Surface Mount Technologies and Devices: Pesky Enclosures and Smart Layouts

The surface of a planet discloses large amounts of information. Mountain ranges, trenches, tectonic plates, and ridges on the Earth’s surface show the continuing effect of tectonic uplift, weathering, erosion, the movement of sediment, climate, and human activity.

Images of Mars tell a different story. Although volcanic eruptions occurred, no plate tectonics allowed surface ruptures to remain open. Endless miles of dry, barren wasteland reveal the impact of gigantic dust storms. Without rain to cause erosion, impact craters remain undisturbed. The small number of rover tracks and the remains of probes leave a minimal human impact on the surface.

In the same way, we can also learn why PCB designs look and operate differently. The use of physically large through-hole components on a board may point to higher power or voltage applications with varying thermal and power requirements. Or, the design may call for through-hole components because of physical stress. From a production perspective, the larger board size, precision drilling, and inserting leads through the holes all push costs higher.

Using surface-mount technologies (SMT) on a board changes everything. Soldering components directly to the board surface reduces weight and space requirements. Because surface mounting also allows high component density and shorter paths, we encounter fewer resistance and inductance problems. Automated processes for component placement and soldering, smaller PCB sizes, and reduce weight keep production costs low.

Can You Identify the Surface Mount Technology?

Before your printed circuit board hits layout, you’ll have to understand it through simulation in SPICE modeling. From the beginning of your device considerations, proper utilization of SPICE tools will give you access to accurate SMT component libraries as well as equip your designs with accurate tolerances and temperature parameters. These options will ensure design security before moving it into layout and facing the tough task of navigating things like form factor and manufacturing specifications.

As you work with your PCB design, your team may discover different types of surface-mount device (SMD) packages and ID codes for passive components, transistors and diodes, and integrated circuits. In terms of flat chip resistor and capacitor packages, common designations refer to the size of the packages. Identifying a specific device requires knowing the package style and then referencing the ID code printed on the device.

However, manufacturers sometimes use unique codes for passive components or the same code for different devices. Because size codes may refer to inches or millimeters and ID codes vary from one manufacturer to another, design teams should consult component libraries for the manufacturer’s specifications.

Although your design teams may recognize the standard designations for passive devices, new manufacturing processes and technologies have introduced another level of size designations for miniaturized components. For example, the smallest possible surface mount tantalum capacitor operates in critical high frequency circuits, has an extremely low equivalent series resistance (ESR), and provides stable temperature and voltage performance.

When compared to a standard 0402 capacitor package (0.04” long and 0.02” wide), the micro-sized 0201 package requires ¼ of the physical space. The size and polarity requirements of the tantalum capacitor translate into the need for reliable schematic and assembly diagrams. Because the 0201 package relies on new assembly methods, designs teams should review specifications for handling and soldering as well as the availability of component handling equipment for the smaller capacitor.

Clear designations of component parameters on a board help manufacturers

SMD Choices in Circuit Board Design

When we move from passive to active components, design teams face a large—and increasing—number of SMD package choices and designations. The most common transistor packages include the Small Outline Transistor (SOT23, SOT89, SOT 143, and SOT-223 Small Outline Transistor). Mini-SOT packages measure about one-half the size of the standard packages.

SMD components will still maintain needs for temperature analysis and wise understandings of board tolerances. The more SMD packages you try and implement, the more necessary working through potential circuit board vulnerabilities like overheating or imbalanced current flow will be.

With integrated circuits, the choices become almost larger than life. Let’s take a quick look at the different package types and designations. Selecting the right components for your design may involve keeping the variety of parts and packages smaller to reduce the amount of equipment setup time. Here’s a few things to know:

• SMD IC packages include Small Outline Integrated Circuit (SOIC) for simple logic ICs and Very Large Scale Integration (VLSI) packages.

  • Within the SOIC category, more than ten different types exist with some having a small number of pins and others have a larger number of pins.

• SOIC designations vary with the outline of the package and the lead types.

  • As an example, Small Outline Medium (SOM) packages have a larger width than Small Outline (SO) packages but remain narrower than Small Outline Large (SOL) packages. SOJ and SOLJ packages utilize J-leads.

With changes in size and functional requirements, manufacturers offer higher density Small Outline Integrated Circuits. The Thin Small Outline Package (TSOP), Thin Shrink Small Outline Package (TSSOP), Quarter-size Small Outline Package (QSOP), the Shrink Small Outline Package (SSOP), and the Very Small Outline Package (VSOP) have higher pin densities within the same small outline package. Type T1 TSOP, QSOP, and VSOP integrated circuits have 56 pins and gull-wing leads.

Keep your transistor necessities close-in-hand when considering for your printed circuit board

Very Large Scale Integration (VLSI) circuits that combine millions of devices on one chip may feature plastic, ceramic (CLCC), or metal chip carriers (MLCC). The VLSI packages include Quad Flat Packs (QFP), Plastic Leaded Chip Carriers (PLCC), Leadless Ceramic Chip Carriers (LCCC), Ball-Grid Arrays (BGA), and microBGAs. Rather than use the inline mounting seen with the simple logic ICs, the VLSI devices have either a square or rectangular footprint. Ball-Grid Arrays have connections underneath the package that establish strong mechanical and electrical connections.

Moving the Board to Production Requires Teamwork

Transitioning from design to production with surface-mount technologies requires an awareness of temperature profiles. When establishing the temperature profile for your PCB, use the manufacturer’s specifications and industry standards to create a reliable solder joint and to keep from damaging the components. You may need to work with the fabricator to set the correct temperature profile, printing, mounting, and reflow guidelines.

PCB designs require good reflow results from preheating and flux activation to reflow and cooling. During the preheating stage, solder paste dries and any unneeded solder ingredients evaporate. Check the manufacturer’s specifications for the proper lead temperature so that the flux properly cleans the bonding surface. Flux activation keeps solder on all areas of the board at a consistent temperature.

The reflow stage increases temperatures each second. Fabricators can prevent the board from warping, bridging, or developing cold solder joints by maintaining the package body above the solder melting point for approximately one minute. Your team should also consult with the fabricator to ensure that hotter and faster reflow processes do not introduce moisture that can cause cracks or delamination. Controlled cooling allows the solder joints to form a physically strong connection and reduces stress on components.

Working through tolerances, board and component sizes, minor thermal management from the beginnings of your schematic design, using an accurate and well-versed SPICE simulator will allow you to accomplish your design and analysis goals. Thankfully, PSpice has been an industry-leading SPICE package for years and will be sure to enable any SMD you or your designers need.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.