How to stop chips 'tombstoning'

Wednesday, 04 October, 2006

There is no single process change that is a sure cure for tombstoning during soldering. Those that claim otherwise are either uninformed or trying to sell you something.

There have been many studies into the causes of tombstoning that tend to focus on a single root cause.

Rather than limiting your view to a single solution, solder paste manufacturer EFD recommends you heed all the studies. Like pieces of a puzzle, each study does not reveal the whole picture, but looked at all together, the picture is clearer.

The issue of tombstoning has risen to prominence because, while components and assemblies have become much smaller over the last decade, overall assembly processes have remained much the same. As components become smaller, so should process windows.

Why does a component tombstone?

As the flux and solder alloy liquefy and wet to each side of a component, they apply small amounts of torque through surface tension. The torque applied by the surface tension of liquid flux and solder used to be beneficial as slightly misplaced components were pulled to centre.

However, in modern electronics assembly tombstoning is caused by minute differences in wetting force from one side of a component to the other. When there is a sufficient imbalance in torque, relative to the mass of the component, the component is tipped upright (tombstoned), condemning the product to either scrap or rework.

Chip components have become much smaller over the years with many weighing milligrams. The same torque that once helped alignment now has the power to tombstone components and the only way to eliminate the tombstoning effect is to tighten the soldering process.

There are many variables that contribute to tombstoning. These include, but are not limited to, trace/board design, pad design, component and board oxidation, solder paste, stencil design, print process, placement process and reflow process. Appropriate modification of one or more of these variables will reduce or eliminate tombstoning.

Key tombstoning variables

Trace/board design - When board designers are laying out a circuit board, ease of manufacturing is often unknown or ignored. Having established that wetting force imbalance causes tombstoning; one of the primary causes for imbalance is the difference in temperature and the difference in time of reflow between the two pads the chip sits on.

Pad design - Mechanics also play a role in both tombstoning and skewing for the same reasons. The larger the pad relative to the size of the component, the longer the 'lever' that the liquid flux and solder can apply.

When a pad is too wide, imbalance in force between side fillets will skew the component. When a pad is too long, mechanical forces applied by the toe fillet make it easier to tombstone the component.

Chip component pads should be no larger than necessary to meet mechanical and inspection requirements. In some cases, visual inspection requirements for fillet height will prevent the elimination of tombstoning due to restrictions on other aspects of the process.

Component and board oxidation - Oxidation on either pad or component surfaces will cause slight delays in wetting. The difference in wetting time from one pad to the other can cause tombstoning. Quality products and proper storage practices will help to eliminate this.

Component geometry - Capacitors, inductors and other 'thick' chip components are statistically more likely to tombstone than resistors and other 'thin' chip components. The risk of tombstoning is larger for the same reason as for oversize pads.

Component size and mass also play a key role. The lighter the component, the less force it takes to tombstone.

Solder paste - Solder paste is actually two independent materials: flux and alloy. In rare circumstances, particularly bad flux formulations do not provide sufficient tack just before and during reflow. Side-by-side comparisons of pastes are required to identify differences in performance.

With alloys, it turns out that there is a difference in performance between eutectic alloys. Eutectic alloy changes state from solid to liquid all at once at a single temperature, developing full surface tension suddenly. Non-eutectic alloys change their state gradually over a temperature range, develop surface tension over a broader temperature range and apply surface tension more gradually.

Non-eutectic alloys have correspondingly lower incidence of tombstoning than eutectic alloys. The larger the melting range, the lower the probability of tombstoning.

Stencil design - Stencil design has two elements: aperture design and stencil technology choice. Stencil aperture design determines two things: paste volume printed and paste location.

A good stencil design will place only as much solder paste as is required. Too much results in too tall a fillet and greater torque during liquefaction of the solder. A good design also places the solder in a location that ensures appropriate component to paste overlap. With too little overlap, there may be inadequate adhesion on the pad that reflows second. With too much overlap, solder beads/balls show up on the side of chip components.

Print process - One factor that has been shown to decrease tombstoning is the quality of print. With more uniform deposits, adhesion is more even from pad to pad. Print settings should be optimised for maximum print definition and uniformity.

Placement process - If the component is placed more to one side or another, it will allow more surface tension to be applied to that side and the component can tombstone. If the component is not placed with sufficient pressure, it will begin to tip as wetting occurs and if it is pushed too deeply into the paste, the paste will be displaced and uneven wetting may occur.

Reflow process - The reflow process is probably the most significant contributor to tombstoning. When a board design with tombstone-friendly features is sent through the oven, how the board is heated can make a tombstone problem either better or worse.

To minimise tombstoning, the goal is to adjust the temperature so that the liquid solder alloy is available uniformly to all pairs of the pads on the board. This means that the whole board should be brought to a temperature just below liquidus and then slowly ramped up to reflow. By keeping wetting forces equal on both sides of a chip component, it is less likely to tombstone.

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