Welcome to the thread for our next Ask Me Anything about IPC Class 3/ES. Ask us questions on:
- Design requirements
- Manufacturing acceptability
- Inspection and testing standards
- And much more!
Question submission window: Through July 31st
What are the exact annular ring requirements for Class 3/aerospace boards? What do I need to do in my design to meet these requirements?
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The annular ring diameter varies based on plating thickness. For internal layers, measure from the edge of the drilled hole, and for external layers, measure from the edge of the wall plating.
Minimum annular ring diameters:
- 50 μm/2 mil for pads on external board layers
- 25 μm/1 mil for pads on internal layers
Be aware that an insufficient annular ring can cause drill breakout if the drill misses the pad center. IPC-6012 states that lifted/fractured rings and drill breakouts are not acceptable in Class 3 boards.
Pad size is determined by the type, layer count, and annular ring dimensions. Use this formula to calculate pad size according to IPC-6012 and IPC-2221 standards:
Pad size = finished hole diameter + 2 (minimum annular ring size) + fabrication allowance
Keep the drill size 4 mil to 5 mil larger than the finished hole diameter to ensure proper copper plating.
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@nikh you can read more here: IPC Class 2 VS Class 3: The Different Design Rules
What are your minimum trace width and spacing capabilities for Class 3 boards?
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Do you recommend some materials over others for class 3?
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How do you ensure the solder mask is properly aligned and free of defects? Can you do other colors than green for class 3? Like red or purple?
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Hi. What is your process for ensuring high-quality plated through holes?
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How do you ensure drill alignment to avoid breakout and meet Class 3 standards?
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Are there differences between Class 3 design and requirements for deep space?
Can you point me to any documentation or additional information for guidance with respect to designs that must meet these rigors?
I am aware of NASA’s technical pages, just looking for anything else I might use to better inform myself. TNX
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3 mils/3 mils
Yes. You are going to want high Tg at a minimum. There are suggestions in our brochure.
I believe our fab team uses laser direct imaging. We can do many colors like green, black, red, blue.
We buy only the freshest holes, straight from the farm. 
Seriously though, the drill machines now are very heavy on computer control and have almost constant feedback. They check a number of parameter either so many times a second or so many holes drilled.
The best source for that is IPC-6012ES, which specifically provides exceptions to Class 3 requirements for use in space and military avionics products, and covers requirements for rigid printed boards to survive vibration, extreme thermal cycling, and ground testing.
This is our brochure: IPC Class 3 Design Guide Download | Sierra Circuits
Takes special material sets, like full ceramic, to withstand the huge temperature shifts – like -85F to + 260F – and information is hard to come by. Best is to talk about your requirements with your fab house.
There is usually a material requirement in the design specs but Allan is right about the high Tg. Polyimide is a high-temp material which is used quite a bit for Mil spec.
Definitely the use of LDI. The laser is a super collimated light source with a very stable output range which enables excellent exposure. Each panel is also aligned and scaled individually. Standard colors are usually OK but purple isn’t one of them. We do mix some custom colors but wouldn’t recommend it for Class3/Mil spec.
Need to start with good drilling/hole wall quality. This is a combination of equipment, correct drilling parameters and maintaining quality of the actual drill bits. Class 3 usually requires Plasma etchback to form a 3 point connection so the Plasma process must be carefully controlled. Over-etched hole walls can be a real problem due to chemical entrapment and plating folds.