A publication of the National Electronics Manufacturing Center of Excellence December 2004

EMPF Director

Michael D. Frederickson
mfrederickson@aciusa.org


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Improving Chip on Board Assembly
A
s the electronics industry is pushed towards reducing costs and space while increasing complexity and reliability, Chip on Board (COB) assembly can reduce the size and number of solder joints, lessen electrical resistance, and provide faster transmission speed. Most importantly, it is very difficult for companies to reverse engineer these designs.

Chip on Board is a hybrid technology that employs bare die devices interconnected to printed circuit boards (PCBs). The connections can be made by wire bonding, tape automated bonding (TAB), or flip chip bonding.

The following are Tech Tips to improve Chip on Board assembly:

1. Before attaching the bare die to PCB, burnish the wire bond pads with a soft wire brush. Cleaning will help to eliminate oxidation build up (Figure 5-1).

2. Use a vacuum tool tip to die attach the bare die, as it is critical to protect the bare die from mishandling. Use of a vision system will help eliminate inconsistent die misplacement. Conductive or non-conductive epoxy can be used for attachment compounds. After die attachment, use plasma cleaner to reduce epoxy bleed-out (O2 plasma cleaning might discolor conductive epoxy). It is recommended to use Argon gas to reduce discoloration in plasma cleaning. Figure 5-2 shows “Target” bare die attachment.

3. Chip on Board wire bond interconnects are critical to creating high performance, highly integrated systems. From design to manufacture, understanding the central considerations in wire bond assemblies is the key to successfully implementing this technology. Figure 5-3 shows 1 mil aluminum wedge bonds.

4. Make sure the unit is properly clamped in the work holder. It is critical that no movement takes place. Verify there is no movement by nudging the object with tweezers. If movement takes place, the unit must be secured during high speed bonding.

5. Make sure the wedge tool is in functional condition. Factors such as bond size, bond pad pitch, wire diameter, harness type, and metallization have an effect on bonding performance. The proper tool selection is essential for consistent wire bonding.

6. Glob Top encapsulant epoxy should be used to protect vulnerable microelectronic components from environmental, mechanical, and electrical damage. The Glob Top epoxy is applied to the aluminum wires and the bare die. The encapsulant will cure in 36-48 hours at room temperature or in 3 hours at 60°C. Figure 5-4 shows black Glob Top over a die component.

Inspecting Chip on Board assembly is paramount for consistent and reliable products. The wire bonds can be analyzed with visual X-ray methods as shown above. Figure5-5 shows the X-ray image of the die and wire bonds. At 0 angle of X-ray transmission, short and open wires are discernable for inspection. Prior to applying electrical power to your COB assembly, X-ray inspection can eliminate costly rework and repairs.

(Figures 5-1 – 5-4 are courtesy of ATC.)





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