ME Labs, Inc.
PIC® a Package
As published in MicroComputer Journal
Integrated circuits (ICs) come in a wide variety of packages and PIC microcontrollers are no different. The package surrounds the actual silicon chip and protects it and allows connections to it.
In some cases, the silicon chip itself is mounted directly to the PC board without any package, only epoxy glopped on to protect it. Fine wires are welded from special pads on the die and connected to pads on the PC board. After personally experimenting with bonding die right onto the board, I have discovered that, in most cases, it is better to use a packaged surface mount part.
Choosing the appropriate package is not rocket science, but it helps to know your options. ICs come in different packages for different uses. The short list is through-hole DIPs, surface mount, PLCC and BGAs.
DIP, DIP, DOOWAH DITTY
DIPs are the most common packages we see, but how many of us know what DIP means (and just how important is it to know?) A friend in college said it stood for Double Integrated Person, while referring to me, of course (hey, it was college). It actually stands for Dual In-line Package for the rows of pins down two sides.
DIPs are common because they are inexpensive, easy to handle and plug into sockets to allow removal for reprogramming or to allow you to easily change them if you let the smoke out. There are several different kind of DIPs. Ceramic DIPs (CERDIPs) are usually used early in a device's life cycle to build the first batches of a new part. They are the most expensive package. Ceramic DIPs are also used to house UV erasable parts. These parts have a quartz window in the top so that UV light can shine directly onto the die to erase the device. Erasable ceramic DIP PICs carry the /JW designator.
Plastic DIPs are the most common and the cheapest parts. Plastic DIP PICs carry either the /P or /SP (skinny plastic - 28-pin, .3" wide) designator.
DIPs can have any number of pins (well any even number) from 4 to 64 or more. The pins are usually on .1" centers, though the 64-pin DIP PICs are on .07" centers. Most parts with 8 to 28 pins are .3" wide. Some 28-pin parts and 40-pin parts are .6" wide. The 64-pin DIP is .75" wide.
DIPs can be used for prototyping a circuit or even for final production, if size is not of concern.
On the surface...
Surface-mount parts are the next most useful to us. Once a prototype is working properly, production units can be made using surface mount technology. This allows a much smaller product to be created and can even save money. Smaller PC boards with fewer holes cost less and many assembly houses are actually charging less to assemble surface mount boards. Not having to form and cut leads can be a real time saver.
Naturally there are a bunch of surface mount packages. The most applicable to us are SOIC, SSOP, MQFP and TQFP. SOIC stands for Small Outline Integrated Circuit. While the body of an SOIC part is still .3" wide, it is flatter and about half the length of a DIP as the pins are spaced at .05". 18 and 28-pin PICs in this package have a /SO designator. 8-pin PICs with the /SM designator are available in a medium SOIC package that is only .208" wide.
Let's get small
SSOP, Shrink Small Outline Package parts are smaller than SOIC parts with a body width of .209" and a pin pitch of .026". These are getting pretty small and can be hard to handle. They are also more expensive than SOICs so most people prefer to use SOICs. SSOP 28 and 20-pin (there is no 18-pin SSOP package) PICs go by the /SS designator.
When you move up to 44-pin surface mount, you move to an MQFP or TQFP. The Metric Quad Flat Pack is 10mm (approximately .4") square with 10-pins on a side spaced at .8mm (about .031"). The pin pitch is kind of a pain for layout, but you can get a lot of I/O into a small space. The MQFP PIC designator if /PQ. The Thin Quad Flat Pack has the same external dimensions as the MQFP, except it is flatter and more money. The PIC designator for TQFP is /PT.
The 64-pin TQFP gets you even more I/O into the same 10mm space. The pin pitch drops to .5mm. While I've layed-out PC boards for pitches this tight, I don't want to be the one to solder on this part.
As the number of pins gets large, the DIP package starts to get unwieldy. But it would still be nice to use a part that can be popped into and out of a socket, if necessary. To this end, 44 and 68-pin PICs are also available in a Plastic Leaded Chip Carrier, PLCC. The 44-pin part is about .65" square and the 68-pin device is about 1" square. A ceramic quad package that has a window in the top gives you an erasable device for the 68-pin parts. Its PIC designator is /CL. The plastic PLCC PICs go by the /L designator.
By the way, the extra pins on 20-pin SSOPs, 44-pin MQFPs or TQFPs, and 68-pin PLCCs are either extra power or ground pins or no-connects.
There are a couple of other PIC packaging alternatives that are not strictly IC packages. Some manufacturers have mounted surface mount PICs along with a 5-volt regulator and oscillator circuit to a small PC board to create a SIP (Single In-line Package) or SIMM (Single In-line Memory Module). While these are larger than the other PIC packages, they can make prototyping easier as all you need to add is power.
Finally, BGAs are Ball Grid Arrays. They are usually square ceramic packages with the bottom covered in small solder balls. It's pretty tough to prototype with these guys. Fortunately there are no PIC BGAs... yet.
ME Labs, Inc.
PO Box 8250
Asheville NC 28814
(719) 520-1867 fax