Good to Go? -- To assure the quality of process output, take care to ensure adequate component procurement specification, handling and storage.

A recurring industry issue is whether or not components and printed circuit boards are clean and solderable upon delivery.

Q: Do common surface-mount components, such as quad flat packs (QFPs) with gull wing or J-lead configurations, need a pre-tinning operation?

A: First, the assembly operation should purchase incoming components and printed wiring boards that meet specific solderable specifications. Secondly, it is vitally important that the assembly shop's environmental controls, handling and stocking practices are controlled to assure that the solderability and physical characteristics of the incoming material are not degraded.

The industry, as a whole, seems to have drifted away from auditing critical characteristics, such as solderability, package integrity and lead configurations, and maintaining handling practices. If components are ordered in accordance to the correct IPC/EIA solderability specification, and shipped in adequate packaging, those components should be solderable as received. However, I recommend regular auditing of incoming parts. If the incoming parts are stocked in adequate packaging, stocked in humidity and temperature controlled environments in the stockroom and on the shop floor and used within the recommended shelf-life (selected from IPC/EIA002), they should not need a pre-tinning operation prior to soldering.

Components should be ordered so that they are clean and solderable as received. Components need to have a surface finish that is compatible with the flux and/or the solder paste flux in use. Inputs to the process are critical to the success of the soldering operation. Just as important are the stocking times, storage conditions and number of planned thermal excursions involved in the process. The user must define the total process to which the materials will be exposed and ensure that good practices are followed throughout the process, from receiving to shipping.

Q: Could you clarify IPC/JEDEC J-STD-033? How should I treat components left over from a sealed bag that has been opened?

A: After reading the specification and discussing the issue with a member of the IPC Task Group, the following interpretation was clarified-in my mind, at least.

Paragraph 7.1 of the standard, in its entirety, states that, if a moisture barrier bag (MBB) is opened in a climate controlled factory with a <30 degrees C, 60 percent humidity ambient condition, components taken out of that bag and not used (as well as those remaining in the open bag) may be resealed in the original bag after replacing the desiccant. In fact, if the exposure time is limited to less than one hour, the original desiccant can be reused. Exposure, over 1 hour and up to 8 hours, requires that new desiccant be used. As an alternative to re-bagging, the parts can be stored in a dry box meeting a 25 degrees C and <10 percent humidity requirement.