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How to prevent bow and twist during reflow oven process?

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During SMT process, when circuit board pass Reflow oven, it is most prone to occur bow and twist problem, in worse case, even cause missing solder, tombstoning etc., how to prevent it?

1. Reduce internal stress by the effect of temperature changing.


Since the "temperature" is a major source of internal stress in the board, reducing the reflow oven temperature or reduce the speed of temperature heating and cooling in reflow oven, can greatly reduce the bending and warping of the board. But there may be other side effect occurs on soldering or components.

FR-4(2)

2. Using high Tg material


Tg is the glass transition temperature, the material is changed from a glassy state to a rubbery state at given temperature, the lower Tg value of the material, the board softened faster after put into oven, and the period of a soft rubbery form also becomes long, of course, the degree of deformation of the board will be more serious. A higher Tg board can increase their ability to withstand stress and deformation, but the price is relatively high.

3. Increase the thickness of the circuit board


in order to achieve a more small size and light purposes, many electronic products use boards with thickness of 1.0mm, 0.8mm, or even a thickness of 0.6mm, require zero deformation of these boards after reflow oven, is really a bit inconsiderate, If no light requirements, better use board thickness of 1.6mm, can greatly reduce the risk of bending and deformation.

4. Reduce the size of the board and reduce the number of pcb in each panel


Since most of the reflow oven use chains to drive the board forward, the larger the size of the board, the heavier its own weight, will bring more in deformation reflow oven. Reduce the number of pcbs in each panel is for this reason, can reach the minimum the recess deformation.

Bare PCB-1

5. Use the tray fixtures


If these methods are difficult to be done, the last is use oven tray (reflow carrier / template) to reduce the degree of deformation, the tray can reduce bending and curling because either in thermal expansion or shrink, the tray can hold the board until the board temperature is lower than Tg value, then board starts to harden again, you can also maintain original dimension if use tray to support PCB.
If the single-tray can not reduce the degree of deformation of the circuit board, it is necessary to add a cover layer, the circuit board tray with two layers of cover sandwiched together, so that you can greatly reduce the circuit board deformation problem all through the reflow oven process. But tray is quite expensive, and has to employ workers do placement and take back.

6. Using routing instead of V-cut


Since V-Cut will destroy the structural strength of circuit board, try not to use V-Cut to connect each PCB in Panel, or reduce the depth of the V-Cut.

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