1
\$\begingroup\$

Question

Is it bad design to alter the manufacturer-recommended VQFN footprint as follows:
• Pin copper width: 0.2 [mm] --> 0.3 [mm]
• Pin copper to copper spacing: 0.2 [mm] --> 0.1 [mm]

and if so:

Is it common that standardized IC packages with many pins (QFN, BGA, etcetera) are not be able to achieve standard pricing with a PCB manufacturer?

Background

• Package: VQFN

I want to manufacturer a board with a VQFN package.

The package pins have:

  • Pin copper width: 0.2 [mm]
  • Pin copper to copper spacing: 0.2 [mm]

enter image description here

• PCB Manufacturer: PCBWay

PCBWay's capabilities specify SMT pin copper width as:

Normal cost ≥ 12 [mil] = 0.30 [mm] Medium cost ≥ 09 [mil] = 0.23 [mm] High cost ≤ 09 [mil]

• IPC-2221B

IPC-2221B specifies minimum copper spacing (under 15 [V] ) as 0.1 [mm]:

enter image description here

• Package: BGA

The IC has an alternative package as a BGA, but:

  • It is not compliant, like VQFN
    • Copper pad width/spacing is better remedied than VQFN, after alteration (shown below)
    • Via pad diameter appears to make this impossible (shown below).
  • It is costlier
  • The thermal properties are significantly worse due to the smaller package size.

enter image description here

After pin copper width/spacing alterations, the copper to copper spacing is much better than the VQFN:
• Pin center to center: 0.50 [mm]
• Pin copper width: 0.25 [mm] --> 0.30 [mm]
• Pin copper to copper spacing: 0.25 [mm] --> 0.20 [mm]

enter image description here

BGA will ultimately require Via In Pad to trace the inner pins past the outer pins.

PCBWay's associated capabilities here are:

Drill diameter minimum : 0.20 [mm] ( @ board thickness ≤ 1.6 [mm] )
Minimum weld ring (outer layer) : 5 [mil] = 0.1270 [mm] (@ Cu Thick : 35 [µm] = 1 [oz/ft^2] )
Thus:
Minimum via pad diameter = (0.20 + 2 • 0.127) [mm] = 0.454 [mm]

This leaves (0.500 - 0.454) [mm] = 0.046 [mm] between copper pads, at best.
Unfortunately, this is also not compliant and BGA is therefore not an option.

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

Copper structure width of 0.2mm and spacing of 0.2mm is not special at all. Almost every PCB manufacturer should be able to do it without problems with 1oz copper.

The according design rules for PCBWay are these: enter image description here

You seem to be looking at "SMT width" which I think somehow corresponds to component assembly capabilities. You might want to ask them just to be sure.

\$\endgroup\$
2
  • \$\begingroup\$ In good faith, I asked PCBWay before posting here. "What does SMT Width refer to? SMT package pin copper width? SMT package pin copper to copper spacing? Something else?" Their official response: "For SMT Width, it refers to the width of IC pads for smt component." \$\endgroup\$
    – kando
    Feb 3 at 19:03
  • \$\begingroup\$ PCBWay's assembly capabilities has its own page. Component size specifies: Passive Component minimum: 01005, 0201,0402. BGA minimum: 0.3mm pitch for rigid PCBs. 0.4mm for flex boards with X-ray testing. Fine Pitch Parts: 0.25mm fine pitch parts. These are absolute minimums though; nothing about increased cost thresholds, if any. \$\endgroup\$
    – kando
    Feb 3 at 19:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.