Solder is a fusible metallic alloy used to join metallic surfaces. A variety of solder alloys are available and are designed for melt temperature and to control the alloy layers in the final solder joint interconnection. Solder joints primarily provide a conductive connection for electrical circuits but, in concert with electronic part leads or attachment pads, must also perform reliably under induced and exposed environmental conditions such as mechanical shock, vibration, temperature induced tensile stress, and vacuum.
Process Engineering Considerations for Quality and Reliability
Solder joints will fail when a pull or bend force on the solder joint overwhelms the tensile strength of the solder itself. The more likely cause of solder joint failure is the growth of small defects in the joint into a large defect which appears as a crack between the two conductors which are intended to be connected by the solder joint. Cracks can create a permanent and sustained disconnect or can make an intermittent electrical connection that appears normal at times and then with vibration or expansion and contraction with temperature, makes the connection appear to fail or to change its electrical resistance. Defects can exist at the crystal lattice level or at the macro level. The time it takes for defects to coalesce and form a crack all the way through the joint is called the time-to-failure and is usually measured in the number of thermal cycles (of a particular temperature range) or time exposed to vibration (of a certain range of superimposed frequencies). Engineering evaluation of the reliability of solder joints based on the type of solder used, the shape of the part lead, the shape of the solder joint, the metallic composition of the formed joint, or other parameters, will test hundreds or thousands of joints in order to predict their life expectancy.
Reliability studies are normally performed for solder joints of a particular design to discover their time-to-failure for a temperature cycling range or vibration range of interest (e.g. for use in cars, or use in spacecraft, or use in medical equipment). Reliability is always relative to the environment of interest and the intended useful lifetime (e.g. two years for cell phones, fifteen years for the James Webb Space Telescope, five years for appliances). Reliability tests are meaningful when the test samples are identical and the sample set does not contain manufacturing mistakes. Early failures of samples that contain manufacturing mistakes are called “infant mortality failures” and are not usually counted in the end-of-life prediction.
It is not unusual that a solder joint with a particular type of macro defect will last a very long time. Researchers have found it extremely difficult to simulate identical macro defects in sufficiently large sample sizes in order to generate reliability results for particular types of macro defects. However, common knowledge about the relationship between macro defects and circuit failure has developed over time from examination of reliability test failures and from hardware returned from use for repair (“field returns”). The manufacturing parameter settings which create these macro defects are also known. Both the appearance of common macro defects (if it can be viewed externally or by X-ray) and the conditions known to create them are the subject of requirements in industry quality standards.
Solder joints which do not contain manufacturing mistakes are considered high-quality. Manufacturing mistakes are known to be caused either by poor planning for or poor control of the variables of the manufacturing process. The work to design the soldering process, including selection of the materials to be used and the process conditions including temperatures, times, and surface preparation, is called process engineering. Successful soldering process engineering will prevent manufacturing mistakes which create macro defects in solder joints which cause early life failures. It is impossible to prevent micro defects in solder joints however they can be minimized by understanding the effects of soldering temperature conditions and trace metals that are present in the solder selected and in the alloy that is formed when the melted solder combines with the leads or surfaces being soldered.
Visual inspection is used to check finished solder joints for macro defects. Many solder joints with externally observable defects can be fixed by reheating the joint (though solder joint reheating can be damaging to a printed circuit board). However, many of the macro defects that cause early life failure cannot be seen externally and the user must rely on the process engineering to know that the solder joints are high quality. Some visual inspection criteria defined in industry quality standards use the appearance of solder joints made under well-controlled conditions as an inference for the absence of macro defects inside of the joint; if the outside looks good, the inside is probably also good. Solder joint quality assurance consists of encouraging manufacturer to employ good process engineering while visually checking the appearance of the finished joints for macro defects or evidence that macro defects may exist internal to the joint.