Thermal-shock cycling

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

Epoxy systems with flexibihsers and properly-selected fillers exhibit high resistance to rapid changes in temperature and do not show signs of cracking or shattering. Rigid epoxy systems can cause severe problems. Better formulated systems can withstand repeated thermal shock cycles from 180°C to -75°C without failure. 

In the present experiments, the low temperature of the cycle is maintained at 850°C, and tliree high temperature differing cycles were tested 850°C (no thermal shock , cycle 1), 950°C (cycle 2) and 1050°C (cycle 3). These reference temperatures are measured at the immediate inlet of samples. 

CVI SiC/SiC composites have been tested in thermal shock with excellent result [2,28]. CVI SiC/SiC generally had good strength retention after thermal shock cycles involving heating up to the desired temperature and then cooling down in water at 20° C. 

Exposure Temperature (°C) of Thermal Shock Cycles Flexural Strength (MPa) % Strength Retained... 

The resistance variation was less than 3% after 1000 h aging at 85°C/85% RH, 100 thermal shock cycles at —65 to +150°C, and lOOOh aging at 150°C. Figure 1.1.20 shows the SEM image of cross-sectional view of the package which was used for thermal shock cycle test. No change was observed in microstructure of the inner metallization. 

FIGURE 57.7 Cycles to failure vs. PTH aspect ratio tor -65 to +125 °C thermal shock cycles. Various hole diameters, board thicknesses, and board constructions. (After Ref 3.)... 

The fracture toughness values determined by the chevron notched specimen technique are strongly dependent on the condition of thermal shock, in particular for the first ten thermal shock cycles, as shown in Figure 7. In this figure, the fracture... 

It was noted that the highest number of thermal shock cycles that was applied in this experiment (24 cycles) led to specimen failure without any further mechanical loading the specimen broke upon quenching into water. Fracture plane analysis showed that the crack had developed along the interface between two fibre bundles. 

Figure 7. The effect of thermal shock cycling on fracture toughness of the hybrid composite.

Figure 8. SEM micrograph showing a typical fracture surface of a composite after 20 thermal shock cycles...

After the first five thermal shock cycles the fracture behaviour is very similar to that of as received composites (compare the corresponding curves in Figures 7 and 10). 

For higher number of thermal shock cycles two typical features arise on the load deflection traces. The first one is a marked pop-in behaviour at loads of about 10 N (marked by circles in Figure 10). The second phenomenon observed is increasing specimen deflection at fracture (maximum) load, as evident when comparing the curves for 10 and 24 cycles in Figure 10. This can be caused by the change of crack tip behaviour affected by damage development at the matrix/fibre interfaces. 

The values of thermal shock cycles temperature - water for fireclay and mullite refractories are within 5-7, and up to 30-50 cycles for silicon carbide refractories. The values of thermal shock cycles temperature - air for fireclay and mullite refractories are within 25-30 cycles, and up to 50-100 cycles for silicrHi carbide refractories. 

To prepare the experiments, a team of product development and materials engineering professionals was formed. Team members determined components that would be the most critical for lead-free soldering. Based on experience gained from examining several hundred PCBs after thermal-shock cycling, the team advised that solder joint problems would be more prevalent when one or more of the following conditions were present ... 

The components that were monitored were those expected to suffer the most thermal-stress damage. Solder joints, including some of these components were cross-sectioned, ground and polished for evaluation after completing 300 thermal-shock cycles. 

Figures 2 and 3 represent typical cross-sections of metal-wrap-tab solder joints. The protective copper coating, in both cases, was OSP and the laminate was FR4. In this case, only the liquid soldering flux was different. These solder joints were rated B, acceptable with only a slight deformation after completing 300 thermal-shock cycles.

Other results relate to a heat-sink-tab solder joint and a metal twist-tab solder joint, both using the SACX alloy on two different single-sided PCB applications. The laminate types were composite glass/paper (CEM-1) and paper phenolic (ERl). The protective coating was rosin-based for both. The failure mechanism was alloy failure therefore, solder joints were labeled D and E, not acceptable. The twist-tab solder joint was lifted and fractured after completing 300 thermal-shock cycles. 

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