what is SMD?

Added Date:2007-06-14

Surface mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices or SMDs. In the industry it has largely replaced the previous construction method of fitting components with wire leads into holes in the circuit board (also called through-hole technology).

An SMT component is usually smaller than its leaded counterpart because it has no leads or smaller leads. It may have short pins or leads of various styles, flat contacts, a matrix of balls (BGAs), or terminations on the body of the component (passives).

History

Surface-mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was done at IBM. Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of the board became far more common with surface-mounting than through-hole mounting, allowing much higher circuit densities. Often, only the solder joints hold the parts to the board, although parts on the bottom or "second" side of the board are temporarily secured with a dot of adhesive as well. Surface-mounted devices (SMDs) are usually made physically small and lightweight for this reason. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of through-hole parts.

Assembly techniques

Where components are to be placed, the printed circuit board has flat, usually tin-lead, silver or gold plated copper pads without holes, called solder pads. Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel stencil. If components are to be mounted on the second side, a numerically controlled (NC) machine places small liquid adhesive dots at the locations of all second-side components. The boards then proceed to the pick-and-place machines, where they are placed on a conveyor belt. Small SMDs are usually delivered to the production line on paper or plastic tapes wound on reels. Integrated circuits are typically delivered stacked in static-free plastic tubes or trays. NC pick-and-place machines remove the parts from the reels or tubes and place them on the PCB. Second-side components are placed first, and the adhesive dots are quickly cured with application of low heat or ultraviolet radiation. The boards are flipped over and first-side components are placed by additional NC machines.

The boards are then conveyed into the reflow soldering oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. This helps minimize thermal stresses when the assemblies cool down after soldering. 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 reflowing solder. One is to use infrared lamps; this is called infrared reflow. Another is to use a hot gas. At one time special fluorocarbon liquids with high boiling points were used, a method called vapor phase reflow. Due to environmental concerns, this method is falling out of favor. Today, it is more common to use nitrogen gas or nitrogen gas enriched air in a convection oven. Each method has its advantages and disadvantages. With infrared reflow, the board designer must lay the board out so that short components don't fall into the shadows of tall components. Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production. Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large scale automation, by focused infrared beam (FIB) equipment.

After soldering, the boards are washed to remove flux residue and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or limonene, derived from orange peels. Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water. When aesthetics are unimportant and the flux doesn't cause shorting or corrosion, flux residues are sometimes left on the boards, saving the cost of cleaning and eliminating the waste disposal.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging. If needed, they are sent to a rework station where a human operator corrects any errors. They are then sent to the testing stations to verify that they work correctly.

Main advantages

The main advantages of SMT over the older through-hole technique are:

* smaller, lighter components
* fewer holes need to be drilled through abrasive boards
* simpler automated assembly
* small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads)
* components can be fitted to both sides of the circuit board
* lower lead resistance and inductance (leading to better performance for high frequency parts)
* better mechanical performance under shake and vibration conditions.
* SMT parts generally cost less than through-hole parts

Main disadvantages

The one major disadvantage of SMT is the difficulty in manual handling due to the very small sizes and lead spacings of SMDs, making component-level repair of devices using it extremely difficult, and often uneconomical.

Reworking defective surface mount components

Defective surface mount components can be repaired by using a rework system. A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:

* Melt solder and component removal
* Residual solder removal
* Printing of solder paste on PCB, direct component printing or dispensing
* Placement and reflow of new component

Sometimes hundreds or thousands of the same part need to be repaired. Such errors, if due to assembly, are often caught during the process. But a whole new level of rework arises when: component failure is discovered too late, design defects go unnoticed until the end user experiences them, high-value products require revisions re-engineering change orders can revive a once-obsolete product, or firmware updates require the change of only a single die to reuse a product. These tasks require a rework operation specifically designed to repair/replace components in volume.

[«]Return

Online Help | Technology Document | Industry News | CIEEC News | Contact Us