Too Close for Comfort

This is a little bit like the old college prank of trying to see how many kids can squeeze into a telephone booth. Pretty soon everyone’s too close for comfort!

In this PCB assembly challenge, someone made a mistake and created a layout for rows of dual-flat no-leads (DFN) SMT packages without taking into account the size of the component bodies. The footprints are too close together, and the bodies of the components are touching.

Because they don’t all fit, as the packages are lined up there isn’t enough room, and alignment issues develop for some of the IC locations. They’re forced off their footprints, while others appear to be acceptable.

Figure 1

Figure 1. With DFN footprints too close to one another, component bodies are actually touching and causing alignment issues, literally forcing others off their footprints.

Figure 2

Figure 2.

As can be seen from the photos (Figures 1 and 2), the crowding causes alignment issues for locations IC1, IC5, IC7, IC9, IC13, and IC15. Locations IC3 and IC11 seem fine.

What can be done? It’s too late to redesign and order new PCBs, and there is no possibility of shrinking the dimensions of the components.

Figure 3

Figure 3. Removal of components in locations IC5 and IC1 have allowed the rest to fit properly.

Luckily, the customer had a solution that worked: removal of the components in locations IC5 and IC1 (Figure 3). This permitted the remaining parts to fit correctly; it made “breathing room” for the rest, and best of all, was accomplished without compromising the functionality of the circuit.

Roy Akber


Picking Packages

A long, long time ago, in a place pretty close to here, picking a form factor was easy. Your CPU came in a 40 pin DIP. Your logic came in 14 or 16 bit dips. You picked resistor sizes based on their current carrying needs. Transistors and other power components got a little more difficult, but not much. It was largely a matter of power dissipation requirements.

Different story now, though. First, there’s through-hole vs. SMT. Then there’s a plethora of options beyond that. So, what really matters? A specific resistor size may come in multiple wattages. Chips come in multiple packages — often from big DIPs all the way down to tiny QFN or BGA packages. Let’s look at a few examples.

Here’s a simple microcontroller: the PIC18F25K22. It’s a pretty typical 8-bit PIC. You can purchase it in four different packages:

  • DIP, $2.05 each, Qty 100, Tube
  • SSOP, $1.86 each, Qty 100, Tube
  • SSOP, $1.90 each, Qty 2,100, Tape & reel
  • QFN, $1.86 each, Qty 100, Tube
  • SOIC $1.89 each, Qty 1,600, Tube
  • SOIC $1.93 each, Qty 1,600, Tape & reel

(DigiKey prices as of the posting date. Some are non-stock items.) There’s also the part presentation to consider; e.g., reel, cut tape, tube.

Next, look at a 1K resistor that might be used as a pull-up. (As listed in DigiKey) through-hole resistors range from 1/20W up to multiple watt packages. SMT parts range from 1/32W up to lots. Simplifying a bit and just looking at 1/4W, you can purchase 0402, 0603, 0805 and 1206 packages. For high volumes, price will be a factor, but for lower volumes, the price difference can be trivial.

If you have plenty of space to work with and you need to build by hand or for some reason need a socketed part, your choice is the DIP. If space is a bit of an issue and you may or may not hand build, then an SOIC is probably your pick. Some people will hand build QFNs and SSOP packages, but that’s not realistic in anything but rare cases.

When size, speed, current or performance need to be at maximums, selection is still not that difficult. You’ll often have far fewer options to choose from at the performance edges. But when there’s headroom all over the place, how do you decide? Why an SOIC over an SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?

Duane Benson
Peter Piper picked a peck of pickled PIC packages.