Round vs Rectangular PTH Calculations

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   Can anyone tell me why there is a difference in the land size between a 1.44mm round and a 0.8x1.2mm rectanular pin?  I am using the proportional enviorment.  The Finished hole calculates out the same 1.64mm, but the land is 2.49mm for round and 2.14mm for rectanular.
I put this on the LP Wizard Forum, but I see Footprint Expert is using the same calculations.  

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rickdehart Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 18 Apr 2012 Location: Austin, TX Status: Offline Points: 184	Post Options Post Reply Quote rickdehart Report Post Thanks(0) Quote Reply Topic: Round vs Rectangular PTH Calculations Posted: 09 Nov 2012 at 12:14pm
	Can anyone tell me why there is a difference in the land size between a 1.44mm round and a 0.8x1.2mm rectanular pin? I am using the proportional enviorment. The Finished hole calculates out the same 1.64mm, but the land is 2.49mm for round and 2.14mm for rectanular. I put this on the LP Wizard Forum, but I see Footprint Expert is using the same calculations.



Jeff.M Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 16 May 2012 Location: San Diego Status: Offline Points: 481	Post Options Post Reply Quote Jeff.M Report Post Thanks(0) Quote Reply Posted: 09 Nov 2012 at 1:35pm
	Since there is currently no component in FPX that allows both round and rectangular leads (just round and square) I checked this out using the Axial calculator for the round lead and a DIP for the rectangular lead with a proportional environment in both cases. The results were as follows: 1.44mm round lead produces as 1.6mm hole and a 2.4mm pad; 0.8 X 1.2mm lead produces a 2.2mm hole (size based on the diagonal of the lead) and a 2.7mm pad. These results are pretty much as expected but not the results you report. Can you explain the method you used to arrive at the values you provided?

rickdehart Members Profile Send Private Message Find Members Posts Add to Buddy List Advanced User Joined: 18 Apr 2012 Location: Austin, TX Status: Offline Points: 184	Post Options Post Reply Quote rickdehart Report Post Thanks(0) Quote Reply Posted: 09 Nov 2012 at 1:47pm
	To keep things simple. I used the Radial (LED) and set b= 0.80, L=1.2 tolerance to +/-0, the round off to 0.01 and the placement to 0.01.

Jeff.M Members Profile Send Private Message Find Members Posts Add to Buddy List Admin Group Joined: 16 May 2012 Location: San Diego Status: Offline Points: 481	Post Options Post Reply Quote Jeff.M Report Post Thanks(0) Quote Reply Posted: 25 Feb 2013 at 9:35am
	It's because the elements that contribute to the Proportional calculation (Fabrication Allowance, Hole- over-Lead, etc.) are based solely on the lead 'b' dimension not the hole size. You'll notice that the Fabrication allowance for the two pads in your example are different. This is to prevent a slotted hole from having a disproportional fabrication allowance from an un-slotted hole with the same drill size.