The Effects of Reduced Alloy Powder Size on Solder Paste Print Performance

Figure 1: an example of type 4 solder spheres. Source

To meet the rapid advance in electronic manufacturing, assembly technology is met with an inevitable need for growth, with solder paste manufacturers now using finer metal powder to meet the need for ever-finer board printing. The evolution of electronics and technology has in recent years favored smaller, finer, and miniature innovations. This improvement and reduced size of electronics are also evident in the solder paste industry. This article seeks to unearth the effects of reduced allow powder size on solder paste print performance.

The Effects of Reduced Alloy Powder Size on Solder Paste Print Performance

But before taking a look at the effects of the alloy powder size on the solder print performance, it is crucial to understand how the solder paste printing process affects the performance of the solder.

The assembly and effectiveness of a printed circuit board are dependent on the solder paste application. This is because each PCB application aims to be accurately deposited on the pads. The
solder paste is screen printed on a stencil, foil, or jet printing for this process to succeed. If not correctly applied, this first process determines the defects or efficiency of the print boards.

To better understand the impact of the reduced alloy powder size, a study in 2011 by Global Solder Solutions sought to know how the smaller sizes were fairing. The study revealed that type 4 powder improved the volume consistency of the paste compared to type 3 powder. While other factors also affected the solder paste print performance, the research showed that the reduced alloy powder size significantly affected the transfer efficiency. Over the years, the powder sizes continued to decrease. From type 3 at 45um, type 4 at 38um, type 5 at 25um, and the smallest type 6 at 10um.

Solder paste is a powder solution mixed into a thick medium, also known as flux. The flux is a temporary adhesive that holds the print board until the solder paste melts to form the electrical
connection.

The solder paste viscosity reduces on abrupt change even during application. Hence, the paste requires energy to be evenly applied in the stencil apertures, and the paste forms a block when no
energy is use.

While the performance of the solder paste is highly linked to the size of the alloy powder, the release of the paste from the aperture is also affected by the size of the powder. Therefore, the correct solder paste should be based on the size of the apertures on the stencil. The rule of thumb in selecting the powder size is a minimum of 5 particles on the width of the smallest aperture stencil, and this rule is also known as the 5 ball rule.


The performance of solder paste on PCB has shown a significant change, with smaller powder particles rolling out higher volumes than the bigger particles since the smaller particles are compact.
The smallest size of the particle makes the spread and use of solder paste faster. However, manufacturers need to develop more accurate and specific stencils to avoid excess rollout, affecting
the solder paste print performance.

Use Consideration

The size of the alloy powder may affect the performance of the solder paste, but another critical factor that plays a significant role in the effectiveness of solder paste is the storage and mixing technique. Both lead-free and Tin-lead pastes should be refrigerated before use to help maintain the paste quality and consistency once it’s ready for use. But before use, the paste should be under room temperature for not less than 8 hours, which helps maintain the high quality of the paste.

Application

Several stenciled print methods apply solder paste on the PCBs. One of the most common and preferred printing methods is squeegee blade printing. However, for solder paste printing to be
effective, it is essential to consider various factors. Besides the size of the alloy powder, some of the other considerations for effective squeegee blade printing include:

The squeegee speed: The speed helps determine how much time is available to roll the solder paste on the apertures and the print boards. In most cases, the squeegee blade uses a speed of about 25mm per second. However, this is relative and depends on the size of the aperture and the solder paste in use.

Stencil separation speed: Another critical factor is the speed of the stencil. A squeegee blade is set at a speed of 3mm per second. This is the speed at which the print boards are separated from the stencil blade after printing. The size of the apertures determines the speed of the squeegee blade. When the stencil uses higher speed, the solder paste is not well released to the apertures leading to the formation of edges of the solder paste layer.

