How Do You Calculate Solder Paste Volume?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, coming to you from the Board Talk cave.

Jim
I think we've got a great question today. This comes from T.A., "How do you calculate the volume of solder paste needed for screen printing PCB assemblies? Is there a simple formula?"

If we had an all-time-great-questions list, Phil, I would put this near the top. This is a great question, "How much paste do I need to get a good joint?" In my opinion, from my experience, there is no formula.

There are no standards to tell you for a given configuration - take, for instance, I've got a 20mil pitch QFP with gull wing leads, .5 millimeter pitch, and from the spec sheet I see that the foot of the lead has certain dimensions, and I design a pad that's 10 mils wide and so long.

How much paste do I have to put down on that pad in order to get a good reliable solder fillets for that configuration? What's the minimum and the maximum? And that's important because suppose that part is on a board with a bunch of big resistors and capacitors, I would tend to want to use a thicker stencil, maybe a 6-mil stencil.

What's the maximum paste that I can tolerate on that pad without getting into bridging and solder balls? On the other hand, suppose that that's the biggest part; I've got nothing but real small resistors and other fine-pitch parts, and I want to use a thin stencil, a 4-mil stencil? What's the minimum solder that I can tolerate on this gull wing lead and still get an adequate fillet?

In my experience, in my reading of the literature, we have never seen any standards for that. When we start designing stencils and talking about area reductions or overprinting, we really need those numbers.

You need to know what's the maximum amount of paste so that you can make your decisions on shaping your aperture sizes when you have these different combinations on the board.

If anybody out there in Board Talk land knows of a resource, or has information that they can share, we would love to hear from you.

Phil
We're probably going to get letters that say, "You boneheads! Of course there's a calculation. But what Jim is saying and emphasizing, is that there are no standards for a given lead, say, on a 20-mil pitch SOIC, that you need this volume of solder there. Jim says it can vary, there are variables. 

Jim
We know there's a range. You look at the IPC in other specifications, they'll show you minimum amount of solder and maximum amount of solder. Below that, you have a chance of getting an insufficient joint; above that, you have a risk of solder balls and bridges.

But the question is, for any given configuration, how do you calculate those numbers? That's what T.A. is asking us, and my answer is I don't know.

Phil
I don't think I've ever seen a direct correlation saying that the ideal volume to get the ideal strength for that particular joint is thus. There is no standard.

But, anyway, regardless of whether you're using re-flow soldering, wave soldering or selective soldering, whatever you do.

Jim
Don't try to solder it like my brother.

Phil
And don't solder like my brother.

I'd like to expand on the comment by Eric S. regarding spreadsheet calculations. For a given joint geometry, a spreadsheet calculation is quite feasible, and can take into account a lot of factors. The basic strategy is to first calculate the metal volume required.

For a gull-wing component, this would be the total volume under the lead (never zero, and we need a reasonable estimate) plus the side fillet volume, plus toe fillet volume if applicable, plus heel fillet volume. Then divide the total volume by the paste metal percent by volume. This is not the metal % listed in the data sheet, which is by weight. The metal % by volume is usually between 50% and 55%, and can usually be approximated at 50% without undue error.

Now we have the (minimum) volume of paste required, and in order to determine the required aperture dimensions, we just need to know the stencil thickness, and account for transfer efficiency. Now lather, rinse, repeat for each different geometry!

ACI has a paper written some time ago, http://store.aciusa.org/EMPF-RB0009-Solder-Joint-Volume-Calculation-Using-a-Spreadsheet-June-1993--P107.aspx

While this isn't a way to calculate solder paste volume, You can calculate the solder paste release ratio for the best possible joint.

Formula = Cross-sectional area / Wall area
Targets = 0.67 or greater for standard stencils, 0.55 for LaserSlic, and 0.45 for UltraSlic material (Fineline Stencils)

With these calculations you will get the best release ratio possible. My solder joints always pass IPC specs.

If knowing the area of solder paste presented and viscosity of the solder paste means we can easily find out the volume of the solder paste.

In the early 90's, EMPF, a joint effort with the military and business on electronic assembly, published a paper entitled Solder Joint Volume Calculation Using A Spreadsheet.

While this was done in Lotus, the necessary calculations were easily switched to Excel. I have used this approach coupled with articles on aperture ratio's for many years with excellent results. There are parts, however that do not follow the formula because the volume of paste needed to properly solder the part is not necessarily the volume of paste to keep the part in position during placement and transportation.

It is, however, a great starting point. When doing the calculations, you can put a min solder height in as well as a max solder height based on the the component metalization end.