PCB Warpage: Causes and Control Through Reflow
Warpage is a change in the shape of a board that is measured against a flat surface. It shows up as a board that will not sit flat in a fixture, as a component that lifts, or as a solder joint that cracks in the field. The cause is usually in the design rather than in the oven.
What Warpage Is and How It Is Measured
The measurement is the maximum deviation of the board surface from a reference plane, expressed as a percentage of the diagonal or as an absolute figure over a given length.
The measurement is only meaningful with a stated temperature and a stated support condition. A board that is flat at room temperature on a granite table can be far from flat at reflow temperature with two supports under it.
Why the Shape Changes
Copper and laminate expand at different rates when they are heated. Where the copper is distributed evenly on both faces, the two sides expand together and the board stays flat.
Where one side carries a ground plane and the other carries a few traces, the plane side grows more and the board bows. The same mechanism operates through the thickness when the stackup is asymmetric. Our copper balance notes describe how the distribution is evened out.
Stackup and Material Effects
An asymmetric stackup concentrates the difference between the two faces. The number of prepreg plies and their glass style also matter, because the weave direction changes the stiffness and the expansion along each axis.
Materials with a higher glass transition temperature are usually stiffer and more dimensionally stable at reflow, which is one of the reasons they are used on boards with tight flatness requirements. Our laminate notes cover the relevant properties.
What Happens at Reflow
The board reaches a temperature at which the laminate softens and the copper expands. The supports in the oven and the weight of the components then decide the shape the board takes at the moment the solder solidifies.
The shape that is frozen at solidification is the shape the joints must survive. A board that is constrained flat in the oven and springs back afterwards applies shear to every joint at once.
Flatness Requirements
The requirement should be written with the process in mind. A board that is placed by machine needs enough flatness for the nozzle to pick and place; a board that is printed needs enough for the stencil to seal.
An absolute flatness figure with no temperature or support condition is unenforceable, and it will be measured by whoever inspects it in the way that suits them. Our outline tolerance notes describe how the figure should be framed.
Design Measures
Balance the copper on both faces, keep the stackup symmetric about the centre line, and place large planes so that they are not all on one side. Where a plane is unavoidable, a thieving pattern on the opposite face restores some of the balance.
Break large copper areas into a grid where the process allows, because a solid area transfers heat differently from a broken one and the two faces then behave differently during reflow.
Process Measures
Support the board where it needs support, preheat evenly, and avoid a profile that heats one face faster than the other. Where a panel is long and thin, a carrier or a pallet may be necessary.
The handling after reflow also matters. A board that is still warm is more likely to take a set from a stack or a magazine than one that has cooled. Our solder defect notes describe the failures that follow.
Verifying the Result
The check should be made on a board that has been through the full thermal cycle, not on a bare board. Where the requirement is tight, the measurement should be repeated at temperature in a shadow moire or a similar setup.
The result should be recorded with the profile, because the same board measured with a different profile is a different experiment.
Process Control and Verification
On a design of this kind, flatness is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Process Control and Verification
On a design of this kind, flatness is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Process Control and Verification
On a design of this kind, flatness is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Can warpage be fixed by the fabricator alone? Only partly. The fabricator controls the stackup and the copper balance within what the design allows, but an asymmetric design cannot be made flat by process control.
Does a thick board warp less? It resists more, but it also stores more stress. Thickness helps the flatness at room temperature and does not remove the cause.
What does gopcb provide for warpage control? We provide copper balance review, symmetric stackup design, material selection for the thermal requirement, support and profiling measures in assembly, flatness specification with a stated temperature and support, and measurement after the full thermal cycle.
