China Topscom Delivered Article About The Solutions of through-hole blockage in the process of pcb boards manufacturing (fabrication) On the China Electronic Manufacturing Week Magzine.
 
the article is posted by Mr.Xiao Wei, Director of Topscom Pcb Layout Design Department,China Topscom provide professional Pcb boards layout design,pcb assembly and manufacturing,full turnkey systems integration box build assembly,contract electronic manufacturing service.
 
Keywords:china pcb boards industry,pcb via hole,pcb boards fabrication process.

 

Conductive hole Via hole, also known as conductive hole, in order to meet customer requirements, circuit board through hole must be plugged, after a lot of practice, change the traditional aluminum sheet plug hole technology, use white net to complete circuit board surface resistance welding and plug hole. Stable production and reliable quality.
 
Via hole pass hole plays the role of connecting lines with each other, and the development of the electronics industry not only promotes the development of PCB, but also puts forward higher requirements for PCB manufacturing technology and surface mount technology. The Via hole plug process arises at the historic moment, and the following requirements should be met at the same time:
(1) there is only copper in the through hole, and the resistance welding can be plugged or not;
(2) there must be tin and lead in the through hole, and there must be a certain thickness requirement (4 microns), and there shall be no soldering ink into the hole, resulting in tin beads in the hole;
(3) the through hole must have anti-soldering ink plug hole, opaque, no tin ring, tin bead and leveling requirements.
 
With the development of electronic products in the direction of "light, thin, short and small", PCB is also developing towards high density and high difficulty, so there are a large number of PCB, of SMT and BGA, and customers require plugs when mounting components, which mainly have five functions:
(1) prevent tin from passing through the component surface from the through hole during PCB wave soldering to cause a short circuit; especially when we put the through hole on the BGA pad, we must first do the plug hole, and then gold plating, so as to facilitate the welding of BGA.
(2) to avoid the residue of flux in the through hole;
(3) after the surface mounting of the electronics factory and the assembly of the components are completed, the PCB must suck a vacuum on the testing machine to form a negative pressure:
(4) to prevent the flow of surface solder paste into the hole to cause virtual welding and affect the mounting;
(5) to prevent tin beads from popping out during wave soldering, resulting in short circuit.
 
The realization of the conductive hole plug process for the surface mount board, especially for the BGA and IC mounting, the through hole plug hole must be flat, convex and concave positive and negative 1mi, there must be no red tin on the edge of the pass hole; the through hole contains tin beads, in order to meet the requirements of customers, the process of the through hole plug hole is varied, the process flow is very long, and the process control is difficult, and oil is often dropped during hot air leveling and green oil solder resistance experiments. Problems such as oil explosion occurred after curing. According to the actual production conditions, various plug hole processes of PCB are summarized, and some comparisons and expositions are made in the process, advantages and disadvantages.
Note: the working principle of hot air leveling is to use hot air to remove the excess solder from the surface and hole of the printed circuit board, and the remaining solder is uniformly covered on the solder pad and unhindered solder lines and surface packaging points, which is one of the ways of surface treatment of the printed circuit board.
 
1, hot air leveling after plug hole technology.
The technological process is as follows: plate surface resistance welding "HAL" plug hole curing. Using non-plug hole process for production, hot air leveling with aluminum screen or ink screen to complete the customer requirements of all fortresses through the plug hole. Plug hole ink can be used photosensitive ink or thermosetting ink, in the case of ensuring the same color of the wet film, plug hole ink is better to use the same ink as the board. This process can ensure that the guide hole does not lose oil after hot air leveling, but it is easy to cause the plug hole ink to contaminate the board surface and uneven. Customers are easy to cause virtual welding when mounting (especially in BGA). So many customers do not accept this method.
 
2, hot air leveling front plug hole technology.
2.1 pattern transfer after plugging hole, curing and grinding plate with aluminum sheet.
This technological process uses a numerical control drilling machine to drill out the aluminum sheet of the plug hole, make a screen plate, and carry on the plug hole to ensure that the plug hole is full, the plug hole ink plug hole ink, or thermosetting ink can also be used, which has the characteristics of high hardness, small change of resin shrinkage and good adhesion with the hole wall. The technological process is as follows: pre-treatment of "plug hole" grinding plate "pattern transfer"etching" plate surface resistance welding.
 
Using this method can ensure that the plug hole of the pass hole is flat, and there will be no quality problems such as oil explosion and oil dropping at the edge of the hole, but this process requires one-time thickening of copper, so that the copper thickness of the hole wall meets the customer's standard, so the requirement for copper plating on the whole plate is very high. and there are high requirements for the performance of the grinding machine to ensure that the resin on the copper surface is completely removed, the copper surface is clean and will not be contaminated. Many PCB plants do not have an one-time thickening copper process, and the performance of the equipment is not up to the requirements, resulting in little use of this process in PCB plants.
 
2.2 Direct screen printing plate surface resistance welding after plugging holes with aluminum sheet.
This technological process uses a numerical control drilling machine to drill out the aluminum sheet of the plug hole, make a screen plate, install the plug hole on the screen printing machine, stop for no more than 30 minutes after completing the plug hole, and use 36T screen to resist welding directly. The technological process is as follows: pre-treatment-plug hole-screen printing-pre-drying-exposure-development-curing.
This process can ensure that the cover oil of the pass hole is good, the plug hole is flat, and the color of the wet film is the same. After hot air leveling, it can ensure that there is no tin on the pass hole, and there are no tin beads in the hole, but it is easy to cause the solderability on the ink pad in the hole after curing, resulting in poor weldability; after hot air leveling, it is difficult to use this process to control the production, and it is necessary for process engineers to adopt special processes and parameters to ensure the quality of the plug hole.
 
2.3 Aluminum sheet plug hole, development, pre-curing, plate surface resistance welding after grinding.
Using a numerical control drilling machine, the aluminum sheet required for plug hole is drilled out and made into a screen plate, which is installed on a shift screen printing machine for plug hole. The plug hole must be full, both sides protruding is better, and then solidified, and the grinding plate is treated. The technological process is as follows: pre-treatment-plug hole pre-drying-development-pre-curing-plate surface resistance welding.
Because the use of plug hole curing in this process can ensure that the oil will not drop and burst through the hole after HAL, but after HAL, it is difficult to completely solve the tin beads in the through hole and tin on the through hole, so many customers do not accept it.
 
2.4 the resistance welding of the plate surface and the plug hole are completed at the same time.
In this method, a 36T (43T) screen is installed on the screen printing machine, and a cushion plate or nail bed is used to plug all the through holes while completing the board surface. the technological process is as follows: pre-treatment-screen printing-pre-drying-exposure-development-curing.
 
This process time is short, the utilization rate of the equipment is high, can ensure that the hot air leveling through the hole does not drop oil, the guide hole is not tin, but due to the use of screen printing to plug the hole, there is a large amount of air in the through hole memory, and during curing, the air expands and breaks through the welding resistance film. resulting in voids, uneven, hot air leveling will have a small number of pass holes to hide tin. At present, after a large number of experiments, our company chooses different types of ink and viscosity, adjusts the pressure of screen printing, basically solves the hole hole and unevenness, and has adopted this process for mass production.