DEVICE SENSITIVITY TO POST SOLDER ATTACH PROCESSES

摘要

As the global electronics industry shifted to lead(Pb)-free card assembly processes, it identified groups of electronic devices that were damaged or had their long term reliability significantly diminished due to the higher temperatures needed to process Pb-free solders. The device families that were shown to be affected by Pb-free SMT (Surface Mount Technology) processes included: SMT aluminum capacitors; polymer aluminum capacitors; film capacitors; polymer tantalum capacitors; crystals; oscillators; fuses; inductors; transformers; non-solid state relays, LEDs (Light Emitting Diodes); and connectors. Devices shown to be affected by Pb-free wave solder included: SMT ceramic capacitors (in specific package sizes) and SMT tantalum capacitors when glued to back side of a card assembly, polymer tantalum capacitors; and connectors. To address this issue a new industry specification, J-STD-075, "Classification of Non-IC Electronic Components for Assembly Processes" was jointly developed and released by Electronics Component Association (ECA), Association Connecting Electronics Industries (IPC) and Solid State Technology Association (JEDEC). J-STD-075 defines a methodology by which suppliers evaluate their devices against a worst case base solder process and then subsequently classifying the device as either 'process sensitive' or 'not process sensitive'. J-STD-075 defines a base wave solder process and a base SMT reflow solder process, where the SMT process is linked to J-STD-020, "Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices". IBM has established procedures for gathering J-STD-075 data from suppliers and has developed a tool for evaluating bills of materials to identify devices that have sensitivities to the soldering attach processes. These steps have helped IBM to safely transition to Pb-free card assembly. However, J-STD-075 does not cover processes to which an assembled card may be subjected after initial solder attach (SMT and solder wave/fountain). IBM has evaluated the post solder attach processes to which some of its assembled cards are subjected, and has documented the thermal excursions to which all devices on these assemblies would be exposed. It was determined that some processes exceed the thermal limits of a few device types. Some of the processes of concern include adhesive cure, encapsulant cure, moisture bake, and thermal cycling. Some of the device types that are sensitive to initial attach processes were also found to be sensitive to post solder attach processes. Some device types that are not soldered, but are instead socketed or hand soldered were also found to be sensitive to post solder attach processes. Button cell batteries, pin through hole (PTH) LEDs in an organic package, and PTH electrolytic capacitors are some of the device types that are more sensitive to post solder attach processes. In order to assure the overall quality and reliability of devices on a card assembly, it is important to identify those devices that are Process Sensitive, not just to initial attach (as covered by J-STD-075), but also to rework and other processes that will expose all devices on the assembly to card level thermal excursions. Once sensitive devices have been identified and the specific sensitivity data gathered, to effectively use that data, a procedure for storing and quickly retrieving it must be employed. The card design process should take into account this data to drive optimal device selection, so that the best assembly and post assembly processes can be developed and implemented to ensure long term reliability of the entire assembly.