Uncontrolled Environment Causing Problems

I supervise 3 high speed no-clean, lead-free SMT lines. Unfortunately these lines are in an area with no control over humidity. We are seeing a high fallout due to insufficient paste application on TSOP components. We have made changes to stencils: we clean stencils routinely and its helps, but still have a fluctuating yield. Is there such a thing as low humidity, low temp lead free - no clean paste? What do you recommend?

D. D.

Expert Panel Responses

Rather than repeating them here, see my response to another similar enquiry having to do with handling of solder paste. There are a number of paste handling issues that could be leading to the high fallout. Make sure all of the recommended paste handling guidelines are being followed. For you, I also ask the following questions:

Are the TSOP pads you speak of fairly small (less than .015" by .015")? If so, have you considered using something other than a standard Type 3 paste particle size? Before anyone jumps up and tries this, one caveat: you better have your printing process down pat, because switching to a Type 4 or smaller paste can drastically increase the number of solder fines found outside of the solder joint. However, for those whose board design allows it, and for those whose print process control allows it, use of Type 4 paste can sometimes drastically improve printing yields as well as overall wetting issues (reduced DPMO) because getting the optimal amount of paste onto the pad is where it all starts. Be aware also that Type 4 or smaller paste may not be as readily available as standard Type 3. Again, QUALIFY YOUR PROCESS carefully, and be sure to carefully inspect for renegade fines using a good X-ray after reflow before attempting this on production CCAs. Some fines can be packed between the edge of the soldermask and the edge of the pad, partially reflowed to the "crusty copper" on the edge of the pad or inside vias, and while somewhat entrapped, may not be acceptable in high-reliability medical, traffic control, avionics, military or other devices. So tread carefully before you jump in and change.
It may make a lot of sense to purchase and install a good humidifier system. The reduction in solder defects and possibly in ESD issues can justify the expense. Make sure your board handling practices do not allow CCAs or PWBs to sit around and soak up moisture from the air, or you will trade one problem for a much worse one (delamination, measling, internal trace-to-via damage caused by z-axis expansion, and CAF).
One more thing I wanted to mention regarding paste printing issues; never wipe solder paste from mis-printed PWBs, as this will pack solder paste into the spaces between the edge of pad and soldermask, as mentioned above, but also into every single exposed via and plated-through hole on the board. Imagine how much fun this is when you discover all of the through-hole parts don't fit later? Instead, use a process that will gently wash away the mis-printed paste, followed by a more robust PWB wash process. And be sure the board is very clean, dry, and at room temperature before attempting to re-print.

Good luck, and keep me posted.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Most no-clean solder pastes are hydrophobic and humidity fluctuations don't play a large role in determining print performance. The solder paste viscosity has a large influence on the fill percentage of the stencil apertures as does print speed and blade pressure. If you are getting insufficient fill on the TSOP stencil apertures, verify that your blade speed is not too fast and/or your blade pressure is not too low. Blade speed and pressure are directly proportional. Increase/decrease one and you typically need to increase/decrease the other when optimizing the print process. Our no-clean solder pastes are specifically formulated with low viscosities, yet are still slump resistant. This allows for improved stencil aperture fill at higher print speeds and provides for a wider process window at the printer. I would be happy to discuss with you this issue in greater detail. Please feel free to contact me at rdervaes@fctassembly.com. The solder paste and stencil are two important variables in solder paste printing, but they are not the only two that impact print performance.

Robert Dervaes
V.P. Technology & Engineering
Fine Line Stencil, Inc.

Robert Dervaes has worked in the electronics industry since 1992 in both design and manufacturing. Over the past 11 years he has established the technical foundation of Fine Line Stencil, Inc. - a premier stencil supplier to the electronics industry.

What is the aperture size and stencil thickness? Is the area ratio > .66?

Bill Coleman
Vice President Technology
Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

Both high and low humidity can have an effect on the life and print capability of any solders. Lead free pastes have improved tremendously over the last few years and our latest material LF721 has shown exceptional open time results in high and low humidity environments. You also mentioned low temp, what temp are we talking of printing or reflow? If it is Low temp reflow, then there are Bismuth containing alloys that are used for low temp reflow, these tend to be Bismuth and Tin (Bi57 is the most common alloy) having a melting point around 138 centigrade. This can be reflowed in a standard tin lead profile or as low as 170 centigrade for reflow peak. This tends to use low temp fluxes also so it becomes a specialized product in most suppliers catalogues. If you mean low temp printing then most solder paste will print at temperatures around 20 C, how ever much lower than this and the material starts to become stiff and harder to print.

Doug Dixon
Marketing
360-Biz

Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.

If these are high speedlines, I would imagine that the processing time (paste printing, pick andplace, travel to reflow) is not long. If you focus only on the printingprocess, a controlled environment in the printing machine itself might help.For better solder release, evaluate finer types (Type IV or V) depending on thepopulation of the board as well as the stencils' design and manufacturingtechnology. You will get a higher volume of flux and solder paste on each pad.Finer granulation of the solder paste in combination with nano-coated stencilsimprove the quality of the solder joints through better paste release andreduction of the stencil cleaning cycle frequency.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

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