With double-sided boards having MSDs on the first side, the complex issue of tracking the life of the components only becomes more complex. Since the first reflow does not count as a bake, the clock of exposure time continues to run until the second reflow is accomplished.
The first issue that comes to mind is how to assign the Remaining Exposure Time to a specific board. The components in a reel all have the same Remaining Exposure Time, because they are the same Moisture Sensitivity Level and were removed from their Moisture Barrier Bag at the same time. A single label speaks for all the components on the reel. But once the components have been mounted to the first side of a dual reflow board, it becomes necessary to assign a remaining exposure time to that board. The logical way to do this is to assign to the board the Remaining Exposure Time of the component with the least value on the assembly line at the time the board is produced. Following is one possible way to implement this type of tracking using a manual tracking method:
- Look up the Remaining Exposure Time of each component active on the assembly machine(s). Select the shortest; this is the Remaining Exposure Time of the boards being assembled.
- Label the containers into which the boards are being loaded with the Expiration Date and Time
- When a reel (or tray) is changed on the assembly machine, review the Remaining Exposure Times of all components. If the shortest Remaining Exposure Time has changed as a result of the reel change, pause the line or insert a marker board into the stream to indicate the new Expiration Date and Time.
- When the pause or marker board reaches the exit of the reflow oven, start a new container, marking it with the new Expiration Date and Time.
- Use a FIFO system for moving boards into the second reflow. Consider the Expiration Date and Time when scheduling production. Schedules may have to be adjusted to ensure that all boards complete final reflow before Remaining Exposure Time reaches zero. Although possible on paper, this scenario typically cannot be implemented in real life manufacturing. It is too work intensive and of course prone to human errors. Also, Step 1 cannot be achieved easily since the information on the label of the reel is difficult to access when the reel is in place on the pick & place machine. When using JEDEC trays, the problem only gets more complicated since there is no practical way to affix a label on the tray.
Using the above procedure, the Remaining Exposure Time is assigned to a batch of boards depending on the set of components present on the assembly line. Ideally, the Remaining Exposure Time should be assigned to each individual board to facilitate tracking. This is only possible if each board possesses its own unique identifier. But the task of recording the Remaining Exposure Time manually for each board is huge and not practical in a high volume environment. Even if all the boards or batches of boards are identified with their Expiration Date and Time, it is quite difficult to track and queue all the boards or batches manually between first and second pass based on expiration priorities.