[sdiy] Low aspect ratio SMD parts? ("pancake" shaped flat parts)

[brianw]

There is a long list of SMD part sizes.

1206, 1020, 1010, 1005, 0915, 0815, 0808, 0805, 0707, 0705, 0612, 0606, 0603, 0602, 0508, 0505, 0503, 0502, 0406, 0404, 0402, 0202, 0201, and I didn't list anything larger than 1206 for size considerations.

In other words, you don't even need to tombstone to add a series component that wasn't part of the original design. Just replace the 1206 with a pair of 0603; replace the 1005 with a pair of 0502; replace the 0805 with a pair of 0402; etc. Granted, some values of capacitance or resistance simply aren't available in smaller sizes, but even in those cases there is often a more expensive part that is smaller than the budget-priced versions. For such hand-worked modifications, the expense of a single part should be inconsequential (compared to buying enough for 10,000 units of a product - at which point you'd just redesign the PCB since that would be more cost-effective).

You could even have a small PCB made - either regular thickness or one of the thinner PCB options - with two pads on the bottom to match the circuit you're modifying, and a three or four pads on the top for your series components. There are plenty of PCB designs with metal on the edges that are designed to be soldered down onto another PCB (see most daughter boards for RF or fancy CPUs).


To your question about SMD with pads on top and bottom - I haven't seen that. But there are resistors and capacitors where the long side is conductive, instead of the usual short side. I recall seeing these with precision current sensing resistors, but that may not be their only application. I've also seen capacitors with four solder pads: two on the usual short ends, and another pair in the middle of the long sides. I forget what those were designed for...

Brian


On Feb 6, 2023, at 7:50 PM, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
> Hi all,
> if you want to modify an SMD circuit by adding a component in series
> with another component, then you can obviously lift one side of the
> existing component, and squeeze in another one between that and the
> pad. For example, if you have a ceramic smd package capacitor, and you
> want to add a series resistor, you can lift one side of the capacitor
> (essentially desolder the capacitor and resolder it in a tombstone
> configuration) and then put the resistor on the free pad, then bridge
> the two sides of the parts that are up in the air.
> 
> However, that's not always possible. Sometimes there isn't enough
> vertical space for this, or maybe you have a larger smd part that
> needs stability, or you have a part that needs to go in series under
> one lead of a SOT package or a ribbon connector.
> 
> What you'd want at that point is eg a resistor that has the footprint
> of a single smd pad, and it's basically a layer of metal (for
> soldering) at the bottom, then a flat pancake of resistive material,
> then another layer of metal. With the operative being that it should
> be flat and wide, like a pancake. Hence "low aspect ratio".
> 
> I was wondering if anyone has ever seen parts like that?
> 
> Cheers