Effective Measures to Improve Solder Mask Plugged Via Manufacturing Technology

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.