How to size a magazine rack to your boards
The four measurements that decide fit, why slot pitch is about components rather than board thickness, and how to leave room for warp.
Sizing a magazine rack looks like a two-number problem: board length and board width. It is a four-number problem, and the two that get missed are the ones that cause returns.
The four measurements
1. Board length, against rack depth
The board sits in a slot that runs the depth of the rack. The board must be shorter than the usable slot depth, and it must be long enough to be supported by the slot rather than rattling in it. On a 355 mm deep chassis, a board of roughly 355 mm is at the limit, and shorter boards are fine provided they still seat in the slot.
2. Board width, against the adjustable range
The side panels move to match board width. Every rack has a range: our gear-adjustable racks typically cover 50 to 250 mm, belt-adjustable covers 10 to 250 mm, and larger chassis extend further. Your board width has to fall inside the range with margin at both ends, because a setting at the extreme of travel is a setting with no adjustment left.
3. Component height, against slot pitch
This is the one that gets missed. Standard slot pitch is 10 mm. That 10 mm is not board thickness. It is the whole envelope: board thickness, plus everything standing on the top side, plus everything hanging off the bottom side, plus clearance.
A 1.6 mm board with a 6 mm electrolytic capacitor and a 2 mm connector underneath is already at 9.6 mm. At 10 mm pitch it will contact the neighbouring board every time the rack is moved.
The fix is not a different rack. The fix is to skip slots: load every second slot and treat a 50-slot rack as a 25-board rack. That is normal practice for tall assemblies, and it is far cheaper than damaged boards.
4. Warp allowance
Boards are not flat, especially large thin ones and especially after reflow. A board that measures 1.6 mm can occupy 3 to 4 mm of slot envelope when it is bowed. Thin boards above roughly 250 mm on a side deserve an extra millimetre or two of planning, or skipped slots.
A worked example
Assembly: 300 mm × 200 mm, 1.6 mm laminate, tallest top-side component 5 mm, bottom-side connector 2.5 mm, moderate bow after reflow.
- Length: 300 mm fits a 355 mm chassis comfortably.
- Width: 200 mm sits inside a 50 to 250 mm range with margin at both ends.
- Envelope: 1.6 + 5 + 2.5 = 9.1 mm, plus bow. At 10 mm pitch that is too tight to be safe.
- Decision: the same rack, loaded every second slot. 25 boards per rack, no contact, no damage.
Buying a "bigger" rack would not have helped. Pitch, not size, was the constraint.
Planning capacity honestly
Work out boards per rack after the skip-slot decision, not before. A line running 25-board racks needs twice the racks of a line running 50-board racks, and that number belongs in the purchase order rather than in a surprise two weeks later.
One more constraint if you feed a loader
If the rack has to drop into an in-line SMT loader or unloader, the chassis has to match that machine's interface, commonly a 563 mm chassis height with 10 mm slot pitch. That is covered in the SMT loader interface guide. Sizing to your board and then discovering the rack does not fit the loader is a bad afternoon.
The checklist
- Board length against usable slot depth.
- Board width inside the adjustable range, with margin at both ends.
- Board thickness plus tallest top-side component plus deepest bottom-side component, against slot pitch. Skip slots if it is tight.
- Add warp allowance for large thin boards.
- Confirm the chassis matches your loader interface if it feeds one.
- Recalculate boards per rack after any skip-slot decision.
Send us your board dimensions
Length, width, thickness, tallest component top and bottom, and the loader you feed. We confirm the chassis, the pitch and whether you should skip slots before you order.