by September 12, 2002 0 comments

What you see of a microprocessor is just the packaging, with the micro- processor and its associated circuits inside. This packaging, among other things, is designed to protect the microprocessor, and acts as its link to the ‘outside world’. We’ll take a detailed look at the packaging technology.

Intel (source: Intel Packaging Handbook) defines a package as “An enclosure for a single element, integrated circuit, or hybrid circuit. It provides hermetic or non-hermetic protection, determines the form factor, and serves as the first-level interconnection externally for the device by means of package terminals.” Sounds impressive? But what does it mean? A package is designed to protect a circuit–in our case, the processor and its associated circuits. The package can be sealed to make it gas-tight (hermetic) or otherwise (non-hermetic). As said earlier, a package acts as the communication medium for the enclosed circuit and also determines the external shape and dimensions of the processor (form factor). But it’s not all that simple either; a package has a lot more to it than a solid plastic case with a few wires (or pins) coming out of it.

Packaging is as important as the 
processor itself 
Keep it up, packaging 
The number of transistors in modern-day products is growing at an explosive rate, making them more and more complex as well as functional. At the same time, they are being shrunk to unimaginable (and in some cases, even invisible) sizes. These advancements would come to naught if the packaging technology were unable to keep pace, because the performance of any communication medium is as good as its weakest link. In other words, while a good packaging design won’t necessarily add something to processor performance, a bad one would certainly rob it of most of its sheen. 
It is important to have an efficient package design as it influences both heat dissipation and the amount of electricity that can flow through a processor. Thus, a lot of effort has been made to keep the packaging technology at pace with the processors and we’ve seen a host of new technologies come up like the OLGA (Organic Land Grid Array) and the PCPGA (Flip Chip Pin Grid Array).

Material change
The most well known packaging type is the PGA (Pin Grid Array). Being among the oldest of packaging technologies, prevalent since the Intel 80386 processor, PGA consists of a flat square with a series of mounting pins sticking out from the bottom. The 386 used a Ceramic Pin Grid Array, which was nothing but a PGA design with a ceramic packaging. This was the simplest of package designs but not without its share of problems, notably the ones faced while inserting or removing the processor. 

As processors become more complex, the number of pins on them also grew. Due to this the regular PGA packaging made it difficult to insert them into a slot without the risk of bending a few pins. Keeping this in mind, motherboard manufacturers came up with a newer ‘Low Insertion Force’ socket design that had less tight holes without affecting the contact points, and eventually the ‘Zero Insertion Force’ design, prevalent even today. It consists of an arm, which when snapped into place, holds the processor at its position. This has allowed the holes to be much looser and made the task of removing/adding a processor easier than ever before.

With a major problem of the package design out of their way, manufacturers started thinking of other ways to improve it, and it was only a matter of time before ceramic was replaced by the material of modern times–plastic. What resulted was the PPGA (Plastic Pin Grid Array), which is used even today in some chips. The earlier Pentiums used a slight variation of the PPGA, called the SPGA (Staggered PGA), which allowed more pins to be crammed onto the package. 

The PPGA, which was used in the earlier Celerons, has been further built upon to get the PPGA2 and the FCPGA (Flip-Chip Pin Grid Array). In the FCPGA style of packing, the die (or the core of the processor) is turned upside down so that it is on top of the processor, as compared to PPGA wherein it lay towards the bottom. This means that the thermal solution can now be applied directly to the die, which allows for much better cooling. The FCPGA is used in Celerons and PIIIs (370 pins) and has been developed to get the FCPGA2, used in the P4 (478 pins), which has an IHS (Integrated Heat Sink) in-addition to everything FCPGA has to offer.

The material used to pack the chips (die) has also undergone a sea change. While earlier PLGA (Plastic Land Grid Array) was used to wrap the chip, which meant that making electrical connections to the die was a tedious process, now the die is wrapped in OLGA (Organic Land Grid Array), which is an “an organic substrate interspersed with copper” and hence, due to its metallic nature, allows for better connectivity to the chip.

Another popularly used packaging type is the PBGA (Plastic Ball Grid Array). The primary difference between PBGA and PPGA is that it has balls instead of pins at the bottom. This removes a lot of handling problems faced in the PGA packaging as there’s no longer a threat of the pins bending or breaking. Also, they are easier to place into the slot and offer a slight performance advantage.

The third popular packaging type is SEC (Single Edge Connector). This packaging is similar to that of memory modules or any other expansion cards present in your computer. Such processors are mounted perpendicular onto the motherboard into a single slot and use “goldfinger contacts” instead of the pins (or balls) to communicate with the motherboard. An example is SECC (Single Edge Short Cartridge) packaging, which was used in the earlier PII (242 contacts) and PIII Xeon (330 contacts) processors consisting of a metal casing as well as a thermal plate which acted like a heat-sink. The SECC2 package, with less casing and no thermal plate, was developed from the SECC and was used in some later PIIs and PIIIs (242 contacts).

A packaging technology, used in some of the laptops these days, is TCP (Tape Carrier Package). This doesn’t involve any ceramic or plastic casing. Instead, the die is covered by a thin, protective plastic sheet and bond patterns and wires are printed on this plastic sheet. This means that the entire processor is as thin as 1 mm and weighs less than 1 gm! Such processors provide excellent performance and thermal characteristics and are ideal for places where space and weight are a constraint. While a lot of work is yet to be done in this technology, it is clearly the way forward as far as the packaging technologies are concerned.

It might not be true for other things in life but as far as processors are concerned, packaging is as important as the product.

Kunal Dua

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