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September 19, 2018

Solder Balling Prediction Formula

We have experienced mid chip solder balling on some passives. Is there a mathematical calculation that can be used to predict solder balling for a give package size? Ideally we would like to design our stencil apertures to ensure no mid chip solder balls. Is there a mathematical way to help with this problem?

R.W.

Experts Comments

I don't believe there is a mathematical way to solve for this type of defect but here are some ideas to minimize or completely remove the mid-chip solder ball by modifying your stencil:
  1. Do not use 1:1 aperture to pad area, minimize the stencil aperture by 10%. Most of the time solder will get trapped under the component and flows out from under it thus generating a solder bead attached to the component.
  2. Use a home-plate design for your pads.
  3. Use a stencil thickness that will not generate excess solder. I see that the most commonly used thickness is 5 mils (depending on your component mix).
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Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
I am not aware of a mathematical formula to predict mid-chip beading on passive components. The stencil design can be changed to minimize this issue. Typically a "U" shape or inverted "home plate" shape can help. The idea is to remove solder paste from under the body of the component which limits the potential of spread of solder. Most stencil suppliers can give more specific recommendations.
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Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

Many variables will contribute to solder balling. The solder paste should be the right type for aperture size. Make sure the paste is fresh and not drying out. The stencil should be cut to the component manufactures recommendations. The printing process needs to be robust applying the proper solder paste volume for the components in question.

This can be confirmed using an automated solder paste inspection process (SPI). Some screen printer have this build in. The printing process may require more frequent under screen cleaning to avoid depositing solder paste on the board mask. If there are no problems found with the printing process, you will need to review your reflow profile. Different paste require different soak times.

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Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

The mid-chip solder ball occurs when solder is squeezed under the component and away from the pad and is unable to coalesce back during reflow, remaining hidden under the component or appearing out to the side of the device. Several causes for mid-chip solder balling have been suggested and include excess solder paste on the pad, poor hot slump and insufficient wetting. The mid-chip solder ball phenomenon tends to occur more frequently with chip resistors due to the fact that they have tinning on three sides and a reduced solderable surface as compared to capacitors, though capacitors may also be subject to this defect. The reported occurrences of mid-chip solder balls have changed as the industry has moved toward more miniaturized components and lead-free solder paste materials.

In order to better understand the relationship between component size and type, stencil design rules and solder paste characteristics to the occurrence of mid-chip solder balls, Henkel conducted an evaluation of these factors to determine the best approach for the elimination of this defect. The study was carried out in two phases; the first studied material slump and aperture reduction and the second analyzed the solder paste characteristics of hot slump, cold slump, wetting-solder-balling and transfer efficiency.

The results of this part of the study revealed that reducing slump either by adding a gelling agent or increasing the metal content in the solder paste did lower the occurrence of mid-chip solder balling, but it was not the only factor; reduction of the stencil's inner aperture wall also played an important - if not more significant -- role.

While paste characteristics are important, reducing aperture area through the use of "u" or "crown" shaped designs has the greatest effect on mid-chip solder balling reduction.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.
Solder balling is a process problem that can occur from multiple causes. Stencil aperture design and stencil thickness are just 2 of them. I do not think that you can mathematically predict solder balling but getting the stencil aperture and thickness as recommended per package size and design is one way to avoid it. Next, ensure that:
  • Components are moisture and oxidation free
  • Reflow profile setting are in accordance with solder paste manufacturer's recommendations and slope to peak is not too aggressive to cause rapid temperature elevation
  • Solder paste is moisture free (out of refrigerator 24 hours before use, no condensation present, mixed to the appropriate viscosity)
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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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