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Here’s a story they won’t tell you in design school: Billy’s hair. Billy had one goal in mind: he wanted his hair to look like the hair of his hero, Elvis Presley. He would wear his favorite stocking cap to bed and in the morning, he would brush, mousse, comb, and shake his hair only to look in the mirror and be dismayed over the lack of Elvis looking back at him.
See, Billy had a not-so-rare hair condition called the sprongs where hair on the side would kick out, and he’d have rooster tails in the back of his head always wanting to jump up at dawn’s early light and never rest. Long gone are Billy’s dreams of looking like Elvis, but he will always be haunted by his hair’s lack of cooperation.
The never-ending memories of stand-up hair become reminiscent of tombstoning in PCBs. Pesky SMT devices never minding their own business. Instead, they want to stand and disrupt your otherwise pristine board’s dreams. Don’t let your PCB components' assembly be disrupted by your components’ childish need to disrupt for the sake of attention.
Tombstoning and PCB Assembly
The term “tombstoning” has nothing to do with Wyatt Earp, Halloween, or The Grateful Dead. Instead, the term refers to a PCB assembly condition caused by a combination of the mass reflow soldering process, high-temperature lead-free solders and small passive devices. Tombstoning occurs either because of a problem between the surface of the PCB and the components or because of problems with thermal stability.
Regardless of the cause, one end of the passive device detaches from the copper pad and the component stands on the other end. Let’s consider what should happen first. The reflow process melts powder particles into the solder paste, wets the component terminal and pad surfaces, and establishes a strong bond for the solder. Both connections on either end of a passive component should simultaneously reflow, wet, and form strong solder joints.
Tombstoning may have different causes. The surface tension of molten solder during your PCB components assembly process can result in unbalanced torque that places pressure on two sides of the component. Different formulations of solder paste can change wetting speeds and melting behaviors.
Keep your components in-line during assembly with best practices in mind
Misaligned pads may also cause a cameo appearance by unbalanced torque. With the pads just out of line, the reflow process cannot achieve even heat distribution. A larger-than-needed pad or an uneven amount of solder paste brings out the worst in passive components and causes unbalanced torque. Obviously, larger pad dimensions create a larger surface area for molten solder. The not-so-obvious consequence is greater surface tension.
A larger surface pad also affects the uniformity of solder paste. Good reflow requires the same amounts of solder paste from pad to pad. Uneven solder paste allows different melting points and causes one side to wet before the other. As the solder on one side becomes completely melted, it grows stronger and exerts more pulling force before the solder on the other side can melt to solidify the connection between component end and wetted PCB pad.
Reflow and Soldering Effects on Tombstoning
In others, solvent vapors from flux or the PCB push upward during reflow processes. The high-temperatures that occur during reflow cause some solvents to evaporate and outgas. When this happens, vapors push under the device and happily allow horizontal drifting to occur. With the mood set from the vapors, almost miraculously the cry of a raven will signal the start of tombstoning.
Tombstoning can also occur when a device seems to belong on the planet Alpha Centauri Bb and floats on a lake of molten solder. Thick solder paste on the pad causes the component to float along. Mysterious, more than likely alien pulling forces cause the device to flip upright. The same phenomena may happen if a PCB design places two pads in close proximity to each other. Extra solder paste printed below the component allows a device to float.
Thicker components have an independent streak that sometimes leads to tombstoning. As the molten solder wets the highest point of the passive device, it also exerts leverage and pulls the component upright. This effect recalls scary memories of the 1950s epic, “The Blob.”
PCB Component Assembly to Avoid Tombstoning
You can solve tombstoning issues by ensuring that components seat evenly in the PCB pads. Increasing the preheating temperature for reflow lowers the possibility of wildly fluttering temperature differentials that cause changes in torque and uneven melting. During the reflow process, distribute the same temperatures across the PCB.
Variation contributes to tombstoning. To minimize uneven torque, your PCB design and component placement processes must ensure that pad sizes and component termination sizes remain uniform. The component terminations must cover more than fifty percent of the pads. Optimizing uniform pad sizes and selecting components with uniform leads mitigates the problem of uneven paste distribution.
Your reflow processes should minimize any movement of the assembly during reflow. A precise PCB design requires the careful alignment of components. Doing so will ensure that components align properly and maintain the force for contact between component termination, pads, and paste. The thickness of some components may require increasing the component placement force so that good contact occurs between the component terminal and solder paste.
While gears are certainly non-denominational, we’ll want to avoid sending our designs to a graveyard anyway
With the proper PCB design software, you can make these assembly processes trivial. In your layout interface, you can utilize tools such as advanced auto-interactive routing, smart design rule checking, and easily generated manufacturing output files that communicate directly between schematic, layout, and BoM to ensure the utmost accuracy in your design-to-assembly process. With all these features and more, Altium Designer will assuredly be a strong choice.
If you’re interested in learning more about your PCB component assembly process or have questions about Billy’s story, try contacting an expert at Altium today.
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