Transcript
Phil
And welcome to Board Talk, Phil Zarrow and Jim Hall who by day go as ITM Consulting. But here as we join you today, we are here for Board Talk and we are the Assembly Brothers. We’re here to help you resolve some nagging problems and mitigate things related to your electronic assembly process.
Well, today’s question Jim comes from JM. We are experiencing difficulties reliably soldering large ground pad on land grid array, LGA, packages using a single large window as a stencil. Should we use all four windows when dispensing solder paste for the center ground pad? The manufacturer data sheets do not provide options for using a multi-window paste stencil. Can you give a reasonable rule to follow regardless of the size of the LGA ground plane when using multiple smaller windows on the stencil?
Yeah, I’m not surprised you didn’t see it on the data sheets of the components because well, some component companies are better fountains of knowledge when it comes to process related things than others. I’m saying sometimes it is hard to find intelligent life in component manufacturers. Here we are talking intelligent life being process savvy. That know our world.
I’m not surprised, well a little bit surprised because a lot more savvy component companies have been pushing away here. But Jim, let’s talk about moving away from a single aperture, a single mound of solder into window paning.
Jim
Phil, first I would like to define the terminology. He is talking about a land grid array. That is basically a BGA without any balls to solder on the bottom of it. You just have an array of circular pads. This one has a central, a large ground plane, which could also be a thermal plane. By definition this is a BTC, bottom terminated component, a little different than the QFNs that we are most familiar with.
The difficulty with soldering this large central ground or thermal plane has been an issue for all of these types of components. The difference with an LGA vs a QFN; probably smaller pitch, more rows, maybe thinner deposit of solder if it has a finer pitch, and so forth. I’m also assuming because they are suggesting what we call window paning that is reducing, taking the aperture and the stencil for that central large plane and instead of having one large opening to print paste over the whole pad you have a series of smaller ones. This is traditionally called window panes or window paning.
The issue it is principally addressing is voiding. Voids in these ground pads or thermal pads can reduce their effectiveness so we want to minimize voiding. Although there is a whole debate about actually how much percentage voiding really matters.
But we are not going to go into that. We are assuming that you have voiding and that you want to reduce it.
Now going through a window pane is one of the most common techniques used to minimize voiding in these large planes. The bottom line is it works sometimes, but not all of the time. Are there standards? If I were forced to say something, most people who use window paning end up with about 70% coverage of the pads. It means that if you look at the total area of the pad when you are done with window paning your stencil you end up with a paste deposit that covers 70% of the pad.
That is a very general rule of thumb. They have been all over the place from 90 to 50%. In terms of shape, the number and so forth that is application specific. You are going to have to find one that optimizes it and one pattern, say a 3x3 window pane may work on one part and not on another. The investigation of soldering these pads, and how to minimize voiding, which is extensive, have shown that one of the biggest factors is the style and size of the package. Such that a 3x3 window pane may work on one LGA and it might not work on another.
It is very frustrating, but that is the reality. There are a number of other techniques. The ultimate being a vacuum oven. That almost guarantees, except in most extreme cases, that you will eliminate voiding. But most of us don’t want to go to the expense and complexity of installing a vacuum oven in our assembly line. One thing that we have found in looking at research is the solder paste formulation can significantly impact voiding.
So, picking the right solder paste formulation has a very good probability of helping reduce voiding. Manufacturers of solder paste formula have done certain things to certain formulations to reduce voiding. That would be my recommendation as a starting point.
Phil
I’ll add to that, as Jim alluded to, what degree of voiding can you tolerate? That is going to come from a design engineering, component engineering. Before you poke yourself in the eye with a sharp stick trying to get down reducing your voiding find out maybe you can tolerate for good thermal dissipation, 35% voiding, 40% voiding. Have a target, what you really need. Make it realistic before you knock yourself out and tear all your hair out.
Beyond that, as Jim mentioned, there are all kinds of experimentation going on with different window paning, circles, squares, zebra patterns. Take a look on the internet and also consult your solder paste company. Chances are, they have been doing a lot of research on this over the last ten of fifteen years.
Jim
I would like to modify your statement to say look at legitimately published research. Just because it is on the internet doesn’t mean it has any credibility. The ones we use are IPC, SMTA. These are reasonably peer reviewed tests that you can have some confidence in.
One comment I recall from one extensive evaluation said that in many cases it will take more than one technique to solve a voiding problem. You may need window paning. You may need to go to a different paste. You may have to change your printing operation. You may have to change your reflow profile. All of those have been shown to, in some cases, have a positive benefit on reducing voiding.
Phil
This has been Jim Hall and Phil Zarrow of ITM Consulting, and also The Assembly Brothers of Board Talk. Thank you for tuning in. In the meantime, however stencil pattern you are using and where you get your information from, here is a bit of information, don’t solder like my brother.
Jim
Don’t solder like my brother.
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