Via holes play a role in contributing to electrical connection between layers in PCB (printed circuit board). Solder mask production for via holes didn't need to be especially emphasized. Along with the development trend towards light weight, thinness and miniaturization of electronic products, however, PCB has started moving towards increasingly higher density. Furthermore, the rapid progress of packaging technologies like SMT (surface mount technology), BGA (ball grid array), QFP (quad flat package) etc. has led customers to lay higher requirement to solder mask plugged via.
Solder mask plugging in via holes features the following functions:
a. It is capable of stopping tin on PCB board from going through via holes and being exposed on component surface during wave soldering after components are mounted on circuit board.
b. It is capable of effectively defeating the issue of flux that is usually residual within via holes and further improving products' security.
c. Vacuum negative pressure state is formed on tester after components assembly.
d. It is capable of prohibiting solder paste on the surface flowing into via hole with empty soldering created.
e. It is capable of stopping solder balls from being generated within via holes and further stopping short circuits from being generated by tin ball ejection during reflow soldering.
Introduction of Solder Mask Plugged Via Manufacturing Technology
Manufacturing process of solder mask plugging for via holes usually contains via plugging, solder mask application on PCB, prebaking, exposure, developing and curing. The manufacturing process is so long and hard to control that product quality can be seldom guaranteed.
Compared with ordinary solder mask application technology, solder mask plugged via manufacturing shares equivalent procedures except silkscreen printing and post-curing. Thus, the essential point to optimize solder mask plugged via manufacturing process lies in reasonable monitoring and management in terms of silkscreen printing and post-curing.
Silkscreen printing for solder mask plugged via primarily comes in two ways: aluminum sheet and silkscreen. The advantages of aluminum sheet include: small-scale deformation and accurate alignment in silkscreen printing while it features longer procedure and relatively a low manufacturing efficiency. Silkscreen printing refers to the process during which solder mask oil flows into via hole through silkscreen printing. The essential advantage of this method lies in high manufacturing efficiency due to synchronous implementation of board solder mask application and solder mask plugging for via holes. However, silkscreen goes through large deformation with alignment difficult to be controlled. When compensation quantity is badly controlled or operators fail to implement agreeable control during silkscreen printing, an issue will take place of insufficient solder mask plugging.
Post curing is primarily associated with curing temperature and time. Boards with plugged via holes depend on segment curing while those without on one-period curing.
Current Manufacturing Technology
• Silkscreen Printing
a. Aluminum sheet. A drill is used with equivalent size with drilling tool to drill holes.
b. Silkscreen. No specific requirement is clarified and operators have no guidelines.
• Post Curing
Parameters for post curing: 80°C for 30 minutes, 120°C for 30 minutes, and 150°C for 60 minutes.
Most Commonly-Seen Issues in Solder Mask Plugged Via Manufacturing Process
Most commonly-seen issues in solder mask plugged via manufacturing process include:
a. Solder mask plugging features bad fullness with copper exposed at via edge.
b. Flatness isn't achieved at via holes with solder mask plugged and solder mask oil is uneven at BGA package.
c. Aperture solder mask oil suffers from bubbles and peel off after the application of HASL (hot air solder levelling) surface finish.
Reasons for Issues and Improvement Measures
Issue#1: Solder mask plugging features bad fullness with copper exposed at aperture.
• Reason Analysis.
Insufficient solder mask oil is used for via plugging, leading some parts of via holes to be covered by no solder mask oil and copper at some parts of via hole exposed. During silkscreen printing process, via holes are called for with the following two situations: same or close via hole diameter and dramatically different diameters. The smaller via holes are, the larger resistance withstood by solder mask oil will be and the more difficult solder mask plugging will be.
• Improvement Measures.
Aluminum sheet aperture should be optimized and operations for solder mask plugged via manufacturing should be regulated.
Issue#2: Flatness isn't achieved at via holes with solder mask plugged and solder mask oil is uneven at BGA package.
• Reason Analysis.
Manufacturing process following allows us to be aware that unflatness doesn't occur at via hole edge after imaging on PCB with solder mask plugged via while it occurs after post curing. Further research indicates that the essential reason for unflatness lies in the fact that volatile substances become expanded in solder mask oil and push solder mask oil away. For higher convenience of printing, when manufacturers are manufacturing solder mask oil, solvent is added to it so that solvent has to be volatilized off as much as possible before high-temperature curing of solder mask oil.
• Improvement Measures.
Parameters for post curing should be modified with curing time suitably prolonged. Volatile substances in solder mask oil should be reduced as much as possible before high-temperature curing.
Issue#3: Aperture solder mask oil suffers from bubbles and peel off after the application of HASL surface finish.
• Reason Analysis.
Without elements taken into consideration concerning solder mask oil, oil bubbles and peel off usually takes place in two situations. One is copper receives bad preprocessing and bonding capability is bad between copper and solder mask oil. The other is insufficient curing of solder mask oil, leading solder mask oil heat resistance to go down. The first reason can be excluded because boards are fabricated with the same preprocessing conditions.
• Improvement Measures.
In accordance with characteristics of solder mask plugged via manufacturing technology, parameters should be modified for high temperature phase in post curing phase with solder mask oil completely cured at aperture of via holes.
In conclusion, effective measures to improve solder mask plugged via manufacturing technology can be summarized as below:
a. The key to control solder mask plugged via manufacturing lies in parameter setting for silkscreen printing and post curing.
b. Stencil opening manufacturing on aluminum sheet plays a significant role in silkscreen printing and diameter of stencil opening should be in the range from 0.4mm to 0.45mm.
c. During solder mask plugging through silkscreen printing, via holes should be filled once to stop air from entering solder mask plugging.
d. During post curing, low-temperature curing time should maintain sufficient to make volatile substances in solder mask completely volatilized.
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Helpful Resources
• Solder Mask and Its Design Tips
• Requirements on Design Files to Ensure Efficient PCB Fabrication
• How to Get Precise Quote for Your PCB Fabrication Demands
