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SMT R-chain layout choices

Hi,

I have an SMT design that must support three R choices at final assembly. R1 < R2 < R3

Differential stuffing is not an option. Boards need to be identical up to final assembly/test.

One option is to install three R's in a series chain: R1, R2 - R1, R3 - (R2 + R1) and short none, the last, or the last *two* R's depending on the configuration desired.

Another approach is to arrange three paralleled resistors such that R1 is formed by R3 || Ra || Rb and R2 formed by R3 || Ra. Then, removing Rb and possibly Ra, as required.

The first approach could be accomplished with solder switches on the board -- but that eats up real estate (this is a *tiny* board!). Bridging R's with shunts seems like it could lead to components becoming "unsoldered" in the process.

The second approach could possibly be accomplished by *cutting* the R's in question (gasp!).

Are there other approaches that I should evaluate?

Thx,

--don

Reply to
D Yuniskis
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Grrrr... s/+ R1//

Reply to
D Yuniskis

You can get zero ohm resistors, but that would require removing and then reinstalling parts. So the 2nd option of removing parallel resistors sounds the least effort.

Depending upon what your requirements, you might be able to use an EEPOT (digital programmable resistor) and have a small header on the board (or simply probed as part of the bead of nails tester) so you program the resistor value after it's assembled.

Dave.

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Reply to
David L. Jones

Stressing the resistors is a bad idea (can cause debris); there are jumper blocks intended to be cut-once for trim, you only need two jumpers for selecting three values. For a dollar, you can get two position DIP switches.

If boards are swapped for field maintenance, the remanufacture of old boards will be easier if you use switches not clipped jumpers.

Reply to
whit3rd

D Yuniskis mulls

Is a trimpot too unstable for the application?

I have seen select-on-test resistors (not SMT) fitted into a pair of single-pin sockets, but that may take up too much room. Of course the resistor can then fit above SMT components, so you win some real estate back.

Snap-off sections of the board.

--
Nemo
Reply to
Nemo

Cut lands to the Rs. Route the lands on the surface, between pads big enough to jumper, and forget the solder mask. A quick cut (one at each end of the trace is better) and it's customized.

Reply to
krw

Correct. Or, adding spare sites in which to install those (as shunts). Two of them wouldn't take up *much* space, but...

Yes. Unsoldering one or two components has to be no more effort than *soldering* one or two shunts! The downside of the parallel approach is it makes testing harder -- i.e., you can only test the R1 condition (all R's installed).

Yeah, I thought of that but I'm not sure how well that would work in this circuit topology. And, these are three *specific* R values so I would have to evaluate the consequences of the pot being set "wrong".

Reply to
D Yuniskis

Yes. I was only offering it up as an "inspiration" for unusual approaches to this problem. :>

Yes. Getting them *small* enough is the issue and, as I mention in another reply, it makes testing harder as you can only test the one "none cut" condition (i.e., if the two shunted R's are incorrect values, you will never know at final test)

Two much real estate and two much money for the value that it provides (you can cut/solder components cheaper than that dollar spent)

Understood. I am hoping to keep costs low enough and reliability high enough that this is not a factor.

Reply to
D Yuniskis

Yeah, I think so. As I mention in another reply, there are a set of three or four *fixed* values that are acceptable here. I don't want to risk dealing with a pot that was set to some *other* value midway between two "legitimate" values.

Now *that* may be an idea! I.e., use a regular 1/8W resistor with pigtails and install it above the other components. This "costs" roughly (labor) the same as soldering/unsoldering components that are already populated on the board. Only needs two pads -- which can be made "far enough apart" just by straddling some larger component, etc.

It also lets the circuit be tested with all of the possible R values (the other approaches have limitations on this) easily.

I will have to stew on that...

Board is already *tiny*. I think it would be hard making a *tinier* portion that you could nick off.

Reply to
D Yuniskis

I think that would be hard to do at this scale (the board is tiny intentionally!). Putting in big pads would use up more space than the SMT R's in the first place.

Reply to
D Yuniskis

The lands only have to be as large as you want them to be. A wire between 0603 pads would be plenty big enough to cut without special tools. The pads would allow a 0-ohm resistor to be attached in case you change your mind. Just be careful not to run any lands underneath the cut area.

Reply to
krw

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