pcb design best practices for more reliable manufacturing
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Pcb design best practices for more reliable manufacturingTRANSCRIPT
PCB Design Best Practices for More Reliable Manufacturing
Duane Benson
The Five Most Common Problems1. Via in pad
2. QFN center pad
3. Footprints and marking
4. Thermal manufacturing issues
5. BOM explosion
Why These Issues?
Things that don’t matter at .1” do matter at .01”
15mm / 0.6”
1206
01005
Why These Issues?
Things that don’t matter at .1” do matter at .01”
15mm / 0.6”
1206
01005
1mm tip
Why These Issues?
Things that don’t matter at .1” do matter at .01”
15mm / 0.6”
1206
01005
0603 package 1mm tip
Why These Issues?
Things that don’t matter at .1” do matter at .01” 2.54mm / 0.1”
0.4mm
1mm tip
Why These Issues?
Things that don’t matter at .1” do matter at .01” 2.54mm / 0.1”
0.4mm
1mm tip
Don’t Fear the New Components
New form-factor components come with a number of challenges, but they can all be easily overcome
FIVE COMMON TRAPS1. VIA IN PAD
Open Via In and Near PadBGA
Open Via In and Near Pad
Vias in or near BGA pads will cause loss of solder balls resulting in a failed assembly
First of all: No.
BGA
Open Via In and Near Pad
Vias in or near BGA pads will cause loss of solder balls resulting in a failed assembly
2nd of all: No.
BGA
Open Via In and Near Pad
Vias in or near BGA pads will cause loss of solder balls resulting in a failed assembly
3rd of all: No
BGA
Open Via In and Near Pad
Just No
BGA
We salvaged this one, but don’t count on it
Open Via In and Near PadFilled and plated over at the board house
Copper plugsConductive epoxyThermal epoxy
The ONLY viable solution for BGA via in pad is to have the vias filled and plated over at the PCB fab house
Open Via In and Near Pad
QFN, DFN, QFP may require vias in the center pad for thermal transfer or for grounding.
Open vias are not acceptable.
QFN and DFN
Open Via In and Near Pad
QFN, DFN, QFP may require vias in the center pad for thermal transfer or for grounding.
Open vias are not acceptable.
QFN and DFN
Open Via In and Near Pad
More options with QFN/DFN than with BGAs. Fill, plate or cap
QFN and DFN
Open Via In and Near Pad
This may not be acceptable with immersion silver surface finish due to corrosion. Check with fab house
QFN and DFN
Open Via In and Near PadQFN and DFN
Open Via In and Near PadQFN and DFN
Open Via In and Near PadPassives & others
Insufficient solder on pad and solder on back side of PCB
Open Via In and Near Pad
Small components are subject to a number of problems that don’t affect larger parts
Passives & others
Open Via In and Near Pad
Basically, just don’t do it. It can lead to too many reliability problems.
Passives & others
Insufficient solder on pad and solder on back side of PCB
IPC-A-610 Class Zero
FIVE COMMON TRAPS2. QFN CENTER PAD
QFN Stencil Layer
QFN Stencil Layer
QFN/DFN solder paste stencil opening is sensitive to a number of factors.
QFN Float
QFN Stencil Layer
QFN/DFN solder paste stencil opening is sensitive to a number of factors.
QFN Float prevented
QFN Stencil Layer
QFN/DFN solder paste stencil opening is sensitive to a number of factors.
QFN Voids
QFN Stencil Layer
QFN/DFN solder paste stencil opening is sensitive to a number of factors.
QFN Voids avoided
QFN Stencil Layer
Segment the stencil so solder doesn’t cover the capped vias.
QFN and DFN
Capped vias will be in between the stencil openings
Space for six vias
QFN Stencil Layer
Segment the stencil so solder doesn’t cover the capped vias.
QFN and DFN
How not to do it
QFN Stencil Layer
QFN Stencil Layer
Stencil layer openings segmented
Vias between the openings
QFN and DFN
QFN Stencil Layer
Can apply to other components as well
FIVE COMMON TRAPS3. FOOTPRINTS AND MARKINGS
Footprint and Marking
While you’re at it, make sure the footprint matches the copper on the part
Footprint and Marking
While you’re at it, make sure the footprint matches the copper on the part
It’s important with thru-hole parts too
Footprint and Marking
Diode and capacitor polarity ambiguity can cause problems in prototype and final assembly
Footprint and Marking
Diode and capacitor polarity ambiguity can cause problems in prototype and final assembly
FIVE COMMON TRAPS4. THERMAL MANUFACTURING ISSUES
Thermal Issues
Thermal mass of large component can cause uneven paste melt or component overheating
Thermal inertia
Thermal IssuesThermal inertia
Thermal IssuesThermal inertia
Thermal IssuesThermal inertia
Thermal Issues
Thermal mass of large component can cause uneven paste melt
Mask too thick can cause part to ride up and not solder well
Tombstoning
Thermal Issues
Thermal mass of larger copper area (even internal layer) on one side can cause uneven paste melt
Tombstoning
Thermal Issues
No thermal relief can cause tombstoning and poor solder joints
Thermal Issues
Meets IPC spacing requirements
But, no mask between parts. All open copper (and no thermal relief)
Thermal Issues
Meets IPC spacing requirements Result
Thermal Issues
Meets IPC spacing, has thermal relief but still has open copper between parts.
Thermal Issues
Meets IPC spacing, has thermal relief and no open copper between parts.
Thermal Issues
Meets IPC spacing, has thermal relief and no open copper between parts.
FIVE COMMON TRAPS5. BOM EXPLOSION
BOM Explosion
Feeders may be a limitation for projects with large BOMs.
BOM Explosion
R18 can be as low as 1KIf current draw isn’t an issue, use 27K
Summary• No open vias in pads• Segment your QFN stencils• Verify footprints• Clarify polarities• Watch for BOM line item creep
blog.screamingcircuits.com
For additional layout advice