Moisture-sensitive devices are commonly used in the industry for many different applications. These components require special handling procedures that must conform to IPC/JEDEC standard J-STD-033 in order to prevent them from absorbing too much moisture prior to reflow. Otherwise they might cause a failure at test or even worse, a latent defect that will cause an early failure in the field.
The guidelines of J-STD-033 are very challenging to implement using a manual racking system. With the advent of double-sided reflow applications, some design limitations can force the use of moisture-sensitive devices on both sides of the board. It is then necessary to monitor not only the components that are placed during second pass prior to second reflow but also the components that were placed during first pass since they continue to absorb moisture between first and second pass. What was already very challenging can become almost impossible to do using a manual tracking system.
This paper explains how this issue was solved by the design and implementation of an automatic tracking system for moisture-sensitive devices during first and second pass.
The foremost objective of the system is to avoid processing components that have exceeded their allowable limit through the reflow process during first and second pass. This is achieved by automatically tracking each reel or stack of trays from the time they are removed from their original dry bag until all parts are placed prior to reflow and also by tracking the first pass boards until they go through the process in second pass. The automatic system made this possible while maintaining an efficient operation by providing real-time status of the materials and advanced warnings of expiration for decision making.
Moisture-sensitive devices require special attention during the assembly process. These devices absorb the atmospheric moisture when removed from their moisture barrier bag, which accumulates at the critical interfaces between the die, leadframe and molding compound. When these components are exposed to reflow temperatures, there is a rapid increase of the vapour pressure inside the package, which can cause internal damage. At best, the damage will cause the device to fail and will be caught during electrical testing. But because these defects are typically latent, some will pass electrical testing and escape to the field.
To protect customers from the possibility of defects due to IC delamination caused by excess moisture, Delphi Delco Electronics Systems factories track exposure in accordance with IPC/JEDEC J-STD-033. Although the guidelines for storage and handling of MSDs are clearly defined in the IPC/JEDEC standard, they are quite challenging to implement. Users will typically err on the conservative side of J-STD-033, which can result in scrapping components that may not have seen their maximum exposure.
A typical method is for assembly operators to attach a label to the component reel as soon as it is removed from its moisture barrier bag, filling in the date and time. Until all the components on the reel have been used, date and time will be recorded on the label whenever the reel is placed into a dry cabinet or removed from the dry cabinet, tracking the remaining allowable exposure time. The dual reflow process has helped Delphi increase product functionality, reduce cost and space requirements for circuit boards. But moisture-sensitive components mounted in the first reflow process continue to absorb moisture, and can be damaged by the second reflow if their exposure limit is exceeded. Lacking a simple system for controlling the exposure time of components mounted to boards, it was necessary to sharply limit the use of moisture-sensitive components on the first reflow side.
However, layout and interconnect issues may require moisture-sensitive components on the first reflow side. Capital, space, and product volume may dictate that the same surface-mount line be used for Reflow Side 1 and Reflow Side 2, precluding a quick flow-through that could use simple Exposure Time accounting.
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.
Faced with implementing the arduous dual-reflow exposure accounting system, Delphi evaluated Cogiscan's MSD Tracking system. This product is already a solution fully compliant with J-STD-033 that provides real-time tracking of all MSD's from the time they are removed from their dry bag until final reflow. But it offered only single-reflow capability.
The definition of a proper material identification methodology is the foundation for the MSD control system. For these reasons, the Radio Frequency Identification (RFID) technology was chosen for the application. RF tags are passive electronic devices that can be read and updated when placed within the range of a reader antenna.
The heart of the system is the Local Control Unit. It is a small footprint station equipped with an RF antenna and a touch screen display. In order to assign a Remaining Exposure Time to each PCB entering the assembly line in first pass, barcode readers are installed at the entrance and exit of the assembly line.
The main objectives of the system are to make sure the product is built per the guidelines of J-STD-033, prevent assembling components that have exceeded their floor life and prevent assembling boards in second pass that have expired components on them. This guarantees the product is free of reliability problems related to moisture-induced defects.
The control of moisture-sensitive components is a complex issue that has several implications in many critical aspects of the material flow and assembly processes. The installation of the Cogiscan system at Delphi Delco Electronics Systems provided an efficient method to control moisture-sensitive devices during the assembly process while providing relevant information for decision-making. The operators adopted the system quickly since all the complexity of J-STD-033 was taken out of their hands.
