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PCB conductive hole plug hole process and the reasons for a detailed explanation

Jan 02, 2018

Conduit hole Via hole aka vias, in order to meet customer requirements, vias must plug hole, after a lot of practice, change the traditional aluminum plug hole process, with white mesh board to complete the solder mask and plug hole. Stable production, reliable quality.


Via hole vias play the role of the interconnection of circuits, the development of the electronics industry, but also to promote the development of PCB, PCB manufacturing technology and surface mount technology put forward higher requirements. Via hole plug hole process came into being, at the same time should meet the following requirements:


(A) copper in the through-hole can be, solder paste plug may not plug;

(B) of the lead-through hole must have tin and lead, a certain thickness requirements (4 microns), no solder mask ink into the hole, resulting in hole tin beads;

(C) lead-through hole must be solder-proof ink hole, opaque, tin ring, tin beads and leveling and other requirements.


With the development of electronic products towards "light, thin, short and small", PCBs have also been developed at a high density and difficulty. As a result, a large number of PCBs are printed on SMT and BGA boards. Clients are required to plug the printed circuit boards Five functions:


(A) to prevent the PCB from wave soldering tin from the through-hole through the component surface caused by a short circuit; especially when we place the hole on the BGA pad, you must first plug hole, and then gold-plated, easy BGA welding.


(B) to avoid flux residue in the vias;

(C) electronics factory surface mount and PCB assembly components in the test after the completion of vacuum suction to be completed:

(D) to prevent the surface of the solder paste into the holes caused by Weld, affecting placement;

(E) to prevent solder wave pop-up wave soldering, resulting in short circuit.

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Conductive hole plug hole technology



For surface mount boards, especially for BGA and IC placement requirements of the via hole plug must be flat, plus or minus 1mil bump, there shall be no red lead on the edge of the via hole tin; pass through the possession of solder beads, in order to reach customers Requirements, through-hole plug hole process can be described as varied, particularly long process, the process control is difficult, often in the hot air leveling and green oil-resistant solder experiments when the oil; oil burst and other issues occur. Now based on the actual conditions of production, various PCB plugging process summarized in the process and advantages and disadvantages for some comparison and elaboration:


Note: The principle of hot air leveling is the use of hot air to remove excess solder on the surface of the printed circuit board and the hole, the remaining solder evenly over the pads and unimpeded solder lines and surface package points, the printed circuit board surface treatment one.




1

Hot air leveling after hole technology



This process is: plate surface welding → HAL → plug → curing. Using non-plug hole process for production, hot air leveling with aluminum mesh or block ink network to complete the customer requirements of all the fortress hole plug. Plug hole ink available photosensitive ink or thermosetting ink, wet film in ensuring the same color, the best plug hole ink and board the same ink. This process can ensure the hot air leveling vias lead-time, but easy to cause plug hole ink pollution board surface, uneven. Customers easily lead to Weld when mounted (especially BGA). So many customers do not accept this method.


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2

Hot air leveling before plug hole technology




2.1 plug holes with aluminum, curing, grinding plate after the transfer of graphics



This process with a CNC drilling machine, drilled brisk hole aluminum sheet, made of screen, plug hole, to ensure full plug hole, plug hole ink plug hole ink, thermosetting ink can also be used, the characteristics must be large hardness , Small changes in resin shrinkage, good adhesion with the hole wall. Process is: pre-treatment → plug hole → mill board → graphics transfer → etching → plate surface solder mask.


This method can ensure that the hole of the via hole is smooth, there is no oil burst when the hot air is leveled off, and the quality of the oil is lost during the hole. However, this process requires one-time thickening of copper so that the copper thickness of the hole wall meets the customer's standard. Therefore, the entire board of copper plating is very demanding, and the performance of the mill plate also has high requirements to ensure that the copper surface completely removed resin, copper surface clean, not contaminated. Many PCB factories do not have a one-time thickened copper process, and the performance of the equipment failed to meet the requirements, resulting in the use of this technology in the PCB plant is not much.

The company is located in:


2.2 After the hole with aluminum sheet directly screen printing surface solder mask



This process with a CNC drilling machine, drilling the plug hole aluminum sheet, made of screen, installed in the screen printing machine plug hole, after the completion of the plug hole can not be more than 30 minutes, with 36T screen direct screen printing surface solder mask, Process: pre-treatment - plug - silk screen - pre-drying - a development of exposure - curing.


This process can be guaranteed through hole cover oil is good, the hole is smooth, the wet film color is consistent, the hot air leveling can guarantee that the through hole is not on the tin, the hole does not hide tin beads, but easily lead to ink on the hole after curing Pad, resulting in poor solderability; hot air leveling vandalism at the edge of the hole foaming oil, using this method of production and control more difficult to process engineering staff to use special processes and parameters to ensure plug quality.




2.3 Aluminum plug hole, development, pre-curing, wear plate after the surface solder mask.



With CNC drilling machine, drill out the required aluminum hole plug, made of screen, installed in the shift screen printing machine plug hole, the plug hole must be full, both sides of the prominent, and then cured, polished plate surface treatment, the plate Process: pre-treatment - plug a pre-bake - Development - pre-curing - surface solder mask.


Because this process uses plug hole curing to ensure HAL after the hole can not fall off the oil, oil explosion, but after HAL, through the possession of tin beads and vias on tin is difficult to completely solve, so many customers do not receive.





2.4 board solder mask and plug at the same time completed.



This method uses 36T (43T) screen, installed in the screen printing machine, the use of pad or nail bed, in the completion of the board at the same time, all through-hole plug, the process is: Pretreatment - Screen Printing - - Pre-bake - Exposure - Development - Curing.


This process time is short, high utilization of equipment, to ensure that the hot air leveling after the hole can not afford to oil, the lead is not on tin, but due to the use of silk screen plug hole in the via hole with a large amount of air in the curing , The expansion of the air, breaking the solder mask, resulting in holes, uneven, hot air leveling will have a small amount of vias hidden tin. At present, our company through a large number of experiments, choose different types of ink and viscosity, adjust the pressure of silk screen, etc., basically solved the voids and uneven vias, this process has been used in mass production.



Plug hole process on the PCB requirements



project

parameter

Remarks

Thickness requirements

> 1.0mm


Plug oil hole diameter

<0.6mm

Mainly by the ink performance. When the plug hole diameter of 0.6-0.8mm should allow a small amount of holes.

Plug the hole to open the window

For the plug hole requires double-sided tenting

For one-sided fenestration of the hole can not guarantee 100% full (not crack).


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