|Ask the Experts|
June 8, 2018
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?
|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:
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 firstname.lastname@example.org. The solder paste and stencil are two important variables in solder paste printing, but they are not the only two that impact print performance.
V.P. Technology & Engineering
Fine Line Stencil, Inc.
What is the aperture size and stencil thickness? Is the area ratio > .66?
Vice President Technology
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.
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.
Engineering and Operations Management
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address