Flipped Capacitors

Different Capacitor Shapes

Most electronic design engineers know SMD capacitor parts come in the “standard” sizes of let’s say 1206, 0805, 0603,0402, 0201 etc. The common element of these parts is that their size is longer along terminals(between 2 pads you actually solder) than along the width of the part. Most times width of the part is half the length. There is a different class of capacitors which are wider than long. These are called flipped or reverse geometry capacitors. So the sizes will be 0612, 0508, 0306, 0204 etc. (To be honest, you might not find a flipped capacitor for every “standard” size capacitor though)

Why do we want to use a flipped capacitor? In one of my older posts, I explained that a capacitor in real life is not a capacitor by itself. It has an equivalent series resistance(ESR) and inductance(ESL). For an ideal capacitor, we would want to have ESRs and ESLs to be zero. Since there is an ESL in a capacitor, you usually see a V-shaped impedance curve with frequency in capacitor datasheets. The impedance value keeps getting low till a point and then it increases due to ESL. Here is where flipped capacitors come into the picture. Flipped capacitors are designed specifically to reduce ESL. When you flip the width and length dimensions of a capacitor, it reduces the soldering pad distances(or the length). Lower travel length means lower inductance and lower conductor length(Not to complicate it more, loop area also reduces, which is a good thing). So current needs to travel a shorter distance. A wider flipped capacitor effectively can have an impedance curve which is similar to 3-4 same standard-size capacitors. That means overall lower BOM count and layout space is lesser. For these reasons, these wider capacitors are starting to gain acceptance in the PCB design space.

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Advanced: NSMD vs SMD -> SolderMask Pad Options

When doing a high-density BGA type PCB routing, you might across these terms. NSMD stands for Non-Solder Mask Defined and SMD for Solder Mask Defined pads. In order to understand the difference, take a look at the image of a 0.4mm Pitch BGA package with 3 selected rows. The first selected row contains NSMD pads, in which the solder mask is removed from an area around the actual physical pad. On the third row, the solder mask opening is smaller and it overlaps the actual pad by some margin. The second row is a compromise where the actual pad size equals the solder mask opening.

Now why are there different ones and what are the pros & cons of each?

NSMD pads are one of the most commonly used ones for large components because it gives the manufacturer a bit more tolerance when they are applying the solder mask layer. In case a slight misalignment happens between the copper layer and solder mask layer in the PCB manufacturing process, having an NSMD pad is beneficial because the pad won’t get hidden under the SM layer as the opening is larger. But these days with a good manufacturer, the chances of misalignment are much lower than before. Now, where does this go bad? Assume you have a BGA part with a pitch(centre to centre spacing between 2 adjacent pads) of 0.4mm or less, now if you have a larger SM opening, you very small width of SM between the pads. That can have a chance of peeling off in the heating/reflow soldering process. Another disadvantage is that you would not be able to route a track between the tracks because the solder balls can potentially bridge to these tracks. The worst-case scenario is that the solder mask width is too thin and the solder balls bridge to an adjacent pad altogether. This might not happen on prototype runs but in large productions, it can affect the yield of the PCBs. Also, another advantage of SMD is that it provides mechanical strength to the PCB pad(the chance of it getting ripped out from the PCB is lesser).

You clearly see that SMD might be preferred over NMSD for smaller pitch parts, but what’s the issue with SMD pads? Since the opening is smaller than the actual pad, for BGA pads you might find that the solder balls might not make full contact with the metal pads as some portion is masked. This can potentially cause mechanical reliability issues wherein the part is not actually soldered properly to the board. Now the other issue is the heat/current transfer. If a pad is designed to carry heat or a particular value of current, reducing the contact area can have issues in the functioning of the part.

So which one to use in your design? As always, it depends on a case-by-case basis. It’s always about balancing your pros and cons. It’s also heavily dependent on how good your manufacturer. Datasheets usually tell you what you need to do. When in doubt and you have no way of taking a decision, follow the datasheet. Keep these in mind for your next high-density design.

PS: I know it’s a slightly advanced post. I tried my best to simplify and explain this, do let me know if you prefer these kinds of posts over normal ones.

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