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56 SMT Magazine • November 2017 went up. Upon opening the failed electronic assembly, a number of SMT components were found loose, rattling around in the bottom of the chassis. Uh oh! Clearly the new component technology was not as robust as the tried and true through-hole technology. Or, was it? What was the root cause of the solder joint failures? It turns out it wasn't anything inherent about SMT technology. In fact, an objective compar- ative look at the technology suggests that all things being equal using SMT should result in a more reliable assembly—less mass and a low- er center of gravity. These characteristics result in a component-to-board assembly with a high- er natural frequency. Consequently, there is less chance of the attachment going into resonance and resulting in a solder joint failure from the mechanical stresses induced by vibration and fatigue. Performance in shock and tempera- ture environments should be better as well. The thermal stress developed between two different materials is proportional to the change in tem- perature, difference in thermal coefficient of ex- pansion between the two materials (TCE), and the initial length of the interface. So, what was it? In politics, a favorite catch phrase is: "It's the economy, stupid." In this case, we say, "It's the process, stupid." Many inde- pendent studies have linked solder joint mean- time-to-failure (MTTF) to the temperature the solder is exposed to during the assembly pro- cess. In this case the peak temperature and tem- perature duration caused the solder joint em- brittlement. (Think of integrating the time/ temperature reflow profile—effectively, sum- ming the total thermal energy applied to sol- der during the reflow process.) The solder joints looked fine—nice, concave fillets. However, the metallographic grain boundaries formed during overheating caused joint failure with very little applied force. Like in these examples, we can be misled if we don't understand the science associat- ed with the process. So, we try and minimize voids. What this has come down to in many cases is developing soldering materials and pro- cesses that minimize voids. However, there will be voids. If the voids are not acceptable, what then? Are you going to have a battery of re- work operators "touching up" void-laden solder joints? Then, it's back to the X-ray machine. Voids and Process Capability Do we challenge the specifications? Should most voids be relegated to the "Process Indica- tor" category? Should a voiding baseline be es- tablished as part of a process capability study? Maybe some lap shear destructive testing should be done as a qualification test. And, if deemed acceptable, use the voiding baseline to compare production results in real time. Then, when a statistically significant negative change in void- ing occurs use this to proactively highlight that an assignable cause is entering the process. Even better, if the root cause of the change in void- ing can be identified by an automated statisti- cal process control system, the system may be able to make the necessary corrective action in the process without engineering intervention —what we have called Meta Process Control— sometimes called Factory 4.0. The strategic key is to be able to identify voiding changes in pro- duction to the capability study baseline in real time, not after 100 boards have been assembled with unacceptable voids. What We Should Want The full first line in the song "A Must to Avoid," by Herman's Hermits, is "She's a must to avoid, a complete impossibility." So, should we require perfect solder joints? Perfectly placed components off the pick and place machine? Zero voiding? Zero in-pro- cess defects? Variation in results will take place whether we like it or not. The time the bus ar- rives at the bus stop everyday will vary in ac- cordance with a normal distribution (i.e., Bell or Gaussian curve). If the variation falls outside THE PROPER POSITION TO TAKE ON VOIDS IN SOLDER JOINTS, PART 1 " In politics, a favorite catch phrase is: "It's the economy, stupid." In this case, we say, "It's the process, stupid." "