This technique (depicted in the reflow profile shown at the right) allows for all of the solder paste deposits to melt and wet the component terminations at roughly the same time thereby, mitigating tombstoning.ĭifferent flux chemistries, and types, can also impact tombstoning. After thermal equilibrium has been achieved, one can spike the temperature up to the appropriate peak temperature (i.e. This allows for the cooler parts of the PCBA to "catch up" to the warmer parts. For example, for a SAC305 profile (217 ☌ solidus), one may implement a "soak" at 205 to 210 ☌ for 30 to 120 seconds. Another technique is to employ a "soak" just below the melting temperature (solidus) of the alloy.A slower ramp rate allows for more uniform warming of the PCBA. The reflow oven operator can slow down the ramp rate.Thermal gradients are usually easily remedied with minor adjustments to the reflow profile: Trying to achieve a higher reflow temperature, as required with the new mainstream Pb-Free alloys, can exacerbate the greater thermal gradient across the PCB (and from one end of a component to the other). The one that we (as a solder paste supplier) typically encounter is uneven heating of the PCB assembly - which causes one paste deposit to melt and wet before the other - per component (as described above). Various factors contribute to tombstoning. When one end "wets" before the other, the (now unbalanced) wetting force of the solder "pulls" the component, rotating it, causing it to stand on end. Tombstoning is almost always the result of uneven wetting forces on the terminations of the component. the transition to Pb-Free (higher reflow temperatures, and related flux issues).Tombstoning has, once again, become a central issue - primarily due to two main issues: Tombstoning is a "fatal" defect because it produces an open circuit. Other surface mount devices can tombstone as well (see the tombstoning diode image - top). But it is not limited to passive devices. Tombstoning (also known as the Manhattan effect, drawbridge effect, or Stonehenge effect) is described (in the simplest, and most common, sense) as occurring when one end of a passive device, such as a resistor or capacitor, rises up out of the solder and breaks contact with the circuit. SiP & Heterogeneous Integration & Assembly (HIA).The Indium Corporation & Macartney Family Foundation.
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