PCB Design Perfection Starts in the CAD Library - Part 4

Part 4 - PCB Design Perfection Starts in the CAD Library

Molded Body Components

 

The next most popular component family on a PCB design layout is the Molded Body Tantalum Capacitor (CAPM). The CAPM components have an “L-Bend” component lead form. Most Molded Body Tantalum Capacitors are metric by default including their standard EIA names –

  • 3216 – 3.2 mm X 1.6 mm
  • 6032 – 6.0 mm X 3.2 mm
  • 7343 – 7.3 mm X 4.3 mm

 

See Figure 1 for the 6032 component and land pattern dimensions. I broke 1 rule to create this land pattern. Instead of a 1.0 mm Land Placement Round-off I used a 2.0 mm Land Placement Round-off to snap the land centers on a 0.5 mm grid from the center of the land pattern. When the land pattern is placed on a 0.5 mm grid, the land centers fall on a 0.5 mm grid. This improves the via fanout seen in Figure 17.

 

 

Figure 1 

 

Figure 2 illustrates the silkscreen and placement courtyard rules and sizes. The illustration shows the component leads on top of the land for graphic representation.

 

 

Figure 2

 

Figure 3 illustrates the via fanout for a 6032 Tantalum Capacitor. If you are going to use the same size via to maintain trace/space compatibility with the rest of the PCB layout I recommend at least two vias. The placement of these vias is critical in accomplish reduced impedance and increased capacitance. It’s important that the vias be placed as close as possible to the capacitor terminal leads. In Figure 17, the 2 vias coming out the side are 0.15 mm away from the terminal lead. The vias coming out the ends on the land pattern are 0.75 mm away from the terminal leads. That’s 5 times farther away than the vias coming out the sides however some EE engineers will request all 4 vias. Since all the traces and vias are snapped to a 0.5 mm grid, this makes copy/paste much easier to manually fanout all of the 6032 Molded Body Capacitors. The dot grid display is 1 mm and the land pattern is placed on a 0.5 mm grid. All the vias in this illustration fall on a 1 mm snap grid.

 

 

Figure 3

 

See Figure 4 for the 7343 Molded Body Tantalum Capacitor I recommend a larger via size with a 1 mm land size, 0.55 mm hole size and 1.3 mm plane anti-pad. This via can carry more current and you only need two (but the EE will ask for a 3rd one). The illustration in Figure 16 snaps all the vias to a 1 mm grid system. These vias are twice the size of the previous vias but all the same trace/space rules apply. The display grid is 1 mm.

 

Because the land pattern, traces and the vias are on a 1 mm snap grid, this improves the copy/paste feature for manual fanout of all of the 7343 Molded Body components in your PCB layout.

 

 

Figure 4

 

Coming Up

Additional brief topical articles will appear in future newsletters. You can also read more detail in my blog, which can be found at: http://blogs.mentor.com/tom-hausherr/

 

Written by Tom Hausherr CID+

EDA Library Product Manager

Mentor Graphics Corporation