pin^2: The way breakout boards should be made

2015.08.23 06:35

Motivation

Integrated circuits generally require auxiliary passive components to function. For example, a voltage regulator may require a bypass capacitor, output capacitor, a pullup enable resistor and a voltage divider to set the voltage level. When using these popular components, I feel the tedium of bending leads, populating breadboards, etc. I noticed that these passive components would connect 2 leads on the IC and then it made sense: I would create pin^2, breakout board that not only held an SMT component, but also contained SMT pads to attach multiple passives. There would be enough pads that every possible combination of leads would be covered (n * (n - 1) / 2), hence the name pin^2.

SOT23-6

SOT23-6 is a popular form factor for many SMT components such as transistors, mosfets, voltage regulators and even EEPROMS. Having 6 pins, there would be a total of (6 * 5 / 2) 15 total pads on the breakout. Since a breakout board has through-hole pins, those pins would also act as vias so I could attach pads to both sides of the boards. I produced 2 types of breakout boards: one for 0402 sized passives and 1 for 0805 passives.

Circuit Constraints

Naturally, some constraints limit what kind of circuits we can populate on these boards.
  1. The circuit must not have more than 6 nets.
  2. Only the SOT23-6 component, termed hub component, can connect to all 6 nets
  3. No more than 1 component must link 2 nets; i.e, no two components are in parallel
This may sound limiting but many circuits satisfy these constraints.

Sample Application: PWM DAC

One way to generate an analog voltage from an MCU is to generate a PWM signal and run it through a low pass filter. In this particular case I wanted to programmatically adjust the contrast on an LCD character screen. I needed to modulate a 5V voltage level using a 3.3V microprocessor, so I elected to use a SOT23-3 NPN transistor. I found the following circuit and modified it slightly for 3.3V and my PWM frequency:

When designing a circuit for this breakout board, I found it easier to label all the nets with which pins I intended to use. This circuit has exactly 6 nets, thus obeying constraint 1. The SOT23 NPN transistor is the hub component and only touched 3 nets. None of the 4 passive components are in parallel, obeying constraint 3. The next step is to determine which pads to solder:

Let's map the pins to their functions: I feel lazy and plan to solder the 0805 components, so let's take a look at the front and back pads for those boards:

The resistors will end up on the front side of the board while the capacitor will end up on the back. Assembly is straightforward; the whitespace on the PCB even allows me to label the pins. Peruse the following gallery to see how much tidier the final result is:

Conclusion

The pin^2 approach has yielded me a PWM DAC adapter that I can add to my prototyping toolbox. Although the pin^2 approach has some constraints, there are still many useful applications for it. If you are interested in this board, please see my tindie store in the references and get yourself a board! If you have any questions please see my about page for contact info or go to the hackaday.io project page.

References

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