PCB Design Perfection Starts in the CAD Library - Part 7

Part 7 - PCB Design Perfection Starts in the CAD Library

Part 7 - SOT143 & SOT343 Components

 

The next most popular small outline transistors (SOT) component families are the SOT143 and SOT343. Both families have four pins with a larger Pin 1. The pin pitch is 1.90 mm.

 

SOT143 Family

There are three different pin configurations for the SOT143 package: Pin 1 is wide pin on upper left, Pin 1 is wide pin on upper right (reverse pin order), and the third has Pin 2 as a wide pin on lower left (custom). Figure 1 illustrates all three of these patterns.

 

Figure 1: The three pin 1 configurations for SOT143

 

Figure 2 shows the 4-pin 1.90 mm pitch SOT143 land pattern via fanout using a via with a 0.50 mm land and a 0.25 mm hole snapped to a 1.00 mm grid. The trace/space rules are 0.10 mm and the routing grid is 0.10 mm.

 

Figure 2: SOT143 via fanout

 

SOT343 Family

The SOT343 is the next most popular component family. This family of components also has four pins, with pin 1 always in the upper left quadrant. Pin 1 is also wider than the other three pins. See Figure 3.

 

Figure 3: Land pattern for the SoT343 family

 

Figure 4 shows the 4-pin 1.30 mm pitch SOT343 land pattern via fanout using a via with a 0.50 mm land and a 0.25 mm hole snapped to a 1.00 mm grid. Like the SOT143, the trace/space rules are 0.10 mm and the routing grid is 0.10 mm. The via fanout direction depends on which layer you need more routing channels.

 

Figure 4: SOT343 fanout

 

Component Leads

Both the SOT143 and SOT343 component families use a Gull Wing component lead. All Gull Wing leaded components have four different sets of land pattern rules. These rule examples are for the “Nominal Environment”.

  1. Pin pitch less than 0.625mm (side goal is -0.02 mm) considered “fine pitch”
  2. Pin pitch greater than 0.625 mm (side goal is 0.03 mm)
  3. Outward Flat Ribbon with pin pitch less than 0.625mm (heel goal is 0.15 mm and side goal is -0.02 mm)
  4. Outward Flat Ribbon with pin pitch greater than 0.625mm (heel goal is 0.15 mm and side goal is 0.03 mm)

 

The formula that calculates the difference between Gull Wing and Outward Flat Ribbon (Mini Gull Wing) is shown in Figure 5.

 

Figure 5: Dimensions for Standard Gull Wing and Outward L-Bend leads

 

We already discussed Chip and Molded Body Assembly Outlines and Ref Des. It’s important to note that the lands (pads) do not get added to the assembly drawing layer for small parts. The 2 most important things on the assembly drawing are the Ref Des and Component Outline. If the part is too small and the lands interfere with the Ref Des, then do not add the top assembly Lands to the padstack. However, if the Lands do not interfere with the Ref Des then we should add the top assembly lands to the padstack.

 

Figure 6 illustrates some of the assembly outlines for the SOT143 component family showing both standard and reverse pin versions of the Assembly Outline, Ref Des, Polarity Marker for pin 1 location and lands (pads).

 

Figure 6: Assembly outlines for SOT143

 

In figure 7, some of the various silkscreen outlines for the SOT component families are illustrated. These include the SOT143 version of the Silkscreen Outline, Polarity Marker and 0.5 mm Post Assembly Inspection Dot for pin 1 location and lands (pads).

Figure 7: Silkscreen for SOT143 components

 

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

Reprinted by permission from iConnect007