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March 2, 2009

What Is the Ideal Humidity for PCB Assembly?

Some experts say that a circuit assembly production facility's temperature should ideally be set between 70 and 78 degrees Fahrenheit and humidity of 40 to 70% when working with electronic assemblies. The facility I work in has a current temperature of 76 degrees and a humidity level of only 16%.

Would that humidity level affect the solderability of our assemblies?

S.D.

Experts Comments

If your facility's ambient RH is only 16% then you are more prone to ESD (Electro Static Discharge) related defects. Humidity in the air helps to increase surface conductivity, which subsequently aids in dissipating electrostatic charges throughout your facility.

ESD literature commonly recommends a MINIMUM RH of 40%, provided that this will not promote corrosion or delamination of the materials used in your products. For a MAXIMUM RH level I would never suggest going higher than 60% RH, for a variety of reasons including the impact of RH on Moisture Sensitive Devices.

MSD floor life is dramatically reduced above 60% RH (Ref: Table 7-1 in IPC/Jedec J-STD-033B.1).

The bottom line is that you want enough humidity to maintain ESD control, but not so much that you induce damage to MSDs. I would generally recommend 40% to 60% RH.

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Mitch DeCaire
Sales Manager (Americas)
Cogiscan, Inc.
Mitch DeCaire, Sales Manager at Cogiscan, has served the electronics manufacturing industry since 1989. His prior experiences include process engineering, business development, and engineering management roles with Nortel Networks, Vansco Electronics, Universal Instruments and Siemens.
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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Solder pastes react differently to temperature and humidity levels. Low humidity levels can exacerbate the driving off, or drying of solvents in solder pastes which in turn affect variables such as:

  • Response to pause (abandon time) how much time can pass between prints
  • Overall stencil life - how long a solder paste remains useable before needing replenished/replaced
  • Open/tack time - amount of time you have to place components
  • Tack force - the pastes ability to hold components after placement

In reflow low humidity levels may produce signs of degraded wetting to component leads and/or spread to the PCB pads. Severe conditions may result in what appears to be un-reflowed solder.

If you are using an older paste chemistry, work with your paste supplier and discuss your manufacturing conditions with them. Many pastes today have the ability to tolerate wider swings in temperature and humidity without compromising product reliability or changing the manufacturing process.

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Mike Scimeca
President
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.

The low humidity could cause the solder paste to dry out prematurely, but your bigger concern with the low humidity is static control. Make sure your operators are using heel and wrist straps and testing them daily.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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