Regardless of an engineer’s experience level, a practically unlimited numberĀ of variables creates challenges to overcome while designing a printed circuit board assembly. During the design phase engineers have to work acrossĀ functional groups to understandĀ the product and regulatory needs. Both are dependent upon product success and market acceptance and both add challenges for the engineer.
1 Spacing From Components to the Edge of a Board
On at least two parallel sides, keep all components at least 3.175mm (0.125ā) from the edge of the board.
Some types of connectors will infringe on these spacing requirements. Charging ports and USB ports are two examples that sometimes
require being flush with the edge of a board.
2 Space Between SMT Components and Thru-Hole Components
Do not place SMT components (top or bottom) any closer than 3.175mm (0.125ā) from a thru-hole lead.
3 Provide Global Fudicials
At a minimum, provide two global fiducials (0.762mm/0.03ā in diameter) on the opposite corners of each board.
4 Provide Fiducials With Fine Pitch Parts
If you are using fine pitch parts/BGAs, local fiducials are helpful in ensuring accurate part placement.
5 Ideal Component Size Range
For the most accurate placements use components sizes between 0201 and 56 x 56mm.
6 Distance Between SMT Parts
SMT parts should be no closer together than 0.3mm (0.0118ā) from part-to-part. The distance from BGAs should be slightly larger at 0.75mm (0.0295ā).
Important Notes:
- Follow IPC standards.
- Max arrays are 14ā x 14ā (larger sizes can be used but require additional tooling and fixturing).
- Double-sided PCBAs with mixed technology should place larger/heavier components on the top side of the board. The weight of the components could have an adverse effect during the reflow process if they are mounted on the bottom.
- Avoid placing BGAs on the bottom side of a board whenever possible.
If need be, never mount BGAs over each other, you will not be able to inspect solder connections with x-ray technology. - Try to place thru-hole components on one side of the board to avoid inefficient thru-hole soldering operations.
- ICs and passive components with uniform orientation will enhance machine placement and inspection efficiences.
- For RoHS PCB
- Higher-Quality FinishāImmersion Gold plating
- Lower-Quality FinishāLead-Free HASL
- If boards are panelized, v-scores are best for depaneling.
- If you use breakaway tabs (i.e. mouse-bites) use 0.508mm (0.020ā) drill holes.
- When a board is panelized use 6.35mm (0.25ā) rails on all four sides with 3.175mm (0.125ā) mounting holes and 1.27mm (0.050ā) fiducials in each corner.
- To improve quality and manufacturing processes, a thorough test plan should be developed and implemented prior to production.
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