How SMT Works

The components are placed on pads or lands on the outer surfaces of the PCB. Solder paste, which is a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process. The components are then placed on the PCB with high-speed pick-and-place machines. The boards are then conveyed into the reflow soldering oven. They enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. The surface tension of the molten solder helps keep the components in place, and if the solder pad geometries are correctly designed, surface tension automatically aligns the components on their pads.

There are a number of techniques for mounting electronic components on a PCB:

The main advantages of SMT over through-hole technology are:
– Smaller components
– Much higher number of components and many more connections per component
– Higher connection density
– Lower initial cost and time of setting up for production
– Fewer holes need to be drilled
– Simpler and faster automated assembly
– Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads
– Components can be placed on both sides of the circuit board
– Lower resistance and inductance at the connection
– Better mechanical performance under shake and vibration conditions

SMT Components

Surface mount components are usually smaller than their through-hole counterparts. Resistors, capacitors, and diodes can be 1/4 to 1/10 of the size through-hole components. In the past, integrated circuits were packaged in surface-mount packages with widths of 6.4 mm, 13 mm or more, with lead pitch spacings (distance between centres of leads) of 2.54 mm or more. In contrast, as of 2008, many components are much smaller, some are chip scale packages with lead pitch spacings of 0.4 mm or less. Components with spacings of 0.65 mm and 0.5 mm are common, and 0.4 mm not uncommon. As of 2015, 0.3 mm and 0.2 mm lead pitch spacings are used on some designs. At these spacings, the leads on fine-pitch parts are thinner or narrower than the 0.5 to 0.8 mm diameter mounting holes commonly used for through-hole parts, making insertion of through-hole parts into these denser surface mount boards difficult or impossible.

Common SMT component package types include:

SMT Manufacturing Process

The SMT component placement process generally follows these steps:
1. Solder paste printing
2. Pick and place
3. Reflow soldering
4. Inspection

Solder Paste Printing

The solder paste is applied to the PCB using a screen printer to coat the pads with solder paste. The screen printer applies solder paste using a stencil that lets solder paste through onto the pads but not onto the rest of the board surface.

Pick and Place

The components are placed onto the PCB with a pick-and-place machine. Modern machines are very fast, placing up to 136,000 components per hour, and are extremely accurate.

Reflow Soldering

After the components are placed, the PCB is conveyed through an oven to melt the solder and permanently attach the components to the PCB. This process is called “reflow soldering.”

Inspection

After soldering, automated optical inspection (AOI) systems check solder joints and component placement. If needed, human operators perform touch-up repairs.

Benefits and Challenges of SMT

The main benefits of SMT are:
– Increased circuit density and functionality
– Increased reliability
– Increased manufacturing automation
– Smaller PCB size
– Faster assembly
– Lower production costs

However, SMT also presents some challenges:
– Fine pitch and high density can create issues like solder bridging
– Thermal expansion mismatches between component and PCB can cause mechanical stresses
– Manual prototype assembly or component-level repair is more difficult and requires skill and appropriate tools and techniques

SMT Industry Trends

Some key trends in SMT include:
– Shrinking component sizes and lead pitches
– Increased adoption of BGA and chip scale packages
– Increased use of Flexible PCBs
– Growing demand for high-mix low-volume production
– More stringent quality requirements
– Environmental regulations like RoHS driving adoption of lead-free manufacturing

Frequently Asked Questions (FAQ)

What is surface mount technology (SMT)?

Surface mount technology is a method for constructing electronic circuits by mounting components directly onto the surface of a printed circuit board (PCB). Components are placed on pads on the surface of the board and soldered.

What are the advantages of SMT?

The main advantages are increased component density, reliability, manufacturing automation, and smaller PCB sizes. This allows faster assembly, increased functionality, and lower production costs.

What types of SMT components are common?

Common SMT component package types include small outline integrated circuits (SOIC), quad flat packages (QFP), ball grid arrays (BGA), and dual flat no-leads (DFN). SMT versions of passive components like resistors and capacitors are also widely used.

What are the steps in the SMT process?

The main steps are:

1. Solder paste printing – solder paste is applied to component pads
2. Pick and place – components are placed on the PCB
3. Reflow soldering – PCB is heated to melt solder and attach components
4. Inspection – solder joints and component placements are inspected

What challenges does SMT face?

Some key challenges include dealing with thermal expansion mismatches, solder joint defects due to fine pitches and high density, and difficulty of manual assembly and repair. Staying up to date with rapid technological changes and more stringent quality and environmental regulations are also ongoing challenges.