Squeegee pressure: For a regular and clean layer, it is crucial to apply equal and sufficient pressure throughout the length of the squeegee blade. The most common setting for the squeegee pressure is about 500 grams of pressure per 25mm. Little pressure during rolling can cause smearing and an incomplete layer on the printing board. Excess pressure, on the other hand, can lead to bulges and scooping on large apertures. The extra solder paste may also cause paste bleeding between the board and the stencil.

Squeegee angle: The squeegee blade is set at 60 degrees and is fixed to holders that ensure the angle remains constant during the process. If increased, the paste deposited will be less than required. When reduced, there will be paste residue on the stencil preventing even application of the paste.

Stencil cleaning: Although most printing machines can be cleaned automatically. It is necessary to clean the squeegee blade both manually and automatically. Leftover solder paste on the blade can cause blockage or smudging. Additionally, other key factors include PCB support, squeegee maintenance, and print stroke.

Solder Paste Evaluation Process

To determine the performance of the solder paste, it is necessary to conduct an evaluation process. This process will help eliminate other underlying factors by comparing the effectiveness of different solder pastes based on different powder sizes.

Several methods are used to perform a paste evaluation, and solder pastes with higher volumes and lower standard deviations are preferred.

Bridging performance, on this test, solder paste with low bridging capacity is preferred. The print tests are run at different print speeds. Some solder pastes perform poorly on higher speed prints; hence, most tests run at high speeds to determine the paste performance.

By rolling the paste through a small aperture ratio, some solder paste rollout well over long periods on the squeegee blade, and to determine the effectiveness, the solder paste is run through a small aperture with pauses along the length.

Reflow performance, this test helps determine the paste’s stability by heating the paste for some time. The paste is heat in a sealed container to about 50 degree Celcius for several days. Aforementioned simulated shipping issues, which in turn affect the performance of the solder paste when used on PCBs.

Depending on the solder paste in use, the solder paste can also be exposed to open air. This can be done during printing on the boards or using a blank stencil. The air oxides the solder paste, and the metal continues to oxide in a continuous cycle, which causes a thickening of the paste, affecting the reflow on the print.

While some pastes are stable and can tolerate mixing under open air, to evaluate the paste reflow performance, it is crucial to perform the methods mentioned below to determine which method work best with a certain solder paste.

Monumental Effects

The key to faultless printing is not only based on the size of the alloy powder. The foundation, equipment, use, and maintenance of the tools also play a significant role. However, when focusing on
the powder size. The smaller powder sizes provide a better transfer efficiency, and the impact is also noted in the production field.

Unfortunately, although the size and reduction of the powder size have improved the transfer and performance of the solder paste, the reduction has also caused cost burden and supply chain cost
increase. By reducing the powder size, the print performance over time requires extended poses, but with the solder paste manufacturers acquiring new skills regularly, there is a need for regular
adjustments and changes to alleviate the effects.

For optimum performance on print boards, the type 6 powder size has shown to be well suited for the new jetting formulas and solder paste dispensers with small apertures to allow deposit variability and effectiveness.

Bottom Line
With an ever-changing and improving need in the electronic and technology industry, solder paste manufacturers need to constantly develop and research how different alloy powder sizes affect the print performance’s effectiveness. And by improving on the few shortcomings, the industry will evolve and be up to par with other electronic manufacturers to ensure efficient and effective products for both manufacturers and end-users.

The research and development in the size of the alloy powder is a great step. However, in line with ensuring wholesome results, it is vital for manufacturers to closely monitor squeegee blade printing as other factors such as application and ensure the industry achieves overall growth.

Author bio

Linda Liu is the overseas marketing manager for MKTPCB , a leading PCB manufacturer that offers high-quality PCB products and services. Since 2012, she has established “first-of-its-
kind” industry-changing and transformational businesses initiatives that increased revenue growth, brand exposure and market expansion for MKTPCB. Linda graduated from Western
University with a bachelors degree in marketing.

Information:
Author: Linda Liu
Title: Overseas Marketing Manager, MKTPCB
Company website: https://www.mktpcb.com/
Company contact email: info@mktpcb.com

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