Controlling MSD for Double-sided Reflow Applications

Estimated reading time: 7 minutes

The dual-reflow process improves design flexibility by allowing SMICs to be placed on both sides of the circuit board, but raises special issues when moisture-sensitive components are involved.

By Michael Blazier and André Corriveau

Moisture-sensitive components begin absorbing moisture immediately when their protective moisture barrier bag is opened; the moisture accumulates at the critical interfaces between the die, lead frame and molding compound. Subsequent exposure to reflow temperatures causes a rapid increase of the vapor pressure inside the package, which can cause internal damage, such as delamination. At best, the damage will cause the device to fail and will be caught during electrical test. At worst, because these defects typically are latent, the device will pass electrical test and escape to the field.

To prevent IC delamination, Delphi Delco Electronics Systems factories track moisture-sensitive devices (MSD) in accordance with IPC/JEDEC J-STD-033A. Although these guidelines are clearly defined, they can be challenging to implement, particularly if there are moisture-sensitive components on the first reflow side of the board.

Figure 1. Line integration.

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The maximum time a reel or tray of these components may be exposed to ambient moisture prior to final reflow is defined by J-STD-033, according to the moisture sensitivity level (MSL) of the particular component. In a single reflow process, the only exposure to soldering temperature follows within seconds or minutes after ICs are removed from their reels or trays and placed on a circuit board, so it is sufficient to track the exposure of the reels and trays until they are loaded on the placement machine. If a product changeover is involved, it also is necessary to track the additional exposure and time in dry storage for all the partial trays and reels that are removed and eventually reloaded on the placement machine. The guidelines on how to account for the desiccating effect of dry storage for previously exposed components also are defined in J-STD-033A.

In dual reflow, however, the ICs placed and reflowed on the first side continue absorbing moisture while waiting for the second assembly/reflow process. The exposure clock is not reset by the first reflow, and the total time of component exposure from opening the moisture barrier bag until the second reflow is complete must be considered.

To know how much safe exposure time remains for components on work-in-process after first reflow, the user must know when exposure commenced for each component. A pencil-and-paper system of tracking exposure history of components board-by-board would be complex and error-prone; at the very least, remaining exposure times must be known for all components mounted on a board, and assign the least of these as that board's remaining exposure time. Faced with implementing such an accounting system, Delphi evaluated an automated MSD control system.* This system is already fully compliant with J-STD-033A, providing real-time tracking of all MSDs from the time they are removed from their dry bag until final reflow.

Material Identification

The definition of a proper material identification methodology is the foundation for the MSD control system. The IPC/JEDEC standard requires that a moisture sensitivity label be placed on the outside of the moisture barrier bag. However, when the individual trays and reels are removed from the bags, this information is lost unless transferred to the carriers. The use of a barcode label is possible for reels but cannot serve as a global identification method because many MSDs are packaged in plastic trays. It also is important that any identification method be fully compatible with the equipment and processes that are associated with the trays and reels. For these reasons, radio frequency identification (RFID) technology was chosen for this application. RF tags are passive electronic devices that can be read and updated when placed within the range of a reader antenna.

Each RF tag contains a unique identifier that tracks and recognizes the specific container. They also contain a programmable memory to store identification and process data directly on the material. This means that the same tags can be used for a networked database in a closed-loop system, as can be used with a stand-alone reader to transfer data throughout the supply chain.

Typically, the tags are initialized in the system when components are released from stock. The initialization process only requires the entry of the part and lot numbers for the components. Generally, this information is on the sealed bag and can be typed or scanned in using a barcode reader. When a new part number is entered for the first time, the system will prompt the operator to enter the sensitivity level and component body thickness. This information is kept in the database and used to apply the rules of J-STD-033A correctly.

Data Capture and Real-time Tracking

After defining the material identification methodology, the next challenge was to provide an infrastructure to capture the data and keep track of the physical location of the materials and real-time status of the required exposure time (RET). The heart of the system is the local control unit (LCU), a small-footprint station equipped with an RF antenna and touch screen display. The LCU is located on the assembly line near the pick-and-place machine(s) that are placing the MSDs. Each time a reel or tray of sensitive components is added or removed from the machine, the operator needs only to place the tag near the antenna; the system will retrieve the relevant information from the tag and database automatically. The operator then performs a simple load or unload transaction using the touch screen. The system automatically keeps track of all the components loaded on the assembly line along with their location (machine and feeder position) and RET.

In order to assign an RET to each printed circuit board (PCB) entering the assembly line in first pass, barcode readers are installed at the entrance and exit of the assembly line (Figure 1). The two barcode reader kits and SMEMA control box are connected to the LCU. LCUs (a minimum of one per line) are networked to a central server that maintains the database and consolidates data from every line.

Process Control

The main objective of the MSD control system is to ensure that the product is built per the guidelines of J-STD-033A to prevent assembling components that have exceeded their floor life, as well as assembly of any boards that may have reached second pass with expired components. The challenge is to perform this while maintaining an efficient operation on the manufacturing floor. The built-in expert system guides the operator through the use of the MSD components, validating that no expired components see any reflow, but also making sure the boards are built with components that have enough remaining floor life for second reflow.

User Interface

In multi-shift operations, several operators are involved in the manufacturing process of a given batch of boards. Adding to this is the complexity of tracking MSDs during the first and second pass. Considering all the other tasks an operator must perform, the user interface should be easy and intuitive.

Because of the RFID technology, the operator has little to do to perform a transaction because the data acquisition is triggered automatically by presenting the tag to the antenna field. Most transactions are performed without any keyboard data entry, and the software is designed with built-in validation that maintains data integrity throughout the process.

Process buttons on the left side of the main user panel give access to process panels, each taking care of a specific aspect of J-STD-033A (Figure 2). The user interface includes a search tool that provides visibility and status of all materials on all the lines, including boards (Figure 3).

Conclusion

The control of moisture-sensitive components is a complex issue that impacts many critical aspects of material flow and assembly pro-cesses. For double-sided boards with sensitive components on the first or both sides, it becomes difficult to ensure that the product is built to J-STD-033A guidelines without implementing an automatic control system.

Figure 2. Process panel example.

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Figure 3. Watch list.

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An automated MSD control system provides an efficient method to manage MSDs during assembly while elevating relevant information for decision-making. Operators adopted the system quickly, since the complexity of J-STD-033A compliance was taken out of their hands. There was a significant reduction of training, especially for new operators, since they no longer needed to be MSD experts, but simply needed to follow instructions provided by the system.

By automating the component-tracking activity, machine operators can focus on other quality and productivity tasks. MSD control has become independent of operator skills and training levels. Most important, implementation of the MSD control system will lead to a straightforward financial justification due to improved quality, productivity and material savings.

*Cogiscan Inc.

References

1. J-STD-033A, IPC/JEDEC, July 2002
2. R.L. Shook and J.P. Goodelle, Lucent Technologies, "Handling of Highly-Moisture Sensitive Components — An Analysis of Low-Humidity Containment and Baking Schedules," IEEE ECTC, 1999.

Michael Blazier may be contacted at Delphi Delco Electronics Systems, Kokomo, IN; E-mail: michael.w.blazier@delphiauto.com. André Corriveau may be contacted at Cogiscan Inc., Bremont, Quebec, Canada; E-mail: acorriveau@cogiscan.com.