A Sticky Situation

Figure 1. Two SMDs have misaligned during reflow due to uneven pad sizes and disproportionate solder liquidus surface tension.

Figure 2.

Figure 2. Temporary solder mask glue applied to two corners of each component prior to reflow keeps them in place.

Figure 3.

Figure 3. Post-reflow, the adhesive is easily removed, and the SMDs are perfectly positioned as they should.

The surface tension of liquidus solder exerts a considerable pulling force on a component during reflow. This is why, once upon a time, small components could be relied on to self-align on SMT pads during reflow. They still can, of course, providing that all things are equal, such as pad dimensions all around. But if they’re not, you can expect problems.

In this case, two components had shifted away from their center location on the PCB footprint during reflow (Figure 1). This was due to the fact that a large SMT pad on one side of the components, see photo, was exerting a stronger pulling force on the component than the ordinary-size pads on the opposite side. More surface area means more pulling force, and consequently component misalignment. It doesn’t matter that the placement machine put the part in the right location beforehand.

Mechanical fixturing simply wasn’t a practical solution. Instead, two dots of temporary solder mask glue were applied to the corners on one side of each component, prior to reflow, to hold it in place. The glue acts as a temporary adhesive and prevents the parts from moving during reflow because it is stronger than the pull of the liquidus solder. After reflow, the glue is easily removed, and the SMDs are perfectly centered. Problem solved!



A Bit More On the LGA

After my last post about LGA land patterns, I received a couple of questions asking for more detail in a few areas.

The LinearTech  LGA apnote (LTM46xx series) shows planes on the mounting layer interconnecting pads that are solder mask-defined. This is supposed to be for heat dissipation. Will smaller copper-defined pads and vias to full internal copper ground and power planes provide adequate cooling?

What about using LGAs on the same layer as BGAs? BGAs have copper-defined pads? We’ve been sending 1:1 soldermask gerbers to the fab house so they can adjust per their process. Can this be done selectively so the SMD LGA pads don’t grow bigger? What kind of Fab Note should be in the “Readme” file?

Also, please warn LGA users to be careful using wizards (e.g., Pads Layout) to generate the pad numbering. Linear Tech’s LGA does NOT follow the standard BGA alpha numeric numbering. I don’t know about other LGA manufacturers’ numbering systems but … double-check the pad numbering and avoid this nasty snake bite!”

First, as far as cooling goes, the answer, unfortunately is “it depends on how closely to the limits you are driving to part.” You will get best results with more surface copper. That said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have lots of surface copper and vias to the internal and back side planes, but that’s not always possible. The vias that are not under the LGA pads can be left open. Any vias in an area to be soldered must not be left open. Ideally, you would have them filled with a thermally conductive material and plated over. You do have some flexibility to reduce the surface copper and replace it with vias to other planes, but ultimately, the final answer will only come from your design testing.

You can have NSMD and SMD pads on the same PCB. How to do it is the big question here. Many fab shops will make their own decision on what is “best” for your PCB in this regard. I would speak with the board house and get its recommendations on how best to specify what you need in terms of NSMD and SMD mixed. You’ll probably have to follow a slightly different procedure for each different fab shop.

I would double-echo the comment about using caution when using wizards to create a land pattern. Not all manufacturers follow the same numbering scheme. You could get bitten badly with this one.

Duane Benson
Who was that soldermask defined man?