Now that we’ve defined what it means to rework a PCB, let’s dive into how it’s done! Here are six steps you can take today to improve the skill of your design.
1.Make Your PCB Larger
Prototypes rarely need to be as small as final designs, so make your prototype PCB larger and put it in an off-the-shelf prototype enclosure. This will allow you to release your design faster, wasting less time on tricky PCB layout issues. To test as part of your system, connect your larger prototype PCB into tight spots in your system using cables. A larger PCB is easier to work with on the bench top, easier to probe, and easier to rework.
2.Use Larger Components
While it may seem cool and cutting edge to use micro BGA components or 01005 package resistors, you probably don’t need them – and you certainly don’t need them on your prototypes. You cannot easily rework parts that are smaller than a grain of sugar. For prototypes, consider using parts no smaller than 0402 unless absolutely necessary, and using parts with exposed leads instead of BGA or QFN packages, where possible.
In the µModule Product Guide, Linear Tech presents a bitter reminder of just how ugly rework can be. (This is what happens when you don’t design for rework.)
3.Allow More Clearance
Rework is easier and more reliable when there is more clearance around parts. For BGA, QFN, or other packages without exposed leads, allowing ample space around them is critical for localized rework reflow heating. For high-density connectors or press-fit connectors, clearance is necessary for tools that leverage against the PCB. For mechanical components such as heat sink snap-in pins, clearance for hand tools such as pliers is important.
4.Layout Strapping Resistor and Jumper Options
The goal of circuit designers is to design everything right the first time. But you can save yourself a lot of time and trouble by designing in strapping resistors to allow you to pull all your setting pins high or low with a simple rework. Or consider adding extra unpopulated filter circuits around your power regulators. You can even design in entire secondary circuits that can be implemented by just moving a jumper from one header to another.
5.Use Programmable Parts
Where possible, use programmable parts to allow rework flexibility. Adjusting firmware is usually easier than cutting traces or re-soldering components. Look for locations where programmable voltage regulators, microcontrollers, or CPLDs/FPGAs might make sense in your prototype, even if it’s not what you intend for your final design. Simple, inexpensive programmable parts allow you to rework your design without risking your hardware.
6.Install All Programming Headers
Not following this guideline has bitten many, many designers. Spend the extra few cents to install every last one of those pesky programming headers. I can hear you all saying, “But I can program that part through the JTAG chain,” or, “We’ve built that part with the same code for a long time – there’s no chance we’re changing it.” Experience says that you’ll thank me later if you install all of your programming headers on your prototypes.