How Heat and Humidity Affect No-Clean Paste

September 7, 2004
/ Print / Reprints /
ShareMore
/ Text Size+
Even with the latest generation of solder pastes, environmental extremes can substantially affect performance.

As electronics manufacturing shifts from the United States to other countries, environmental issues such as temperature and humidity control are influencing the assembly process. When qualifying solder paste for domestic assembly lines, engineers rarely test the material's performance in hot, humid conditions. That's because these factors are usually tightly controlled within the manufacturing facility.

However, temperature and humidity fluctuation may create problems when manufacturing is relocated to places without good environmental controls. Temperature and humidity can cause paste-related defects during both printing and reflow. These defects include cold slumping, solder balling, voiding and grainy joints.

Although modern solder pastes are much more resistant to local environmental conditions than their predecessors, significant differences in performance can still be observed when temperature and humidity are not controlled. When new products are manufactured at a variety of worldwide sites, substantial problems with product quality can occur. By identifying potential problems at an early stage, process adjustments can be made prior to implementation.

Temperature and humidity can vary significantly from location to location and from season to season. In cold climates during the winter months, relative humidity can fall to below 10 percent. In contrast, in humid climates in the summer, relative humidity can exceed 70 percent. Depending on the plant's location, temperatures, in Fahrenheit, can easily vary from the mid-60s to the low 90s. Even with the latest generation of solder pastes, these environmental extremes can substantially affect solder performance, an important consideration when setting up evaluation programs.



Evaluating Paste

A simple way to determine the susceptibility of solder paste to humidity is to look at solder balling performance. Recently, we tested four pastes for solder balling: two no-clean pastes and two water-soluble pastes. For each paste, we printed a deposit 5 millimeters in diameter on a ceramic coupon using an 8-mil stencil. The deposit was then conditioned in a temperature and humidity cabinet before being reflowed on a hot plate at 250 C for 5 seconds.

Both of the no-clean solder pastes performed well at a low relative humidity. However, Paste A had much less resistance to high humidity than Paste B. After humidity aging, Paste A produced significantly more solder balls.

A similar but more pronounced effect was seen when we repeated the test with water-soluble solder pastes. Even the best-performing product, Paste D, survived only a few hours at high humidity. Moreover, survival time will be even less with deposit sizes that are closer to those used on actual production assemblies.

This simple test can quickly and easily differentiate between solder pastes that may, at first glance, appear the same. Having identified that a paste is susceptible to humidity aging, more detailed research can determine the effect of humidity on the assembly process.

Many water-soluble solder pastes have a strong affinity for water. This hydrophilic nature makes water-soluble pastes susceptible to humidity-induced slumping. When tested at 47 percent relative humidity, the two water-soluble pastes may appear indistinguishable. However, after exposure to only 30 minutes at 80 percent relative humidity, some pastes can exhibit significant slumping that is more than sufficient to cause solder bridges.

The effect of temperature on a solder paste's rheology (the flow characteristics at low and high shear) is seldom characterized beyond its long-term effect on storage. Rheological changes caused by relatively small temperature increases can dramatically affect process yield by reducing print definition and increasing bridging, solder balling, and mid-chip beading. Such problems can easily be exacerbated by suboptimal printer settings.

The reduction in low-shear viscosity observed over a temperature range as small as 5 C or 9 F can increase the frequency of print bridges on a component with a 16-mil pitch. To maintain print quality, engineers would have to increase the frequency of wiping the underside of the stencil by 20 percent.

Tack force, abandon time, and stencil life may also be affected by the ambient temperature, so these parameters should also be considered when determining process guidelines.

These simple tests show that pastes with very similar performance under moderate temperature and humidity can show dramatically different performance when run under high humidity and temperature. Thus, before approving a solder paste for global use, electronics assemblers must consider the likely local environmental conditions at individual manufacturing sites around the world.

Did you enjoy this article? Click here to subscribe to Assembly Magazine.

You must login or register in order to post a comment.

Multimedia

Videos

Image Galleries

Behind the Scenes at Ford's Michigan Assembly Plant

People are the heart and soul of the 2012 Assembly Plant of the Year. This slideshow shows some of the men and women who build three different types of electrified vehicles alongside traditional gas-powered cars on the auto industry’s most flexible assembly line—Ford’s Michigan Assembly Plant in Wayne, MI. Photos courtesy Ford Motor Co.

Podcasts

Tim Hutzel, President, Tim Hutzel American Business Services and Dave Lippert, President, Hamilton Caster & Mfg. Co have deep and unique perspectives on reshoring. That’s the primary reason they’ve been invited to deliver the keynote address at 2014’s The ASSEMBLY Show.

More Podcasts

Assembly Magazine

assembly cover september 2014

2014 September

The 2014 September Assembly includes articles about engine assembly plus much more. Check it out today!
Table Of Contents Subscribe

Assembly Plant Age

How Old Is Your Assembly Plant?
View Results Poll Archive

THE ASSEMBLY MAGAZINE STORE

welding.gif
Welding: Principles & Practices

This text introduces students to a solid background in the basic principles and practices of welding.

More Products

Clear Seas Research

Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications,Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

Assembly Showrooms

ASSEMBLY Showrooms

STAY CONNECTED

facebook_40px twitter_40px  youtube_40pxlinkedin_40pxgoogle plus