Joint voids

G. Echeverria, D. Santos, P. Chouta, C. Shea, and R. Love, Effect of Lead-Free Processing on Solder Joint Voiding presented at IMAPS, Pasadena, California, May 11, 2004... 

Overall rework time was about eight minutes for lead-free and six minutes for SnPb profiles. In most cases, board temperature 150 mils from the reworked component was above the liquidus-reflow temperatures, which was also the case during SnPb rework. Time above liquidus (TAL), which was often close to 90 seconds, combined with higher peak temperatures for lead-free solder rework, let to increased solder joint voiding. More solder paste development work is needed to support the elevated lead-free solder-temperature profiles. 

In some cases, smaller voids can increase reliability by changing the crack pattern. Studies have shown that there is no reliability reduction when voids are present to up to 25% by volume in the joint. Voids can act as stress relievers, due in part to the compressive nature of air pockets. ... 

The effect of the bondline thickness on single-lap joints is well-documented in the literature. Most of the results are for typical structural adhesives and show that the lap-joint strength decreases as the bondline increases (da Silva et al. 2006 Adams and Peppiatt 1974). Experimental results show that for structural adhesives, the optimum joint strength is obtained with thin bondlines, in the range of 0.1-0.2 mm. However, the classical analytical models such as those of Volkersen (1938) or Goland and Reissner (1944) predict the opposite. There are many theories that attempt to explain this fact and this subject is still controversial. Adams and Peppiatt (1974) explained that an increase in the bondline thickness increases the probability of having internal imperfection in the joint (voids and microcracks), which will lead to premature... 

Casey, W. Reduction of BGA eutectic ball solder joint voiding. Proceedings of Surface Mount International, 1988 541-548. 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

In 1976 the Swedish government stipulated that no new nuclear reactors should be charged until it had been shown how the radioactive waste products could be taken care of in an "absolutely safe manner" (8). Consequently, the nuclear power industry (through their joint Nuclear Fuel Supply Co, SKBF) embarked on a program referred to as the Nuclear Fuel Safety (KBS) Project (8). In one of the schemes (9) a repository for spent nuclear fuel elements in envisaged at a depth of 500 m in granitic bedrock. The repository will ultimately contain 6000 tonnes of uranium and 45 tonnes of plutonium. The spent fuel elements will be stored in copper cylinders (0.8 m in diameter and 4.7 m in length) with a wall thickness of 200 mm the void will be filled with lead. 

In practice, clean (very clean ) cleaved or otherwise smoothed metal surfaces should be made to effect a firm mechanical contact using a strong force but one that is still insufficient to cause macroscopic deformation even at an elevated temperature. This will have to be been done, usually, in vacuum or at least in an inert atmosphere. The problems of hard to get to (inaccessible) joints and possibly objectionable thermal conditions and the resultant undesired microstructures such as Kirkendall voids, for instance, are minimized, if not eliminated all together. Thus, good-quality, distortion-free joints requiring no additional machining or other posttreatment can be achieved. 

Sheet or panel (metal, plastic, etc.) wall siding. Seal tight all lap or butt joints. Make certain that caps, covers and flashings don t have hidden voids due to siding configurations (flutes, corrugations, etc.)... 

Loss of theoretical adhesive strength can also arise from the action of internal stress concentrations caused by trapped gas and voids. Griffith11 showed that adhesive joints may fail at relatively low stress if cracks, air bubbles, voids, inclusions, or other surface defects occur as a result of the curing process. 

Since real surfaces are not smooth or perfectly flat and most epoxy adhesives are viscoelastic fluids, it is necessary to understand the effects of surface roughness on joint strength. A viscous liquid can appear to spread over a solid surface and yet leave many gas pockets or voids in small surface pores and crevices. Even if the liquid does spread spontaneously over the solid, there is no certainty that it will have sufficient time to fill in all the voids and displace the air. The gap-filling mechanism is generally competing with the setting mechanism of the liquid. 

Pastes and Mastics. Bulk adhesives such as pastes or mastics are the simplest and most reproducible adhesive to apply. They produce heavy coatings that fill voids, bridge gaps, or seal joints. They can be in the form of high-viscosity extrudable liquid or a trowelable mastic. 

Voids in bond (i.e., areas Joint surfaces not properly that are not bonded), treated... 

Other NDT Methods. Radiography (x-ray) inspection can be used to detect voids or discontinuities in the adhesive bond. This method is more expensive and requires more skilled experience than ultrasonic methods. The adhesive must contain some metal powder or other suitable filler to create enough contrast to make defects visible. This method is applicable to honeycomb sandwich structures as well as metal and nonmetal joints. 

Thermal transmission methods are relatively new techniques for adhesive inspection. Heat flow is determined by monitoring the surface temperature of a test piece a short time immediately after external heating or cooling has been applied. Subsurface anomalies alter the heat flow pattern and, thereby, affect the surface temperature. The surface temperature difference can be detected by thermometers, thermocouples, or heat-sensitive coatings. Liquid crystals applied to the joint can make voids visible if the substrate is heated. 

Designating the contact area by A, and the void area by Ar, we may write for the heat flow across the joint... 

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