PCB Design Perfection starts in the CAD Library - part 10

Part 10 - PCB Design Perfection Starts in the CAD Library

Part 10 - SMD Bottom and Flat Lead Forms


Recall in last column, the pin (component lead) pitch and the overall body height are continually shrinking. This is why the SSOP and TSOP land pattern names have to be dropped from the standard — S = Shrink for Fine Pitch and T = Thin for low profile height. If these two values are constantly changing,  where is the line drawn? Whose part is Thin or Fine Pitch and by what measure? The Gull Wing lead has hit the wall at 0.4 mm pitch.


Most assembly shops will try to convince you to swap that part out of your design for a larger pin pitch however, no-lead SON and QFN lead styles are being produced and manufactured at 0.4 mm pitch with no problems. The finer pitch parts have more I/Os and a smaller footprint with a much lower profile than J-Lead or Gull Wing packages, so it's obvious that the component industry is going to be no-lead or bottom only flat lead or side lead packages.


The "Corner Concave" lead form is primarily used for the oscillator component family. It's perfect for oscillators because it only has the four leads that are necessary for the standard oscillator requirements. See Figure 1.  

Figure 1


"Cylindrical End Cap" goes with the Metal Electrode Lead Face (MELF) component family for resistors and diodes. I just can't figure out why the industry still produces round components, but engineers continue to design them into their schematics. In some cases these components can easily roll off the workbench or circuit board before they have been soldered into place because of their cylindrical shape and small size. As such, there is a joke that suggests an alternate meaning for the acronym MELF : " M ost E nd up L ying on F loor".


Figure 2

  Mostly, this happens if the mechanical pressure of the SMD placer nozzle is too low. If the MELF resistors are placed into the solder paste with enough pressure this problem can be minimized. But be careful if SOD-80 glass diodes are used; those types are different compared to MELF resistors and the mechanical robustness is limited. See Figure 2.


Every PCB designer is familiar with the Gull Wing lead (See Figure 3), but it has 2 separate rule sets that are defined by the pin pitch:
  1. Less than 0.625 mm pitch (fine pitch)
  2. Greater than 0.625 mm pitch


These component families include:

  1. Ceramic Flat Package (CFP)

  2. Ceramic Quad Flat Package (CQFP)
  3. Quad Flat Pack (QFP)
  4. Small Outline Diode (SOD)
  5. Small Outline Package (SOP)
  6. Small Outline Transistor (SOT)
  7. Transistor Outline (TO)

Figure 3


Figure 4

  The "Inward Flat Ribbon L" is used for the Molded Body component family. This includes polarized and non-polarized capacitors, inductors, resistors and LED's. The most popular is the tantalum capacitor. See Figure 4.


The "J-Lead" is one of the original SMT leads that became popular with the PLCC (Plastic Leaded Chip Carrier) and then the SOJ (Small Outline J-Lead). This lead form was very popular because the leads were stable and easy to manually solder. And the solder joint was easy to inspect. However, with the advent of high speed technology, lead-free solder, low profile fine pitch component packages, this lead form will be one of the first SMT leads to become obsolete. See Figure 5.  


Figure 5


The "Outward Flat Ribbon L" lead is used to reduce the footprint size of SOT and SOP components. It's similar to a gull wing lead, but the lead bends downward immediately coming out of the component body and then is bent flat. The flat lead is very compatible with lead-free solder alloys and takes up less PCB real-estate. Since there is no heel and these components are so "low profile", the land is usually trimmed at the nominal component body. If the land (pad) protrudes under the component body, it will end up with solder on the bottom of the component during reflow. See Figure 6.


The Outward L lead also has 2 separate rule sets that are defined by the pin pitch:
  1. Less than 0.625 mm pitch (fine pitch)
  2. Greater than 0.625 mm pitch

Figure 6


Figure 7

  The "Rectangular End Cap" is used for discrete resistors, capacitors, inductors and LED’s. This lead type is by far the most popular due to the component count. An average PCB has 80 - 90% of the total part quantity using the rectangular end cap lead form. These components are easy to manually solder and easy to rework if necessary. However, the new DFN (Dual Flat No-lead) component with bottom only terminations is better for lead-free solder and part placement density. See Figure 7.


The "Side Lead" comes in 3 different lead styles (See Figures 8, 9 & 10):

  1. Concave
  2. Convex
  3. Flat


The side lead is on the outside perimeter of the component body and normally runs from the bottom to the top of the component. It is used widely for chip array's and LCC (Leadless Chip Carriers) and has 2 different sets of solder joint goals depending on the lead pitch:

  • Pitch is less than or equal to 1 mm
  • Pitch is greater than 1 mm


Figure 8

Figure 9

             Figure 10


Now we covered all the component lead forms. Next week, we’ll dive into the various component families and relate their lead forms back to this post. It's going to be interesting to find out what new component lead will be invented by a component manufacturer in the years to come, but when they do, the IPC-7351 land pattern committee will be there to develop the optimized solder joint goal chart.


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