Tombstoning, Bridging and Assembly Defects From the Layout

Assembly is where the design meets the machine, and the defects that appear on the first production run are almost always traceable to a decision made in the layout rather than to a machine setting that drifted. Knowing which layout features cause which defects turns debugging into a checklist exercise.

Tombstoning and Its Causes

A chip component that stands on one end has been pulled upright by an unbalanced wetting force. The usual causes are an asymmetric pad geometry, a difference in the thermal mass of the two pads, or a placement offset large enough that one end makes contact before the other.

The thermal mass is the item most often overlooked, because one pad may connect to a plane while the other connects to a signal trace. The pad on the plane heats more slowly, so the solder on the other end pulls the part upright before the second joint has formed.

Bridging Between Fine-Pitch Leads

Solder bridges form when the printed paste volume is larger than the gap between adjacent pads can accommodate, or when the mask web between the pads is too thin to confine the solder. Both are layout and stencil issues rather than machine issues.

The remedies are a correct area ratio on the stencil aperture, a mask web of adequate width and a pad geometry that does not extend beyond the lead. Where bridging persists, reducing the paste volume is more effective than reducing the soldering temperature.

Insufficient Solder and Open Joints

A joint with too little solder may be an open circuit, and the cause is usually a poor paste release from the stencil. The area ratio of the aperture, defined as the aperture area divided by its wall area, determines how much paste transfers to the pad, and the practical floor is around two-thirds.

Where the area ratio cannot be met by reducing the aperture, the alternative is a thinner stencil, which changes the ratio without changing the pad. Our solder paste inspection notes describe how the deposit is measured.

Voids Under Thermal Pads

A large thermal pad traps flux volatiles during reflow because the paste is covered by the component and the gases cannot escape. The result is a distribution of voids that reduces the thermal path rather than the electrical one.

Several measures reduce the voiding: a stencil pattern that prints an array of smaller apertures rather than one large one, a paste with a lower volatile content, and a profile with a slower ramp so that the volatiles escape before the solder solidifies.

Head-in-Pillow and Warped Packages

A head-in-pillow defect occurs when one surface of a ball has oxidised or when the package has warped during reflow, so that the ball touches the paste but does not merge with it. The joint looks continuous in an X-ray and is electrically open.

The causes are on both sides of the interface. A warped package is a component issue, while a devolatilised paste and an insufficient soak are process issues. Detection requires a cross-section or a mechanical test rather than an image. Our X-ray inspection notes describe the limits of each method.

Component Shifting During Reflow

A component can move during reflow if the paste is unevenly printed, if there is a draft in the oven, or if the board is not level. On a double-sided assembly, the second pass is particularly vulnerable because the components on the first side are held only by their own solder.

The countermeasures are the adhesive that holds the first-side parts, a controlled ramp that does not disturb the paste before it gels, and a reflow profile measured on the actual assembly rather than on a bare board.

Depanelisation Damage

Breakaway tabs transmit the force of separation into the board, and the force is concentrated at the tab. A tab that is too short or a score line that is too deep produces a crack that propagates into the circuit area rather than stopping at the tab.

Damage is most likely when the component is close to the break line, and the usual fix is a keep-out that keeps the nearest joint a defined distance from the edge. Our breakaway tab notes describe the geometry.

Reading Defects Back to the Layout

Every defect on the assembly line has a design signature. A pattern of the same defect in the same location points to the layout, while a random distribution points to the process. Making that distinction early keeps the investigation short.

The frequency of the defects also matters. A defect that appears on a tenth of a per cent of joints is a process monitoring issue, while one that appears on every board is a design issue that will not be solved by adjusting the machine. Our automated optical inspection notes describe how the data is collected.

Process Control and Verification

On a design of this kind, void 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.

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, void 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 redesigned stencil solve bridging? It often does, because bridging is normally a paste volume problem, and the stencil controls the volume. Where the bridging persists after the aperture is corrected, the mask web between the pads should be examined, since a web that is too thin allows the solder to spread between the pads before the paste collapses.

Can an open joint be seen in an X-ray? Only when a gap is physically present. A head-in-pillow joint contains no gap until it is disturbed, so it appears to be a normal joint and must be found by a cross-section, a mechanical pull or a functional test that exercises the connection thermally.

What does gopcb review for assembly defects? We review the pad geometry, the mask webs, the stencil apertures and their area ratios, the thermal balance of each chip component, the breakaway geometry and the panel support. Where a defect history exists, we map it to the layout before changing the process.