I see a lot of inconsistencies when it comes to PCB footprints in different software.
So in this video, I’ll show the fundamental building blocks of a QFN footprint (land pattern).
By the end of this video you will know what to include in your land pattern regardless of your PCB design software tool.
Hi, do you understand the different layers that create a footprint? Let’s look at the top pains in design for manufacturing PCBs. Do you know what they are? Well according to this article and from personal experience, the top five problems are:
- Incorrect PCB footprints and land patterns
- Component location and rotation
- Component clearances
- Insufficient solder mask coverage
- Errors in the bill of materials (BOM)
So we really need to understand how to make a good footprint and that will help us use our software tools better. OrCAD does a really good job at this, but as should any good ECAD software tool.
Let’s unpack the structure of a good land pattern
- Silkscreen Pin 1 Polarity – this really helps PCB manufacturers assemble the part in the right direction, whether they’re using a machine or soldering the part manually. If you don’t have this marker, and the assembler places the part incorrectly, then it won’t work.
- Assembly Pin 1 polarity – this marker performs the same function but for the automated machine assembly of your components. This is just for the machine. So silkscreen is moreso for post-inspection and for people.
- Also in general, anything silkscreen, is for humans to observe while assembly details are for the machines
- IPC-7351 Land Size Calculation – the land is exposed copper for the device pin to ‘land’ onto.
- For example look at these surface mount chips – some of their lands are showing
- For the processor, the lands are covered in solder
- This requires specific calculations you can find in the IPC
- 1:1 Scale Paste Mask – What’s a paste mask? – Solder paste is solder material that you spread across the exposed copper pads
- Center of gravity origin – this is helpful for like a pick and place machine so it knows about which axis to rotate a component.
- IPC-7351 thru hole vs surface mount
- 1:1 Solder Mask
- Prevents soldering
- Protects traces
- Negative film
- Expansion
- 50% Paste Reduction on thermal pad broken into squares
- Why? It can get messy
- PErfect amount for soldering
- IPC-7351 3-Tier placement courtyard external boundary with 0.01 mm line width
- Least
- Nominal
- Most
- Assembly Outline Mapped to Maximum Component Body with 0.1 mm (4 mil) line width
- Useful for assembly placement
- Silkscreen outline line width 0.12 mm (~5 mil) visible after assembly
- Part should be here
- Helpful for inspection
- 2 Ref Des – Silkscreen & Assembly
- Silkscreen ref des – goes outside the land pattern
- ASsembly ref des – inside the land pattern
- Benefit? You can match the parts on the PCB with the parts on the schematic. You can check your design is right
- 0.15 mm Land to Land Gap DRC checked
- Thermal connections to the PCB
- Helps with soldering onto the PCB
- Make sure it’s not too wide or small
- 0.12 mm silk to land gap
- it can lead to several problems during soldering, assembly, and inspection if not far enough away from solder mask
- If too close to pads, it could melt onto the pad and cause problems soldering the pin into or onto the pads
Okay so we’ve covered all the important parts of a QFN surface mount land pattern, which covers most of footprints anyway. Every feature here prevents problems with assembly and manufacturing your printed circuit board. So try making a footprint that has all this, whether you use OrCAD, Altium, KiCAD, it’s up to you. One thing is for sure, you’ll reduce problems for your PCB design by at least 40% the more careful attention you spend on making or using quality footprints.