Solder shock

These materials were conditioned for 60 minutes in 15 psi steam and then solder shocked for 20 seconds at 288°C. Their moisture absorption was measured and compared to their... 

FIGURE 10.11 Moisture absorption versus solder shock resistance for several materials. 

The solder shock test is one of several methods to assess the thermal resistance of copper-clad laminates. It is easy to perform and represents another key test during the early assessment of a material. There are a number of different methods to choose from, which will be described in detail in Section 12.5.2. During the initial assessment of the material, it is important to choose at least one of the described test methods to make certain that the material meets the minimum requirements, especially if the material is nsed in higher-temperatnre lead-free assembly processes. Aside from solder shock testing of bare laminate material, it is also recommended that the PCB engineer consider PCB-level temperatnre shock as well as repeated reflow testing with a particnlar focns on resin-reinforcement delaminations. This will ensnre that not only the raw material bnt also the completed PCB will be able to withstand the required temperatnre regime. 

Solder shock laminate Pass/fail IPC-TM-650, As received ... 

Missing component Placement, soldering Shock, abrasion, too little glue... 

Of the common metal finishes, HAST is the only one which can directly reduce reliability of the board. In a typical HAST process, the board receives a severe thermal shock when it is dunked into a bath of molten eutectic Sn-Pb solder. The PTHs can survive only a certain number of solder shocks without failure this process uses up one of these thermal cycles before the board leaves the fabricator. 

CuNiAu boards fabricated either with the NiAu as the Cu etch resist or by the SMOBC process followed by electrolessly plating Ni and Au can confer improved PTH reliability. There are two mechanisms for the observed improvement the enhanced rivet effect provided by the Ni and the elimination of Cu dissolution during solder shocks such as wave soldering or PGA rework. For high-aspect-ratio holes, electroless Ni confers an additional benefit because the plating thickness in the barrel is more consistent than for conventional electroplating. 

Laminates based on Rhone Poulenc s Keramid 601 polyimide resin are fabricated in a conventional laminating press, and processed in a manner similar to that used for epoxies but with an extended cure cycle or post-cure. The room-temperature mechanical and electrical properties are similar to epoxy laminates, as shown in Table 9.4. At elevated temperatures, the polyimides exhibit exceptional stability. In particular, the thermal coefficient of expansion in the Z axis does not change significantly up to approximately 240°C, as shown in Fig. 9.11. Exhaustive tests have shown that polyimide-based multilayer boards can withstand repetitive thermal cycling at elevated temperatures (>150°C) without cracking of plated through holes. Similar excellent results were also obtained in solder shock tests (10 s at 288°C in molten solder). The thermal stability of these materials is retained at temperatures of approximately 200°C for continuous exposure in air, which has qualified them for military applications. 

Uses. Lithium fluoride is used primarily in the ceramic industry to reduce firing temperatures and improve resistance to thermal shock, abrasion, and acid attack (see Ceramics). Another use of LiF is in flux compositions with other fluorides, chlorides, and borates for metal joining (17) (see Solders). 

Most commercial wave soldering machines have a preheating section before the PCB or assembly is immersed into the solder wave, the thermal shock is reduced As the ambient temperature has already been raised before the solder wave, the thermal gradient in the reed switch seal is reduced. 

Most industrial glasses have linear thermal expansion coefficients between 30 10 /K (thermal shock resistant borosilicate gla.ss) and 80 to 90 lO /K (window gla.ss). In composite systems, such as used in the electronics industry for fusing (soldering) glasses to metal conductors or casing components, the thermal expansion coefficients of the glass and the metal have to be precisely matched. 

Lastly, adhesives are used to dissipate stresses that may be generated from thermal excursions, mechanical shock, vibration, or moisture. Specially formulated adhesives are effectively used as underfills for flip-chip devices and ball-grid-array packages to compensate for mismatches of expansion coefficients among the solder, the silicon chip, and the ceramic or plastic-laminate substrate. Low-stress adhesives are also used to attach fragile devices such glass diodes and to dampen stresses due to vibration. 

For hermetically sealed microcircuits several screen tests should be performed before delidding the package. A particle impact noise detection test (FIND [MIL-STD-883 Method 2020 or MEL STD-750D, Method 2052-2]) is recommended. This shake test combines a series of shocks and vibrations and, through an attached transponder, acoustically detects any loose particles (for example, chips of cured adhesive, solder, wire, ceramic, or loose plating) that have detached and can cause shorted or open circuits and damage or breakage of thin wires and connections. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Uses Epoxy-a lic in photo resists, solder masks, conformal coatings, overprint varnishes, paper coalings in clear coatings-type applies. Features Enhanced impact resist., flexibility, and resist, to thermal shock Properties Gardner 7-8 color m.w. 1500 vise. 5800 cps (65 C) acid no. 2.4 tens. str. 2700 psi tens, elong. 38%... 

The adhesion of specimens subjected to environmental exposure tests was evaluated prior to and subsequent to the contact. Wet thermal shock testing consisted of five cycles each for IS minutes in boiling water and 2 minutes in ice water. The maximum transfer time between the two baths was 30 seconds. Thermal and temperature/humidity exposures were performed in controlled atmosphere chambers for 200 hours. Simulated solder tests comprised immersing the test. specimen in a silicone oil followed by solder flotation. The solder temperature varied from 232 to 288 C and the contact time was S or 10 seconds. In some cases, two flotations were performed on the same sample. The effect of a heat treatment at 135 C prior to solder testing was examined the heat treatment time varied from 0 to 16 hours. ... 

There are other clues as to how well a sensor will withstand shock and vibration. Excessive shock and vibration cause sensors to fail by breaking electronic components or solder joints inside the sensor. Sensors with many internal components and/or open spaces inside the enclosure are more susceptible to failures than less complex sensors that have any open space inside the enclosure filled with epoxy or some other substance. Sensors that are epoxy-encapsulated are injected or filled with epoxy in one of the final manufacturing steps. Eliminating open space within the sensor enclosure makes the sensor more rugged. Often this step also seals the sensor against the environment. Another process similar to epoxy encapsulation is potting. A sensor that is potted or has potted electronics has been filled with a substance that hardens and protects the internal components. 

Figure 12.4 Thermal shock cracks in PTC-ceramics. (a) In a quenched specimen (b) In a PTC resistor after soldering. The reason for the...

The vacuum space is sealed at the lower end of the can by a threaded, flanged stud which mates with the internal threads of the retaining member for the stack of stainless steel washers. The flange on the stud is soft soldered to the end of the retainer and a polytetrafluoroethylene disk located between the upper end of this stud and the lower end of the stud which supports the stack of washers absorbs some of the shock produced when a specimen fractures in the cryostat. 

The ability to resist a performance degradation when subjected to mechanical shock is a critical property that solder replacement ICAs must possess. There are ongoing efforts to develop ICAs that exhibit acceptable impact strength, ie capable of passing the standard drop test used to evaluate the impact strength of components attached to a PCB. Among the methods are ... 

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