Calculating Solder Paste Usage

Posted by Christopher Nash on Monday, August 20, 2012

Calculating the exact amount of solder paste needed for a given circuit board or for a full production run can be difficult for several reasons, including:

  • print deposit variations
  • paste left on the stencil and squeegees after the build is complete
  • bead size needed for the build (which depends on the squeegee size)

The theoretical volume of solder paste can be calculated for each board using the Greely Formula and a simple volume calculation.

The Greely Formula:

Specific Gravity of the flux vehicle is generalized to 1. 

Example:

  • Solder Paste: SAC305, Indium8.9HFA, Type 4.5, 87.75%
  • Aperture Size: 0.012” Square
  • Stencil Thickness: 0.004”

 

 

 

 

Solder Paste Specific Gravity = 4.14

 

Volume for this aperture can be calculated using the following formula:

Length x Width x Height

0.012” x 0.012” x 0.004” = 0.000000576 inches3

 

To get the theoretical weight of the solder paste for the 0.012” square aperture you must multiply the solder paste theoretical volume by the solder paste density. 

 0.000000576 inches3 * 4.14gm/cm3 = 0.000009439cm3 * 4.14gm/cm3 = 0.0000391gm

 

To calculate the theoretical amount of solder paste that will be used for each board, the weight of solder paste for each aperture on the board will need to be calculated.  Once all of the weights have been calculated they can be added together which will result in the amount of solder paste per board. 

Of course this is the theoretical value and not an actual value.  The easiest way to determine the actual paste weight per board is to weigh a board before the paste has been printed and then again after the paste has been printed.  The difference is the actual solder paste weight or consumption of solder paste per board. Of course, there will be some margin of error even in this calculation due to the weight tolerances of the board and the variations in solder paste deposits from print to print.

Let me know if I can help you calculate or estimate the amount of solder paste your project will consume.
Chris

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Compatibility of Conformal Coatings versus No-Clean Solder Paste Flux Residues: When To Clean?

Posted by Christopher Nash on Friday, January 28, 2011

Conformal CoatingsConformal coating compatibility with no-clean flux residues has been a major topic for years – becoming even more popular recently with companies looking to cut manufacturing costs and processes.

Unfortunately, no industry standards definitively determine or define “compatibility” between conformal coating materials and no-clean flux residues. This does not mean that people are always automatically cleaning the flux residues before conformal coating their boards. Nor does it mean that most people are shooting in the dark with their decision to clean or not. I have a number of customers who conformally coat over no-clean flux residues, smart customers who have taken the time to do their due diligence and create their own standards and test methods to determine compatibility. These companies also run the tests and apply their standards on their materials.

Here at Indium Corporation we look at materials compatibility from three different viewpoints:

The first is the simplest: visual appearance. Does the coating look like it has adhered to the board, components, and/or flux residue? A lack of adhesion will usually result in bubbles, crazing, and a number of other visual defects or anomalies.

The next step/test that could be taken to determine compatibility would be to measure actual adhesion. “Tape Tests” are often used to measure adhesion. However, these Tape Tests all have a number of imperfections and variables that accumulate to result in a lack of both accuracy and repeatability. For example, many of these Tape Tests are very operator dependent and rely upon the speed at which the tape is removed, the angle at which it is removed, the force in which it is removed, etc. Different operators can, and do, have very different results. Tape Tests are also dependent on the tape. Variables include: age, shelf life, tack strength, adhesion to certain materials, tape brand, tape width and/or length, even test temperature. Another issue with this method has to do with the conformal coating material. What happens when the conformal coating material is a silicone? There isn’t much that will stick to a silicone, so using the Tape Test with this material is probably worthless. Here are several Tape Test procedures that are present within the industry:

Note that the ASTM test method neuters itself by stating all the possible flaws that could be present.

The third viewpoint is electrical reliability using Surface Insulation Resistance (SIR). Even this is not easy to conclusively determine because there are two different test methods for SIR. One each for:

Another issue with SIR testing involves 3D parts. The National Physical Laboratory (Britain) and the SMART Group (a British trade association) have been trying to conquer these SIR challenges for years. They do have a draft of a standard written, but nobody wants to stick their neck out, without further testing and proof, to release this standard; so it has been in limbo for quite some time. For further reading on this topic, read this paper, authored by my colleague Andy Mackie and me. It provides further insight into the issues that our industry is facing with regard to compatibility. Note the chart, in this paper, that highlights the differences between the two IPC SIR test methods.

Now that you know Indium Corporation’s position on conformal coating compatibility with no-clean flux residues, it is time to consider the point of view of the companies who manufacture and supply conformal coatings. The majority recommend that, when in doubt, clean the no-clean flux residue before conformally coating. This takes any and all of the guessing out of the equation.

As stated earlier, there are companies that conformally coat over no-clean flux residues with success. Some of these companies are very successful and well-known aerospace, automotive, and military organizations. I am sure they have created their own standards and test methods to mitigate risk and doubt, while assuring performance and reliability. Unfortunately, they have not yet shared their methods and/or experiences with the rest of the industry, so we cannot yet use their expertise to guide us as we converge on one industry standard. If you are one such organization, I’d love to hear from you.

We have not done much work with conformal coating “compatibility” trials due to the vast number of conformal coating materials commercially available and the vast number of no-clean flux formulations that we offer. Even if there were a standard that would define what it means to be “compatible”, the sheer number of conformal coating products, when combined with the sheer number of no-clean solder fluxes, creates an extremely large matrix of necessary tests.

In conclusion: When in doubt, clean.

If you have any other questions or concerns please feel free to contact me at any time.

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Recent Electronics Assembly Trends

Posted by Christopher Nash on Friday, March 5, 2010

shot clockI love watching a good basketball game, and one of my favorite local teams is the Syracuse Orangemen. If you go to a Syracuse home game, notice the shot clock – it was made with Indium Corporation solder. There are a lot of places you can see our products in your everyday life. That smart phone in your pocket, the electrical components in your car, the thermal interface in the computer in front of you. That’s one of the things that makes this job rewarding, being part of so many various applications.

 

In addition to learning about these different applications, we also get a good reference for what assembly trends are developing, and which material technologies are becoming more popular. 

 

I’ve watched the halogen-free trend explode and fade, as it was adopted by some large OEMs and their contract manufacturers, but has not spread to most other companies. Another trend that is fading away from the spotlight is Pb-free die-attach solder, since the EU has not found a suitable replacement and has pushed back the exemption deadline. 

 

A long-existing topic that has had recent mention is solder jetting. The trend towards soldering smaller components is not new or surprising, but for smaller components (01005s and 0201s) we have seen a trend towards dispensing instead of jetting – which seems to suit those applications.

 

For small component printing, transfer efficiency is critical. Outside of solder paste optimization, “nano-stencil” technology is an upcoming technology that may take-off and improve paste release characteristics. Solder paste is being used in some other creative ways too, like low temperature alloy dipping paste for rework operations. Manycompanies are now using or evaluating specialized solder applications to replace components without fully reflowing the rest of the components on the board.

 

Integrated preforms are finding their way into more and more applications recently as well. These connected preforms are being used to reduce the need for component pallets and selective soldering operations.


All these applications are great ways that our customers are taking soldering technology to the next level, using materials and assembly methods that were not common before. I look forward to learning how you’d like to use solder in your application!

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