Choosing Configuration Jumpers - Solder bridges, 0-ohm resistors, DIP switches, pin jumpers

Question

I'm working on a development board, and need to let users set some configurations.

It will be used by students and engineers who are trying to build circuits on a breadboard; I'm not dealing with consumers. Usually, the settings will stay the same, but it's possible that every new project could use a different configuration.

I will be dedicating some pins to interfaces like USB and Ethernet, but I'd like to give users the option of using those pins for a different purpose. Some kind of configuration will be required. The options I've considered so far are:

Solder bridges:

Either 0603 resistor packages to allow 0-ohm resistors to be used, or nearby pads for a solder blob.
Pros:

• Cheapest option possible
• Smallest PCB area required
• No accidental changes
• Customizable by soldering directly to pad

Cons:

• Requires soldering iron to make changes
• Possible to damage board with repeated soldering/desoldering
• 0-ohm resistors require having those parts on hand.

DIP switches:

Tiny mechanical switches in an IC package.

Pros:

• Easiest to change
• Fairly durable

Cons:

• Most expensive option by far
• Might be changed by accident
• Large area on PCB
• Lowest current of the options
• Hard to make changes to PCB

Pin Jumpers

Removable Jumpers for .1" headers like those found on PC motherboards and drives.

Pros:

• Less expensive than DIP switches
• Easy to make changes to PCB
• Good balance between easy-to-change and semi-permanent
• Easy to see configuration

Cons:

• Large PCB area required
• Tallest profile; usually .5" or so required vertically
• Jumpers might be lost

Electronic Bus Switching

Use FETs or a bus switching IC like the TI 74CBT series, and control with an EEPROM/microcontroller. Suggested by Brian Carlton.

Pros:

• Small PCB area
• Configurable in software
• Can put both to High-Z or connected

Cons:

• Requires another couple ICs; medium cost.
• Less current than other options
• Has real resistance
• Can now confuse hardware bugs with software bugs and vice versa

The solder bridge option makes me worry about weakening the pad with repeated resoldering and delaminating it from the PCB. How many times can a good soldering tech change a part on 1-ounce copper with an ENIG finish? Would covering the edges of the pad with soldermask and adding thermal reliefs (for adhesion, not heatsinking) on several sides of the pad increase the durability?

Am I missing anything? What configuration methods do you like to use on a dev board?

Answer 1

For straight-up development boards (for your internal use), I go with a solder jumper or put two back-to-back (3 pads) to make an SPDT switch (here's a footprint I use). If it's small enough, it's fast to both close and open with a touch of solder or desolder braid. Using an actual resistor makes it much more difficult to rework with a standard iron.

If this is a product (as in, the Atmel STK500 development board is a product), you should use something like jumpers or DIP switches, because you don't want some dumb user poking around your board with a 1000°F iron. I'd tend towards DIP switches if you have more options or you are going to put it in an enclosure, otherwise jumpers would be cheaper.

The main question should be "is this something that will be changed as part of normal use?" If the answer is yes, requiring a soldering iron and skills is inappropriate. If it's something that an end user might modify 1-5 times (or preferably someone skilled, e.g. a lab tech), a solder jumper might be OK.

Answer 2

I've grappled with this question a few times myself. Clearly, there is a time and a place for all of these techniques. That being said, there are no hard and fast rules or conventions I know of that are standardized (or even necessarily broadly agreed upon). My take is:

• The solder-bridge/0-ohm technique is appropriate for building in an "option" into a board that is "supported" but not the typical use case. This is often called a "population option" so the idea is generally you either add the bridge once or not at all. It's not typically used for settings that change more than a couple times. An example might be an alternative signal routing through an optionally populated transceiver.
• The jumper/header technique is appropriate for situations where you want to be able to "break-in" to a signal, or for a very few "this-or-that" type settings. Also, think of using this when the setting changes infrequently. Example maybe a current probe break-in point or a "voltage select" setting.
• The DIP switch technique is appropriate for configuration settings that may change often and require a beefy/permanent user interface. Example maybe "address bits" for an IC.

Again, not an authorative answer, but my opinion / rules of thumb.

Answer 3

Solder pads are out. Just say NO. You absolutely do not want to force anyone to use a soldering iron in order to be able to use your board, EVEN IF they are good with a soldering iron.

The EEPROM/FET idea is also not a good idea, because it is not readily observable. What is the state of that FET? You need a SW suite to find out, and maybe that is not enough: what if something funky happened between what you told SW you wanted and what actually happened at the FET?

So your choices are DIP switches or pins and jumpers. You could also do pins with a wire wrap gun. I would slightly prefer the DIP, but take your pick. Any of these three is miles better than solder/SW.

Answer 4

How often will it be changed? If only very occasionally or even just once then a solder pad is fine. If you expect it to be changed often I would go with the dip Switch. The jumpers are somewhere in between.

Answer 5

Don't use DIP switches for something going out of house. Users will change them.

I would put solder holes for jumpers, but only install them on the prototype if you are changing them frequently. But for what you describe (i.e. board variants) I would go with putting the settings in your EEPROM -> can set in software, less area.

Answer 6

For things which only need to be changed "one way" [i.e. changed once but not changed back], I've sometimes seen boards with a physical wire soldered between two points and marked for cutting. That's may only work well for through-hold boards, but with the right placement equipment it might work with reflow. (I've seen through-hole resistors reflowed by using a cutout under the resistor body so the leads would sit flat on the board; if the jumper wire would stay in place during reflow I wouldn't see any reason it couldn't work).