Phil
Welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers here at Board Talk, but by day going as ITM Consulting. And let's look at what kind of question we have today.

Jim
J.R. writes, "I'm getting concerned about voiding in the central ground planes of my QFN components. What can I do about that and how concerned should I be about it?"

First of all I want to say that although many of the components we deal with technically are QFNs, the new IPC term is BTC, Bottom Terminated Components.

Phil
So on these bottom terminated leadless components, sometimes known as QFNs, voiding issues have been around for a while. One question is how much voiding is actually permissible.That's defined by the conductivity and as well as the mechanical strength. We've seen cases where 65% voiding is permitted. 

But if you want to try to reduce voiding there are a number of different methods. One methodology is the use of preforms. Indium Corporation has some excellent papers on this. The flux is on the outside of the preform, as opposed to mixed in with the solder paste in normal solder paste.

Jim
What does a preform look like? 

Phil
The preforms we're talking about not the old-fashioned donuts we used to remember. These are basically solder slugs and they are introduced on tape in real and put down by the pick and place machines.

Jim
So it's a rectangular square piece of solid solder about the size or slightly smaller than the pad with flux on the outside.

Phil
So the flux has less chance to get entrapped as it might in solder paste. So that's a method worth experimenting with. Your result may vary, but it's well worth a try.

Jim
There are an infinite number of variations of the window pane technique where you print solder paste on less than a 100 percent of the area using some pattern of apertures within the standard within the area of the pad. That's a whole day's work, perhaps we will talk about that another day.

Phil
So thanks for listening. And whether you're soldering bottom terminated leadless components or big half-watt resistors, whatever you do ...

Jim
Don't solder like my brother.

Phil
Please don't solder like my brother.