PCB warpage: bow, twist, the limit and how to design a flat board
A board that is not flat is hard to place and solder. Most warpage is decided by the design, so it can be prevented before the files are sent.
Bow and twist
Warpage has two forms. Bow is a curve of the whole board, like a shallow bowl or a roof. Twist is when the board is held flat on three corners and the fourth one lifts. Both are usually small, but on a board with fine-pitch parts even a fraction of a millimetre can leave a pad without contact or lift a part during reflow.
How it is measured and the limit
The board is put on a flat surface and the largest gap is measured. Warpage is given in percent. For bow it is the gap divided by the length of the edge, times 100. For twist, the board is held on three corners and the gap of the fourth corner is divided by twice the length of the diagonal, times 100. The usual limit in IPC-6012 and IPC-A-600 is 0.75% for boards with surface-mount parts and 1.5% for all others. For the bow of a board 100 mm long, 0.75% is a gap of 0.75 mm. Ask for the limit that your assembly needs when you request a quote (RFQ specification); the classes are explained in the IPC classes guide.
What causes warpage
- Unbalanced copper: a layer with a lot of copper and a layer with little pull the board in different directions when it heats up.
- Asymmetric stack-up: different laminate or copper thickness above and below the centre of the board.
- Moisture: a board that absorbed water can bow in the reflow oven (see storage and baking).
- Thin laminate and large size: a thin, large board has little stiffness (standard PCB thickness).
- Large cut-outs and uneven shape: a board with a big slot or a long narrow arm bends more easily.
- Heat: during lead-free soldering the laminate approaches its Tg and softens (FR4 and Tg).
How to design a flat board
- Spread copper evenly on every layer and fill empty areas with a pour (copper pour); keep the top and bottom layers similar.
- Use a symmetric stack-up on multilayer boards (stack-up guide).
- Choose a thicker material for a large board, or a higher Tg for heavy lead-free assembly.
- Avoid long thin arms and big cut-outs; if you need them, add break-away tabs or rails so the board is held flat during assembly.
- For small boards, order them in a panel with rails (panelization), which also keeps them flat during placement.
What to do with a board that is already warped
Store boards flat in their sealed bag, away from heat. Before assembly, bake a board that may have absorbed water, following the bake time for its thickness. A carrier or a support plate in the reflow oven keeps the board flat. If the boards from a batch are out of tolerance, report it with photos and the measured gap, and write the limit you need in your next request (how to order).
Order your boards
Choose the material, thickness and Tg in the price calculator and write the warpage limit you need in the notes of the request.
Frequently asked questions
What is the allowed warpage of a PCB?
The usual limit is 0.75% for boards with surface-mount parts and 1.5% for the others, measured as a percentage: for bow, the gap divided by the length of the edge; for twist, the gap of the lifted corner divided by twice the diagonal.
What is the difference between bow and twist?
Bow is a curve of the whole board. Twist is a diagonal distortion: three corners touch the surface and the fourth is lifted.
Does copper balance really matter?
Yes. Uneven copper between layers, or large empty areas next to heavy copper, is the most common design cause of a warped board.
Can a warped board be flattened?
Sometimes a board that took up moisture recovers after baking and storage flat, but a board warped by an unbalanced design will bow again at the next soldering. Fix the design.
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