Solder Paste Print Volume vs. Alignment?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting, here to answer your questions on surface mount process, technique, equipment and materials.

Jim
Here's an interesting, high level global question: Which of these factors is more critical to the solder printing process, solder paste volume or solder print alignment?

Phil
A very good questions. Well, I'll answer with a question. What's more important in your car, the engine or the brakes? The answer to your question is: they're both equally important. You can't really have one without the other if you want to have good printing.

I'm a little curious what spurred this question, because generally, by adjusting for solder paste volume and height, you should not be sacrificing anything in alignment, and vice versa. They are independently controlled parameters. Even if they were interrelated, you shouldn't have to sacrifice one over the other.

Jim
That's a really good point, that they are pretty much independent variables when you set up a printer.

Phil
Inadequate alignment could result in the same problems you would see with inadequate volume.

Jim
Stepping back farther, if the alignment is off, when you place the component - you might only partially be in to the paste. The component could move before it got to reflow because it wasn't adequately held, there wasn't enough tack surface contact with the paste.

If it's off between adjacent fine pitch pads you could start to get bridging. Of course volume is important, particularly on area array packages, where the joints are under the package and you can't visually inspect them, and they're very difficult to repair and touch up.

Phil
So the bottom line is that you want to do your design of experiment in your printer set up properly. You want to make sure that all of the proper parameters are set up, alignment, volume, print height. Ideally, all of these are checked at least on an AQL basis with post-print inspection.

On that note, whether you're reflow soldering and you've got your print parameters right or not, or you're wave soldering or selective soldering, whatever you do ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother. 

Gentlemen,

It is always interesting listening to your board talks.

As above subject is known to be "the yield" parameter of most importance for the bottom line result of any printed circuit assembly process, my curiosity was called for.

I do agree on all of your general comparisons and I'm fully aware that these Board Talks mainly are aiming at "the proper focus of any process". In that sense I'd like to add some comments.

First, the parameters of importance have to be isolated from each other to describe the influence of the others, one at a time.

Theoretically three axes symmetry for the final paste deposit is of huge importance, as the following reflow implements a number of forces, that influences the final position of any component, some more, some less, depending on physical size and footprint of the components - see below.

Component - When judging influence of volume / alignment it will always be done "thru observing the mounted components", which leads to an important parameter for the components invoked: The quality of components in the sense that they are supposed to be ideal with respect to footprint symmetry and wettability (yes I do know that its imaginary).

Volume - Volume @ 100% perfectly aligned: Mainly affects mechanical behavior - product quality

Alignment - Alignment @ 100% correctly dosed volume: Mainly affects bottom line - in the sense that 100% yield after reflow (which of course is a city out in No-where) reflects the lowest possible production cost (on that specific process).

And yes production of printed circuit assemblies is still achieved thru a number of compromises, which again is different from production setup to production setup.

Hoping that my way of thinking can inspire somebody to a yield full compromise:-)

Med Venlig Hilsen / Kind Regards.

Proper solder paste brick placement depends on proper stencil to board alignment, board pattern accuracy and stencil aperture location accuracy. See: http://www.lpkfusa.com/SMTStencil/whitepapers/index.html

If you do not place the solder brick in the right spot you may get incomplete joints, solder balls, or even shorts. And remember that lead free solders do not wick or creep as well as the leaded solders, which means you more likely end up with incomplete joints or even bare copper, depending on the surface finish.

If the solder brick is placed in the proper location, but an unpredictable and insufficient amount of solder was released from the stencil the result may be incomplete solder joints or misaligned components. The cause for this is typically related to an area ratio that is too small, poor aperture wall surface or a poor choice of stencil material.