PCB Tech - Why plug the PCB through the hole

As electronic products develop towards the direction of "light, thin, short and small", PCB also develops towards high-density and high difficulty, so a large number of PCB with SMT and BGA appear, and customers require plug holes when mounting components. After a lot of practice, change the traditional aluminum plug hole process, with white mesh to complete the PCB board surface resistance welding and plug hole. Stable production and reliable quality.

Via hole conduction hole plays a role in the inter-connection conduction of lines, the development of electronic industry, but also promote the development of PCB, but also put forward higher requirements for printed board production technology and surface mounting technology. Via hole plug hole process came into being, and should meet the following re

quirements:

(a) there is copper through the hole, welding resistance can plug but not plug;

(b) there must be tin lead through the hole, there is a certain thickness requirements (4 microns), no solder ink into the hole, resulting in tin beads hidden in the hole;

(c) through hole must have solder ink plug hole, opaque, no tin ring, tin bead and leveling requirements.

As electronic products develop towards the direction of "light, thin, short and small", PCB also develops towards high-density and high difficulty, so a large number of SMT and BGA PCBS appear, and customers require plug holes when mounting components, which mainly have five functions:

(1) To prevent the tin from the through hole through the component surface caused by short circuit during PCB wave soldering; Especially when we put the hole on the BGA pad, it is necessary to do the plug hole first, and then gold-plated to facilitate the welding of BGA;

(2) avoid flux residue in the conduction hole;

(3) After the surface mounting and component assembly in the electronics factory is completed, the PCB shall be vacuumized to form negative pressure on the testing machine;

(4) Prevent the solder paste from flowing into the hole to cause virtual welding and affect the mounting;

(5) to prevent over wave soldering tin beads pop up, resulting in short circuit.

Realization of conductive hole plug hole process

For surface mounting board, especially for BGA and IC mounting, the plug hole of the conduction hole must be smooth, convex and concave plus or minus 1mil, no red tin on the edge of the conduction hole; Conductive hole hidden tin beads, in order to meet the requirements of customers, the conductive hole plug hole process can be described as varied, the process is particularly long, the process control is difficult, often in hot air leveling and green oil solder resistance experiment oil. After curing, oil explosion and other problems occur. According to the actual production conditions, various PCB plug hole processes are summarized, and the process and advantages and disadvantages are compared and elaborated:

Note: The working principle of hot air leveling is to use hot air to remove excess solder on the surface and hole of printed circuit board, and the remaining solder is evenly covered on the pad and open solder line and surface seal decoration, which is one of the surface treatment methods of printed circuit board.

1. Plug hole process after hot air leveling

The process flow is: plate surface blocking welding -HAL- plug hole - curing. Non-plug hole process is adopted for production. After hot air leveling, aluminum screen plate or ink screen is used to complete the plug holes of all fortress as required by customers. Plug hole ink can be sensitive ink or thermosetting ink, in order to ensure the consistency of wet film color, plug hole ink is best used with the same ink board. This process can ensure that hot air leveling after the through hole does not drop oil, but easy to cause plug hole ink pollution board surface, uneven. The customer is prone to virtual welding when mounting (especially in BGA). So many customers don't accept this approach.

2. Hot air leveling before plug hole process

2.1 Graphic transfer is carried out after plug hole, solidification and grinding of aluminum sheet

This process process with CNC drilling machine, drilling out to plug hole aluminum sheet, made of screen, plug hole, ensure that the plug hole full plug hole, plug hole ink plug hole ink, also available thermosetting ink, its characteristics must be hardness, resin shrinkage change is small, and the hole wall binding force is good. The process flow is as follows: pretreatment - plug hole - grinding plate - graphic transfer - etching - plate surface resistance welding.

By this method can guarantee the smooth conduction of plug hole, hot air leveling there would be no oil, the side of hole blasting off the quality problems such as oil, but the process requirement of disposable thickening copper, make this hole wall copper thickness meet customer standard, so the whole board high demand for copper plating, and also has a high requirement on the performance of the grinding machine, to ensure that the resin on the surface of the copper thoroughly remove, such as copper surface is clean and not contaminated. Many PCB plants do not have the one-time thickening copper process, and the performance of the equipment is not up to the requirements, resulting in this process is not used in PCB plants.

2.2 Block welding of screen printing board surface directly after plug hole of aluminum sheet

This process process uses NUMERICAL control drilling machine, drill out the aluminum sheet to plug hole, made into a screen version, installed on the screen printing machine plug hole, after the completion of the plug hole parking shall not exceed 30 minutes, with 36T screen screen direct screen printing board surface resistance welding, the process process is: pretreatment - plug hole - screen printing - pre baking - exposure - development - curing. With this process can ensure that the conduction hole cover oil is good, plug hole flat, wet film color is consistent, hot air leveling can ensure that the conduction hole is not tin, tin beads are not hidden in the hole, but easy to cause the ink pad in the hole after curing, resulting in poor solderability; After hot air leveling, the edge of the through-hole bubbles and drops oil. It is difficult to use this process for production control, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of the plug hole.

2.3 Aluminum plug hole, development, precuring, grinding plate surface welding.

With CNC drilling machine, drilling the aluminum sheet required plug hole, made into a screen, installed on the shift screen printing machine plug hole, plug hole must be full, both sides of the protruding is better, and then after curing, grinding plate surface treatment, the process is: pretreatment - plug hole a pre-drying - developing - pre-curing - surface welding. Since plug hole curing in this process can ensure that oil does not drop or burst through the hole after HAL, but tin beads hidden in the hole and tin on the through hole after HAL cannot be completely solved, so many customers do not accept it.

2.4 Plate surface resistance welding and plug hole at the same time.

This method uses 36T (43T) screen, installed on the screen printing machine, the use of pad or nail bed, in the completion of the PCB board at the same time, all the through hole plug, the process is: pretreatment - screen printing - pre - drying - exposure - development - curing. The process time is short, high equipment utilization, can guarantee after hole out oil, hot air leveling guide hole is not on the tin, but because of using screen printing to plug hole, in the hole memory with a large number of air, when curing, air inflation, to break through resistance welding membrane, have holes, uneven, hot air leveling will guide hole is a small amount of tin.