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Back to the basics: a reader asks Les to help with some basics of the assembly process.

I recently received a request to answer several questions concerning solderability testing and no-clean flux. The sender writes, "We operate a small, low-volume shop, and management is planning to switch to no-clean liquid flux. On the shop floor, we have questions that have not been answered and concerns that have not been addressed. Would you please provide some comments with respect to the following questions?" After reading each question, I determined that the sender needed some background information on the basic principles of testing and flux issues.

Q: At present, we do not require our suppliers to provide solderability testing data, nor do we conduct any solderability testing on a regular basis in our facility. Should we begin solderability testing on incoming components and boards?

A: I recommend you require solderability testing be conducted by your suppliers, with documented results, and that those results be provided to your facility on a lot basis. Verify incoming material solderability characteristics and examine the stockroom periodically, since solderability can be degraded as a result of the facility atmosphere, storage practices and length of shelf time before use. Such practices, as part of a well-controlled process, should be considered regardless of the type of flux being used! With the inherent reduced activity of no-clean fluxes, good solderability surfaces are even more essential, as is attention to the overall cleanliness of input materials, tools and equipment. However, a general tendency exists across much of the industry to ignore solderability testing until a major problem arises--a big mistake, but c'est la vie!

Why test for solderability? Discovering poor solderability, at the time of soldering a product, can require costly rework, touchup and/or scrap. In addition, rework can result in reduced product reliability. How big is the solderability problem in the industry? Some say the problem is small became only a small percentage of connections may exhibit solderability problems. Others say that up to 75 percent of all defects may be solderability related. Large or small, poor solderability issues must be resolved to achieve six sigma performance in the operation.

Q: How does preheat time and temperature affect the performance of no-dean flux?

A: Preheat time and temperature and general soldering temperature profiles are often more critical with no-clean fluxes than with the more active organic acid fluxes. Too much preheat, for an extended period of time, can reduce the effectiveness of the flux and may also result in undesirable oxidation of surfaces to be soldered. Conversely, too little preheat may result in reduced flux activation and leave unreacted flux residues on the surface--with the potential for causing reliability problems in service. Care must be taken to understand the characteristics of the no-clean flux and the need for a sufficient time/temperature profile to both preheat the surfaces to be joined and to activate specific flux ingredients.

No-clean fluxes offer less fluxing power at the surface to be soldered and, as a result, understanding of the need for fluxing and awareness of the capability of the materials being used are mandatory. The correct temperature profile is required for the specific type of flux being used. The correct slope for temperature increase, with time, is necessary to assure the flux does not react too soon. In addition, the maximum temperature must not be too high or the flux constituents may be charred instead of activated. Conversely, the profile must not be too short, yet the temperature achieved must be high enough to assure that no unreacted flux residues are left on the assembly surface. Seek council from technical representatives of the flux supplier.

Q: Will we still need our water cleaner if we make the change to a no-clean flux?

A: A facility should maintain a replacement cleaning capability in the event the supplier, or the facility itself, loses control of a process. A backup cleaning process may also be needed if incoming or in-process materials require cleaning to maintain special testing, conformal coating or required end-item cleanliness levels. A cleaning capability may also be required to support maintenance of solder paste stencils and recovery of solder paste misprints. I should note that not all no-clean flux residues are removable with a water cleaner--some turn white when exposed to hot water.

Send your process, technology or training question to les.hymes@worldnet.att.net. Please type "ASK LES" in the subject line and indicate your name and company or institute affiliation. All questions may not be answered.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; (625) 544-4646; e-mail: les.hymes@worldnet.att.net.
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Title Annotation:Ask Les
Author:Hymes, Les
Publication:Circuits Assembly
Geographic Code:1USA
Date:May 1, 2002
Words:759
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