Volume 1, No. 2, 4th Quarter 1993

Basic Design Considerations for Surface Mount Technology

By: Lawrence A. Genereux

There was a time in the not too distant past when Printed Wiring Assembly (PWA) design considerations primarily involved form, fit and function. A robust electronic design, assembled using standard through-hole technology, would stand the test of time with little or no special mechanical consideration given to the design of the PWA. This, unfortunately, is not the case when the Printed Wiring Board (PWB) is assembled using Surface Mount Technology (SMT).

The fundamental difference between through-hole technology and SMT involves 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 can not 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 PWA. 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.

For its fundamental failure mode, SMT does offer several advantages not available from through-hole technology, each adding to the number of components which can be located on any given PWB. Of primary consideration is the ability of SMT components to be mounted on both sides of the PWB, while through-hole technology components can be mounted on only one side 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 than would be possible with the relatively long leaded components required for through-hole technology. When space is limited, SMT is the obvious choice.

When considering the application of SMT to an armament system, several special considerations must be made to ensure the reliability of the PWA in such a system. These include:

1. The Coefficient of Thermal Expansion (CTE) of the PWB and the components to be mounted thereon must be closely matched. This is of particular importance for leadless SMT components, such as leadless chip capacitors. Leadless capacitors are soldered directly flush with the lands of the PWB, without the stress relief which is normally afforded by the component leads. 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.

2. Gold plating must never be employed as a soldering surface for SMT on either a component or the PWB. Once considered to be an excellent soldering surface, gold plating has been shown to cause catastrophic solder joint failure by causing gold embrittlement which leads to the total disintegration of the connection.

3. The solderability (the degree to which a surface can be wet by molten solder) of the soldering surfaces must be as close to ideal as possible to ensure the optimum solder connection quality for SMT. At the design stage, solderability 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 fusing process. Solder plating without fusing is not desirable. Additionally, due to the extremely rapid oxidation of its surface, nickel should not be used as a soldering surface.

4. When designing circuitry for SMT applications, it is important to select components which can withstand soldering temperatures and dwell times. Unlike through-hole technology soldering processes which generally apply direct heat only to the component leads, SMT soldering processes generally heat the entire component, including the body, to soldering temperature. When using vapor phase soldering techniques, one of the most common SMT soldering methods, the components on the PWA can be heated to as much as 430°ree; F for one minute or longer. It is essential that potential component damage from this source be considered when specifying component parts.

5. Do not design the PWA to be any smaller than necessary. A clever layout of the components on the PWB may reduce the spacing between the components, thereby reducing the overall size of the board. However, if this is done solely because it can be done, and there is no need for the PWA to be that small, producibility will suffer needlessly. As the inter-component spacing decreases, the number of acceptable methods which can be employed to solder the assembly also decreases. Additionally, tight component spacing on an SMT PWA can complicate the removal and replacement of a defective component should it become necessary to repair the PWA. When SMT solder connections are initially made on a PWA, they are soldered en masse. However, 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.

   This article is by no means a complete treatise on the subject of SMT design. The intent herein was to initiate the design engineer to some of the common pitfalls of SMT design.

   For additional information contact: US Army Armament Research, Development and Engineering Center, (ARDEC) High Reliability Soldering Technology Center, (201) 724-4466 or 7319.

   About the Author: Lawrence A. Genereux is the Program Administrator of the Picatinny Arsenal ARDEC High Reliability Soldering Technology Center and MIL-STD-2000 Certification School.

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   From the Reliability Analysis Center Webserver. Comments or questions can be submitted using e-mail or telephone at (315) 337-9933. (Last update: December 1, 1993, jmc)