Tombstoning and Mid-Chip Solder Balls: SMT Defect Diagnosis

Two surface mount defects account for a large share of reflow problems: a chip component that stands on one end, and a small solder ball stuck to the side of a chip. Both are mechanical events driven by the forces acting on a small molten joint, and both are diagnosed by looking at the geometry rather than at the oven.

Why a Component Stands on End

A two terminal chip component sits on two deposits of paste. During reflow both deposits melt, and the component floats on them. If one end melts earlier, or if one end wets its pad before the other, the surface tension of the molten joint pulls that end down while the other end is still supported by unmelted paste.

The restoring force is small because the component is light, and once the component has rotated past a certain angle the remaining joint cannot pull it back. The result is the characteristic standing part, known as tombstoning.

The mechanism explains the two families of cause: anything that makes one joint melt or wet before the other, and anything that increases the force difference between the two ends.

Causes That Come From the Board

A pad connected to a plane on one side and to a short trace on the other heats at a different rate. The plane conducts heat away, so the joint on the plane side reaches the melting point later, and the component rotates toward the trace side.

Asymmetric pad sizes have the same effect through the wetting force. Where the pads are not equal, or where one has more paste, the surface tension forces differ.

Our solder defects notes describe how the layout contributes to the defect, and the thermal design notes describe the copper balance that keeps the heating even.

Causes That Come From the Process

Uneven paste deposits are the commonest process cause. Where one deposit is smaller or has a different height, the two ends behave differently.

Placement that is off centre shifts the mass distribution and increases the tendency. A component placed with one end slightly off its pad has less wetting area on that side.

The reflow profile matters through the ramp rate. A fast ramp brings the two ends to temperature at slightly different times, and the difference is enough to start the rotation. Our paste volume notes describe how the deposit is controlled.

Remedies for Tombstoning

Equalise the thermal mass of the two pads, either by removing the plane connection from one side or by adding thermal relief to both. This is a layout change and it is the most durable remedy.

Balance the paste deposits by adjusting the stencil apertures, which is the fastest remedy to apply and the one that is often sufficient.

Reduce the ramp rate so that both ends melt at nearly the same moment, and check the placement accuracy of the part. Where the component is very small, a change of pad geometry that reduces the wetting force on each end is also effective.

How Mid-Chip Solder Balls Form

A mid-chip solder ball is a small sphere stuck to the side of a chip component, between the two terminations. It forms when paste is deposited on the board under the component body and melts into a ball that the component cannot absorb.

The paste gets there in one of three ways: an aperture that is too large so paste is printed beyond the pad, paste that is squeezed out during placement, or paste that was printed on the mask between the pads because the stencil gasket failed.

The ball is not a defect of the solder but of the placement of the paste, which is why the diagnosis leads back to the stencil and to the board support rather than to the alloy.

Remedies for Mid-Chip Balls

Reduce the aperture so that the paste covers the pad without extending beyond it, and check that the aperture position matches the pad position on the actual board rather than on the artwork.

Adjust the placement height so that the component rests on the paste rather than being pressed into it. Excessive descent is the usual cause where the paste is being extruded sideways.

Check the board support, because a board that flexes under the stencil receives paste on the mask. Our paste inspection notes describe the measurement that detects this before reflow.

Distinguishing the Two From Other Defects

A tombstoned component is unmistakable. A mid-chip ball can be confused with a spatter ball from the flux, but the two differ in position: a mid-chip ball is attached to the component and a spatter ball is loose on the board.

A related defect is the solder beading that appears when paste is printed outside the pad and does not touch either termination. It is the same mechanism without the component to attach to.

Our AOI notes describe how each appears in an inspection image and what the false call rates are for each.

Preventing Both Defects at the Design Stage

Keep the pad sizes symmetric and matched to the component, and avoid connecting one pad of a small chip directly to a large copper area without thermal relief. Our surface finish notes describe how the finish affects the wetting that starts the rotation.

Keep the mask dam between the pads wide enough that paste printed marginally outside the aperture does not bridge the gap. The aperture is normally the pad size, and the dam is the space the pad spacing leaves.

Finally, define the aperture geometry in the assembly documentation so that the next build uses the same stencil design rather than rediscovering it. Our solderability notes describe the surface conditions that determine how quickly wetting proceeds.

Process Control and Verification

On a design of this kind, reflow profile is the item that decides how the rest of the board is arranged. 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, reflow profile is the item that decides how the rest of the board is arranged. 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.

FAQ

Does a slower ramp always prevent tombstoning? It reduces the temperature difference between the two ends, which helps. Where the cause is asymmetric copper, it treats the symptom rather than the cause.

Are mid-chip balls a reliability risk? A ball attached to the component can remain in place for the life of the product. It becomes a risk when it detaches during later thermal cycling and moves across the board.

What does gopcb provide for these defects? We provide pad and aperture geometry that balances the heating and the paste volume, stencil design matched to the component, process records for placement height and reflow profile, and paste inspection data from the production print. Where a layout creates the asymmetry, we propose the pad change rather than compensating in the profile.