Ask the Experts
May 26, 2010 - Updated
May 24, 2010 - Originally Posted

Humidity Inside Our Screen Printer

We notice solder paste is drying out after being inside our screen printer for a short time. Humidity inside our screen printer is 35%, while humidity outside our screen printer is 50%. How can we increase humidity inside our screen printer to help slow the drying of our solder paste?

K. L.

Expert Panel Responses

Some of the older generation pastes were more susceptible to drying out in low humidity environments and slumping in high humidity environments. This was especially true with water soluble pastes. Over the last few years we have seen most pastes formulated to withstand lower and higher humidity environments. In the case of water soluble pastes we now have a number of products on the market that are water washable where they have some ingredients that aren't completely water soluble but have surfactants that are able to allow the water to wash away the residue. This has led to water washable pastes that are less hydroscopic. Also on the No Clean front there has been a change to higher boiling point/low evaporation solvents. This has allowed for pastes to have greater open and abandon times. Typically these new generation pastes recommend a RH of 25-65% so both your printer and facility would be within specification. My suggestion is to look at the paste rather than increasing the humidity in the printer.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

Check with your Solder Paste Printer Manufacturing Company, they may have a fan or A/C unit built into the system to cool the operating mechanics and electronics of the printer. If this is the case, this might be the cause of your paste drying out. You might need to attach a deflector of shield near the paste bead to protect it from this airflow.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

The solder paste needs to be robust enough to withstand fluctuations in humidity without drying out / slumping. Increasing humidity inside the printer could cause slump / bridging during reflow. Typically water-wash pastes are more prone to dryout / slump as they are hygroscopic. No-Clean pastes would be more resistant as they are non-hygroscopic.

Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.
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