Prominent component shipping methods are tape and reel, tube, and tray form. Components are also available in bulk if required. But the bulk components are generally used only for very limited, simple components such as resistors and capacitors. ‘Tape and reel’ form is used for all surface mount components such as resistors, capacitors, diodes, SOTs, SOICs, PLCCs, SOJs etc. One major disadvantage of ‘tape and reel’ is that minimum quantity requirement, which may be of the order of a few hundreds of components, is to be met. Therefore tube packages are preferred by users where quantities involved are small. Because the chip resistors and capacitors are inexpensive, their inventory costs are negligible and therefore, they are available in bulk and ‘tape and reel’ form only. Tube package is used for components like PLCCs, SOICs (SOs in short) when the quantities involved are small. The selection process may also be affected by the pick and place equipment available with the user.
Various packaging styles are shown below.
SOT-23: Line Diagram
4. Component procurement guidelines
Component procurement specifications are important for any surface mount component as the technology is in transition stage. There are certain general guide lines, as given below for developing in-house specification.
- deal with a limited number of vendors, establishing specs in cooperation with the preferred vendors. The current trend is to rely on one or atmost two vendors.
- Only limited number of packages that enjoy widespread support should be selected for use. Limiting the number of part types provides such benefits as increased purchasing leverage, reduced inventory cost.
- The component selected should be qualified for performance and reliability, solderability, component tolerance and compatibility with land pattern design, and compatibility with processes and equipment used in manufacturing.
- Surface mount standards, even when they exist, are not supposed to be used as procurement specifications. The general intent of most standards is to accommodate the needs of all member companies. Therefore the industry standards should be used as guideline, not as a substitute for in-house procurement specifications.
In summary, surface mount components serve the same functions electrically as their through-hole counter parts. However, they have smaller package area and better electrical performance. At this stage, all the components are not available in surface mount form and therefore, the designer has to choose rightly between conventional and surface mount components. As the surface mount technology is in transition, one must develop in-house procurement specifications to take advantage of SMT.