Jumpering in difficult areas.

Increasing complexity and time-to-market pressures make the requirements for functional circuits more demanding. If an error exists during substrate development, traces are usually cut and jumper wires are added. Ball grid array (BGA) components make node access substantially more challenging than in cases where all component terminations and pads are exposed--a conventional jumper wire connection to the interior of a BGA footprint may be nearly impossible.

A new, patent-pending method for repairing circuits in difficult places, such as beneath BGA components, incorporates a low profile, narrow self-adhesive flexible circuit that does not interfere with BGA balls and joints. The pressure-sensitive adhesive provides mechanical stabilization. The ends of the jumper may be soldered to the substrate with a high-melting solder before the components are loaded for the minimum disruption to normal assembly processing.

Problem Description

An inner layer short, between two ceramic ball grid array (CBGA) signal pads (Figure 1), was discovered in the production of a complex printed circuit assembly (PCA). When the problem was discovered, over 5,000 PCBs were affected in the supplier's work-in-progress (WIP). Due to lead time con straints, customer demand and financial exposure, the defective boards had to be used without jeopardizing the quality and delivery of the final product.

[FIGURE 1 OMITTED]

Investigation

Several potential techniques were investigated in the solution evaluation phase. After experimentation, the self-adhesive repair flex circuit patch was recommended as the most feasible option (Figure 2). The solution required: 1) electrically isolated CBGA pads; 2) alternative paths for the electrical signals associated with the ball sites; 3) the utilization of standard surface-mount processes and equipment; and 4) that all reliability and performance specifications associated with the product be satisfied.

Evaluation

The $1.00, four-layer flex circuit has a 3M VHB layer that provides adhesion and is capable of withstanding soldering temperatures. Fifty fully assembled evaluation printed circuit assemblies (PCAs) satisfied all mechanical and electrical performance specifications. A sample of the modified modules passed the standard environmental qualification procedure for this product, which included thermal cycle, temperature-humidity, mechanical vibration and mechanical shock tests.

Implementation

The process flow used is seen in Figure 3.

[FIGURE 3 OMITTED]

Challenges

* Design--The flex patch thickness needed to be controlled (less than 4 mils), and flex openings maximized, to ensure good solder joint quality.

* Manufacturability--The flex patches were stored in nitrogen to minimize moisture-induced swelling. The patch needed to be aligned carefully to the PCA because a poorly aligned flex could lift the CBGA balls from the pads and cause open solder joints. Additional solder paste was applied in areas near the flex to compensate for the larger CBGA ball-to-pad separation distance. This task was accomplished by using a dual-head dispense machine with different dot profiles for various areas of the CBGA pads. The dispensed dots were inspected prior to CBGA placement. Special attention was applied to the flex opening areas to ensure that no trapped air pockets, which could interfere with solder joint formation, existed. The reflow profile was developed to minimize temperature differences between the CBGAs and PCA. A large temperature difference could result in a solder jumping effect--the solder paste wets and migrates to the hotter side of the interface first and creates an open joint.

* Quality Assurance--A special x-ray inspection procedure was implemented to inspect the joints using rotated angles. The solder ball size was monitored to detect ball diameter expansion (less than 20 percent) that is indicative of an open solder joint. An Ersascope inspection step was implemented to verify the outer row solder joint quality.

Conclusion

Over 2,500 PCAs were successfully reworked utilizing the flex circuit patch technique (Figure 4). The manufacturer was able to meet this product's time-to-market goals through the recovery of these defective boards.

[FIGURE 4 OMITTED]

Marvin Wong is a senior R&D engineer and Albert Yeh is a senior process engineer at Agilent Technologies' Colorado Springs Technology Center, Loveland, CO; e-mail: [e-mail] and [e-mail].

Implementation Contributions: Barn/Welsh, Joe Groshong and Dennis Burdick, all with Agilent Technologies.