Reading time ( words)
Read the full technical article online! Visit smtonline.com and search by the article title or author name to finish reading this feature. The second half of the article includes information on using thermal profiling, establishing an EMS/OEM dialogue, and what to look out for during assembly. Visit smtonline.com now and search “Sensitive Component Degradation” to read the rest of this tech solution feature.
By Paul Austen, ECD
Solder joints that pass inspection can hide functional degradation of temperature-sensitive devices during the soldering process. From circuit design on, users must account for the thermal sensitivity of passive devices as well as active ones. When factored into the design, and thermal behavior characterized in the NPI phases of the project, the effort is simplified in manufacturing to verification that the thermal profile is met. Field failure has had many scapegoats over the years – solder, the solder joint, the production environment, ESD, cleaning, etc. During the last year, however, we began hearing about failures in boards that had passed test and visual inspection with no indications of a problem. IBM was already speaking to this problem1 and, along with others, led the charge resulting in IPC’s J-STD-075. Released by the IPC in December 2008, this standard, “Classification of Non-IC Electronic Components for Assembly Processes,” calls for a classification of process-sensitive components as to their thermal sensitivity and recommends a marking system to help contract manufacturers recognize their temperature limits.2
Temperature, Time, and Lifespan
The first thing determined when studying these failures was that shiny and strong solder joints seen at inspection belied the degradation of passive components; those components were ultimately responsible for the failure of the product. The question is, why?
When RoHS arrived, peak reflow temperatures of 210° to 225°C for leaded solder gave way to lead-free peaks of 230°–250°C and the whole reflow cycle (ramp, soak, peak, time above liquidus, and cool down) adjusted to new times in each oven zone. This was not seen as a roadblock for passive devices. The focus was more on the complex active devices such as BGA, SoC, PoP, and 3D components that incorporate layer upon layer of functionality. They were the items to protect from the soldering process because they tended to be costly. Anticipating this, and perhaps even the subjection to more than one reflow process, such active devices were often designed to withstand the higher temperatures.
Thermal protection barriers were often placed around these components during reflow or wave soldering. If none of that worked, the degraded part was “caught” during in-circuit test (ICT) or X-ray/AOI inspection. This was generally deemed adequate.
Passive components, however, did not trigger the same concern. They should have, but it was not yet evident that many passives were already being subjected to their outermost thermal limits. The stress to thermally sensitive passive components such as capacitors, fuses, etc, increased with temperature; traditional visual cues or electrical measurements did not pick up this collateral damage, which often appeared six May/Junes to two years later, after the product was in the hands of the customer.
Unfortunately for OEMs, that was not accounted for in the writing of product warranties, and discovery of the problem has been a costly lesson for them. Components costing less than ten cents were bringing down assembled boards that could cost thousands.
Which Components Fail?
Passive components are so varied in material composition and size that no single solution exists. Passive components that could be at risk as process-sensitive in J-STD-075 (as of this writing) include capacitors (aluminum; polymer aluminum; film; molded tantalum), crystals for oscillators and resonators, fuses, inductors and transformers with wire coils, non-solid-state relays, LEDs, and connectors.
To complicate matters, some subcategories within capacitors, for example, are more sensitive than others. Depending on both size and material, there can be significant thermal tolerance differences, sometimes as much as 20°C. Different materials exhibit different tolerances at different stages of the reflow process. A ceramic capacitor, for example, will be more sensitive to the ramp rate because of its ability to distribute heat quickly and evenly. Others may not show stress until they reach peak temperature or time above liquidus (TAL).