modules/usb_a_plug_smt.fpd: enlarge signal pads by 0.2 mm

- modules/usb_a_plug_smt.fpd: move the far edge of the signal pads out by
  0.2 mm, moving their center by 0.1 mm
- modules/INFO: explain why Molex' calculation seems to be questionable

modules/INFO

@@ -66,6 +66,22 @@ F: mmcx-f-edge
 N: 73415-0961
 
 # USB A plug for SMT
+#
+# Note: the signal pads are enlarged by 0.2 mm and their center is shifted by
+# 0.1 mm with respect to the reference.
+#
+# According to the data sheet, the leads terminate at (2.00+/-0.10 mm)/2+
+# 2.48 mm = 3.48+/-0.10 mm from the center. The reference footprint puts the
+# edge of the pads at (2.00+/-0.10 mm)/2+2.6 mm = 3.6 mm+/-0.10 mm, so the
+# maximum lead end could be at 3.58 mm while the minimum pad edge could be at
+# 3.5 mm.
+#
+# Assuming that the pad is intended to extend a bit beyond the lead, we need
+# to add 0.2 mm to compensate for these tolerances. (The pad tolerance may not
+# be intended to be used this way, however, we still have to consider small
+# errors in registration, which also amount to about 0.1 mm, so the calculation
+# remains valid.)
+#
 F: usb_a_plug_smt
 
 # SOT-323 package with counter-clockweise or clockwise pin assignment

modules/usb_a_plug_smt.fpd

@@ -3,7 +3,7 @@
 frame pad {
 	table
 	    { h, w }
-	    { 2mm, 1.2mm }
+	    { 2mm+2*enlarge, 1.2mm }
 
 	__0: vec @(w/2, h/2)
 	__1: vec @(-w/2, -h/2)
@@ -18,7 +18,9 @@ frame pads {
 	    { 3, -1mm }
 	    { 4, -3.5mm }
 
-	__0: vec @(0mm, 2.6mm)
+	set enlarge = 0.1mm
+
+	__0: vec @(0mm, 2.6mm+enlarge)
 	__1: vec .(x, 0mm)
 	frame pad .
 }