering;

(2) Avoid flux residue in the vias;

(3) After the surface mounting of the electronics factory and the assembly of the components are completed, the PCB must be vacuumed on the testing machine to form a negative pressure to complete;

(4) Prevent surface solder paste from flowing into the hole, causing false soldering and affecting placement;

(5) Prevent the tin beads 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 holes must be flat, convex and concave plus or minus 1 mil, and there must be no red tin on the edge of the via hole. Since the via hole plugging process can be described as diverse, the process flow is particularly long, and the process control is difficult. There are often problems such as oil drop during hot air leveling and green oil solder resistance experiments, and oil explosion after curing. According to the actual conditions of production, the various plugging processes of PCB are summarized, and some comparisons and explanations are made in the process and advantages and disadvantages.

Note: The working principle of hot air leveling is to use hot air to remove excess solder from the surface and holes of the printed circuit board, and the remaining solder is evenly coated on the pads, non-resistive solder lines and surface packaging points, which is the surface treatment method of the printed circuit board one.

1. Hole plugging process after hot air leveling

The process flow is: board surface solder mask-HAL-plug hole-curing. Non-plugging process is adopted for production. After hot air leveling, aluminum sheet screen or ink screen is used to complete all the via hole plugging required by customers. The plug hole ink can be photosensitive ink or thermosetting ink. In the case of ensuring the same color of the wet film, the plug hole ink is best to use 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 plugging 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 holes, solidify, and grind the board to transfer graphics. This process is

Use a CNC drilling machine to drill out the aluminum sheet that needs to be plugged, make a screen, and plug the holes to ensure that the via hole is full and the plug hole ink plugs the hole ink. In addition, thermosetting ink can also be used, but 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: pre-treatment - plug hole - grinding plate - pattern transfer - etching - board surface solder mask.

This method can ensure that the plug hole of the via hole is flat, and there will be no quality problems such as oil explosion and oil drop on the edge of the hole when leveling with hot air. However, this process requires one-time thickening of copper to make the copper thickness of the hole wall meet the customer's standard. Therefore, the requirements for copper plating on the entire plate are very high, and the performance of the plate grinding machine is also very high. It is necessary to ensure that the resin on the copper surface is completely removed, and the copper surface is clean and not contaminated. Many PCB factories do not have a one-time thickening copper process, and the performance of the equipment does not meet the requirements, resulting in not much use of this process in PCB 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 the aluminum sheet that needs to be plugged to make a screen, install it on the screen printing machine for plugging, and park it for no more than 30 minutes after completing the plugging, and use 36T screen to directly screen the surface of the board. The process flow is: pre-treatment-plug hole-silk screen-pre-baking-exposure-development-curing

This process can ensure that the through hole is well covered with oil, the plug hole is flat, and the color of the wet film is consistent. Hot air leveling can ensure that the through holes are not tinned and no tin beads are hidden in the holes, but it is easy to cause the ink in the hole to be on the pad after curing, resulting in poor solderability. After the hot air leveling, the edge of the through hole will bubble and remove oil. . It is difficult to use this process method to control the production, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of plug holes.

2.3 The aluminum sheet is plugged into the hole, developed, pre-cured, and then soldered 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: pre-treatment-plug hole-pre-baking-development-pre-curing-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 pad or a bed of nails, and when completing the board surface, all the vias are plugged. The process flow is: pretreatment-screen printing- -Pre-baking--exposure--development--curing. The process time is short, and the utilization rate of the equipment is high. It can ensure that the through holes will not lose oil and the through holes will not be tinned after the hot air is leveled, but because the silk screen is used for plugging, There is a large amount of air in the via hole. During curing, the air expands and breaks through the solder mask, causing cavities and unevenness. There will be a small amount of tin through holes for hot air leveling.

The above is the secret of PCB via clogging that I explored with you today, and there are related solutions, so have you learned it?