This advanced ASIC power package is defined in Freescale Application Note AN2467 as: "The PQFN is a surface mount plastic package with lead pads located on the bottom surface of the package. All PQFN packages have either been designed with a single exposed die pad (flag) or multiple exposed die pads, depending on device requirements and intended application. The industry standardization committee, JEDEC, has given a registered designator of MO-251 to describe the family of single exposed pad PQFN packages." The particular device data sheet we'll use in this example is the MC33922/MC34922 dual 4.0-A power H-Bridge.
Top Copper: To prepare the footprint, open the PCB-layout program's library editor and create either a new part in an existing library or a new library for the new part. Then select the menu option creating a new package. At this point, most PCB-layout software packages will present a 2D, layered drawing screen.
Now turn your attention to page 19 of the data sheet for the packagedimension drawing for the 29-Terminal PQFN plastic package, Case #1469-02 . Note the dimension units, and set the grid on your drawing screen to the same units (millimeters in this case). Also note the dimensions of the package outline (10 by 10 mm). For reference, draw this outline around the drawing origin, using a temporary line that you will later delete.
The next step is to obtain the dimensions of the portion of the terminals (leads) that will sit in intimate contact with the PCB, since you'll need these to determine the size of the corresponding top copper pads. In this case, there are four terminal sizes: 16 terminals are 1.525 mm (nominal) long by 0.565 mm (nominal) wide; eight terminals are 0.775 mm (nominal) long by 0.565 mm (nominal) wide; four terminals are 1.005 mm (nominal) long by 0.565 mm (nominal) wide; and one terminal (the exposed heatsink pad) is a huge 9.4 mm (nominal) by 6 mm ( nominal) with approximately 1-mm radius bites taken out of each corner.
Finally, note the pin-center-to-pincenter spacing around the part's periphery— nominally 0.8 mm on centers. You now the basic dimension data needed to drawing the package footprint.
Now, for the first guidelines:
Guideline 1: For closely spaced, finepitch terminals, create the pad size width slightly less wide (say, 0.05 mm) than the lead/terminal width, and extend it outward about 0.15 mm longer than the lead/terminal length.
This helps preserve a minimum solder mask between the terminals and provides an externally visible solder fillet at the end of the terminals. The pad widths can be made wider for the production design. For prototyping, though, you want to make sure the risk of solderbridging between terminals is minimized so you don't waste time troubleshooting shorts caused by excess solder (a likely scenario with the hand-stencil-applied solder-paste process).
For the 16 1.525- by 0.565-mm terminals, this guideline equates to a PCB pad size of 1.675 by 0.515 mm. Thus, create this pad size in your PCB-layout software (for example, by selecting "change," then "Smd," and typing in 1.675 x 0.515). Do the same for each unique pad dimension (except the large heatsink pad; we'll get to that later).
Guideline 2: Create the footprint as it would be viewed looking down at the top copper of the PCB.
This means the pin 1 location and direction of counting will be as if you were looking down through the top of the part with X-ray vision. Many PCBlayout programs automatically number the pads in the order in which they're created on the drawing. Make sure to place the first pad in the pin 1 location, and increment in the direction of pin numbering as viewed from the top of the package.
To place these pads, you need to temporarily change your snap-to-grid spacing so that it equals one-half the nominal pin-center-to-pin-center spacing (in this case, 0.4 mm). Arrange the pads so that they're oriented identically like the terminals on the package. Remember to position them such that the length extends beyond the package perimeter by an additional 0.15 mm. Continue placing the pads going around the perimeter in pin-number order, remembering to change pad sizes when required, and re-orienting the pintopin baseline space when continuing onto adjacent sides |
