As we discussed in the previous post, “Assembly” in electronics generally refers to method by which structure and interconnects are formed between the IC and its substrate. Assembly technologies serve several purposes.
First, assemblies provide electrical connections between the IC and the substrate for signals and power. Additionally, assemblies provide a mechanical support for the fragile IC, designed specifically to reduce stress on the chip. Assemblies are designed to protect the IC from any environmental elements. Assemblies also provide a path for heat to be dissipated from the IC. Assembly technologies can be broadly placed in two categories – wire bonding and flip chip.
Wire bonding is the oldest and most common assembly technology. In a wire bond assembly, the IC is mounted to the substrate with the “active face” – the face where the circuitry has been built – up. Small wires arch from the inputs and outputs (“I/Os”) on the outside edges – known as the “periphery” – of the IC to a specific on the substrate. These fine wires are typically made of gold, silver, or copper; and are bonded using localized heat created by ultrasonic vibrations. This is known as thermosonic bonding, which can be seen more clearly in this picture.
The primary reason wire bonding is so widespread is because of cost. Since the process has existed for such a long time, a large infrastructure exists to build assemblies. Therefore, wire bond packages are cheap to implement. Wire bonding also has other advantages, including highly flexible interconnects, a high rate of success in forming the interconnect (yes, these assemblies often go wrong), easily programmed thermal cycles, and a high reliability. However, the process also carries severe disadvantages. Since each wire must be bonded at each end in sequence, the process can be slow and throughput is very low. Additionally, the physical length of the wire interconnects creates higher electrical resistances and energy losses, costing power and producing excess heat. Since wires can only be bonded to the periphery of the IC, the overall I/O density of the package is very limited. The wires also must connect to the area of the substrate surrounding the IC, requiring a large substrate footprint and wasting the space underneath the IC. There is even a potential for “wire sweep” – where the tool catches and pulls multiple wires, causing catastrophic damage to the assembly before it is even complete. These serious disadvantages created the need for new assembly technologies.
To ease some of the problems associated with wire bonding, new processes were explored. One of these is Tape Automated Bonding, or TAB. Developed to address the throughput issue associate with Wire Bonding, TAB involves “gang” bonding of wires. However, while this assembly addressed certain limitations of wire bonding, like the throughput, others issues, like energy losses and wire sweep, still remained. TAB did pave the way for a more advanced assembly: the flip chip.
Today, flip chip has emerged as the best alternative to wire bond. The defining feature of the flip-chip package is a “flipped” IC, with the active side facing downward or toward the substrate. Flip-chip interconnects range between solder bumps, stud bumps, copper pillars, and conductive adhesives. Notably, the interconnections are all formed in a single assembly step. This is possible because the IC’s solder bumps and the substrate’s pads are added to each before the assembly.
Flip Chip assemblies have several advantages over wire bonding. First, the flip chip enabled chip-scale packaging by eliminating the need for interconnection space surrounding the IC. This means that the entire package for your device can shrink to something roughly the scale of the IC, paving the way for more compact electronics. The flip chip also enabled a significantly higher I/O density between IC and substrate, since the interconnection array can cover the entire surface area of the flip chip. This is why the flip chip assembly is said to be “fine pitch” – because the distance between the interconnects can be quite small. The interconnects in a flip chip assembly are much shorter than a wire bond, meaning that electrical losses and heat generation will be less severe. Flip chip assemblies have many options in terms of the interconnect structure, including stud bumping, solder bumps, and conductive adhesives. Most notably, flip chip has a higher throughput than wire bonding, since all interconnects are made at the same time. Though it is usually considered more expensive than wire bonding, with the right volume and at the right scale, it can be made to be cheaper.
In our next posts, we will discuss some of the most pressing challenges to flip chip assemblies and electronics in general, including intermetallic formation and CTE mismatch.
