Basic Design Considerations for Surface Mount Technology

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Basic Design Considerations for Surface Mount Technology

  1. 1. Hilaire Ananda Perera http://www.linkedin.com/in/hilaireperera Basic Design Considerations for Surface Mount Technology The fundamental difference between through-hole technology and Surface Mount Technology (SMT) involve the means by which the components are secured to the PWB. In through-hole technology, component leads are routed through holes in the PWB and soldered in place via a solder connection which forms on both sides of the PWB. This type of solder connection forms a solder “plug” which cannot pull free of the PWB. Additional mechanical reliability is afforded by the common practice of clinching the component leads to the PWB prior to soldering. These effects combine to produce a very reliable PWB. When SMT is employed, the component leads are “lap soldered” flush to lands which lie on the surface of the PWB. Since the component leads do not pass through the PWB, the metallurgical strength of the solder connection alone holds the components in place. This places additional stresses on the solder connection, leading to a higher probability of failure. SMT does offer several advantages not available from through-hole technology, each adding to the number of components which can be located on any PWB. Of primary consideration is the ability of SMT components to be mounted on both sides of the PWB. Additionally, the leads of many SMT components are configured to occupy no more surface area than the component body itself, thereby allowing SMT components to be mounted in closer proximity to each other. When space is limited, SMT is the obvious choice. Several special considerations must be made to ensure the reliability of the SMT employed PWB. These include: a. The Coefficient of Thermal Expansion (CTE) of the PWB and the components to be mounted must be closely matched. This is of particular importance for leadless SMT components. Mechanical stresses imparted during the cool-down phase of the soldering operation and during normal thermal cycling, are absorbed by the solder connections, causing accelerated cracking of the solder and premature joint failure b. Gold plating must never be employed as a soldering surface for SMT on either a component or the PWB. Gold plating has been shown to cause catastrophic solder joint failure by causing gold embrittlement which leads to the total disintegration of the connection c. At the design stage, solderability (the degree to which a surface can be wet by molten solder) is addressed when considering the material surfaces to be used for the solder connection areas. The optimum surface would be hot solder dipped or solder plated using a fusion process. Due to the extremely rapid oxidation of its surface, nickel should not be used as a soldering surface. d. When designing circuitry for SMT applications, it is important to select components which can withstand the required soldering temperatures and dwell times. SMT soldering processes generally heat the entire component, including the body, to soldering temperature. When using vapour phase soldering techniques, the components on the PWB can be heated to as much as 220 deg. C for one minute or longer. e. Do not design the PWB to be any smaller than necessary. As the inter-component spacing decreases, the number of acceptable methods which can be employed to solder the assembly also decreases. Tight component spacing on a SMT PWB can complicate the removal and replacement of a component during repair. When repairs involving component replacement are required, it is necessary to reflow the solder connections of the defective component without disturbing the solder connections of adjacent components. Extremely tight component spacing can make this impossible, rendering the design unrepairable.

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