Common of Surface Mount Devices (SMDs)

The taxonomy of surface mount devices (SMDs) is so expansive and ever-changing that covering it in full would be impossible. But here are several types that are very common and very important to know.

MELF (Metal Electrode Face Bonded): Consisting of two terminals bonded to a cylindrical body, these SMD components are less expensive than flat chips but require special handing during assembly. Furthermore, one of their biggest disadvantages is their tendency to roll off solder pads during assembly. Generally speaking, they come in the form of diodes, resistors, and capacitors.

SOT Transistors and Diodes: These are usually rectangular and easy to place, though they're a bit outdated. The most common SOTs are SOT 23, SOT 89, SOT 143, and SOT 223. Its most common packaging is tape & reel.

Integrated Circuits (ICs):

Small outline Integrated Circuit (SOIC) – These are good SMT alternatives to the duel in-line package (DIP), due to their dramatically reduced size. In general, they take up 30 – 50% less space and 70% less thickness than an average DIP.

Thin Small Outline Package (TSOP) – TSOPs are low profile packages with fine-pitch leads. TSOPs are typically meant to accommodate large silicon chips in high density packages (RAM or flash memory ICs), largely because of their low volume/high pin count.

Quad Flat Pack (QFN) – QFNs are high lead count packages (44 – 304). Its leads are typically gull wing. There are many kinds of QFNs, and they are one of the most common surface-mount ICs.

Plastic Leaded Chip Carrier (PLCC) - Connections are made on all four edges of a square package with a relatively high pin count. PLCCs can have roughly 18 – 100 leads (usually J-leads). Many of them can fit into IC sockets and can be easily replaced in the field. PLCCs have long been a popular option. 

Lead-less Chip Carrier (LCC) – Not to be confused with PLCC, LCCs have no leads. Rather, LCCs are soldered directly onto PCBs by their (castellation) solder pads. These are usually designed for Mil Spec because, with no leads to damage, they're quite "rugged." LCCs are great for high temperature and aerospace applications.  

Pin Grid Array (PGA) – PGAs are typically square or rectangular, with pins arranged underneath the package. They're design was highly influential on the now ubiquitous BGA. 

Flip Chip – Flip chips are bare die packages, with small bottom-side solder bumps that act as leads. They are soldered directly onto the PCB.

Ball Grid Array (BGA) – BGAs are perhaps one of the best performing SMT packages in use today, due to their high densities. The BGA is a descendent of the PGA, yet instead of pins, it has solder balls that can be placed directly onto the PCB. Because of their high density, BGAs are typically used to house microprocessors.

Advantages: SMT allows for smaller PCB size, higher component density, and more real estate to work with. Because fewer drilling holes are required, SMT allows for lower cost and faster production time. During assembly, SMT components can be placed at rates of thousands—even tens of thousands—of placements per hour, versus less than a thousand for THM. Solder joint formation is much more reliable and repeatable using programmed reflow ovens versus through techniques. SMT has proven to be more stable and better performing in shake and vibration conditions.

Disadvantages: SMT can be unreliable when used as the sole attachment method for components subject to mechanical stress (i.e. external devices that are frequently attached or detached).

Overall, surface mounting will almost always prove more efficient and cost-effective than through-hole mounting. It is used in more than 90 percent of PCBAs today. However, special mechanical, electrical, and thermal considerations will continue to require THM, keeping it relevant well into the future.

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