Overview

This part of IEC 62137 describes the selection methodology of an appropriate test method for a reliability test for solder joints of various shapes and types of surface mount devices (SMD), array type devices and leaded devices, and lead insertion type devices using various types of solder material alloys. The test methods indicated here are intended to assess durability of joints of a device mounted on a board, but not for testing the mechanical strength of the device itself. Lead-free solders have other properties than the usual eutectic solder or nearly eutectic tin-lead solder. Reliability of solder joints made of lead-free solder may be reduced by the composition of the used solder, the shape of the terminals/lead wires and the surface treatment. An introductory chapter Terms and definitions deals with definitions and used terms such as pull strength for SMD, that is the maximum force used to break the joint between a lead wire and a substrate, shear strength for SMD, that is the maximum applied force to break the joint of an SMD mounted on a substrate when a force is applied parallel to the substrate and at a right angle to the side of the specimen, and torque shear strength for SMD, this is the maximum torque at the SMD applied parallel to the substrate surface to break the solder joint between a terminal/lead wire and the land on the substrate. Factors influencing the reliability of the joint are explained, for example, during use of SN96.5AG3Cu.5 solder. This solder has a higher melting temperature and is harder as eutectic lead tin solder; it is only hardly workable in solid condition. As a result, the stress caused in the joint is higher than for the eutectic tin-lead solder. Due to accelerated conditioning these properties can lead to breakage of the solder joint. Surface treatment of the terminals/lead wires of the SMDs may not only influence the test results for the drop test, but during other tests as well. For this reason they should be taken into account during all tests. The selection of the suitable test method deals with the stress of solder joints at the application site and test methods and the selection of test methods using the shapes and terminals/lead wires of electronic components for surface mounting devices and components for insertion mounting. Common aspects of the test methods are given; this includes the mounting of the components and materials used, solder conditions such as reflow soldering, wave soldering, accelerated conditioning and the selection of test conditions and evaluation of test methods. In the chapter Methods of the proof test, the strength test of the solder joint of SMDs, cyclic bending stress test, fatigue test at mechanical shear stress and cyclic drop test with the steel ball are considered. ((Figure)) During the cyclic drop test, mounted SMDs are tested in portable devices. During the test, a test substrate with a mounted SMD with the mounted side downwards is clamped into the substrate securing rig and is dropped from a certain height onto an impact test rig. The number of drop procedures until breakage of solder joint takes place is detected by electrical discontinuity by using a measuring instrument for electrical resistance. Breakage of the joint is caused by the stress in the test substrate which is introduced by the impact of the load during collision with the test substrate. ((Figure)) During the cyclic drop test with the steel ball, a test substrate with a mounted SMD and the mounted side downwards is clamped into the substrate securing rig and a steel ball is dropped from a certain height onto the back of the test substrate. The point where the ball strikes the large SMD should be near the component's circumference, as the weakest points for mechanical damage are found there. Breakage of the solder joint is detected by electrical discontinuity. Annex A explains the conditions of rapid temperature change. The time at which a test specimen shall be subjected to effective examination of an environment is decisive. The stress acting on the solder joint decreases over time by causing ballistic stretching in the solder joint. The initial change of the stress is very high, but over time the stress decreases. Fatigue of the solder joint by repeated temperature change strongly depends on the non-ballistic stretching. The stress caused by the various thermal coefficients of expansion of the termination/lead wires of the test substrate and component during the test with rapid temperature changes in the solder joint decreases over time; non-ballistic stretching takes place causing fatigue of the solder joint. For durability testing of the solder joint the joint shall be maintained for some time at a high or low temperature. It is not appropriate for the evaluation to maintain the joint at a temperature at which stress relaxation is insignificant. Annex B describes the electrical continuity test for solder joints. With this test, durability of solder joints is evaluated by monitoring the electrical continuity through the joint without mechanical stress. This test method is particularly suitable for testing durability of the solder joint for components with several terminations such as BGA and LGA, for which a durability test of the solder joint such as a shear strength test cannot be performed. The torque shear strength test is described in Annex C, the simple flexural strength test is described in Annex D and the cyclic drop test with the steel ball is described in Annex E. The cyclic steel test with the steel ball is a simplified test for SMDs such as BGA, LGA and QFN in portable devices as an alternative to the cyclic drop test (for jogs). With this test the durability of components mounted on a board is not evaluated as such, but rather the relative comparison of the correlation of durability of a joint between mounted components subjected to stress exercised by dropping a steel ball. The evaluation methods for fillet lifting of a solder joint with a component for insertion mounting are explained. There are three types of fillet lifting: lifting of the fillet between lead wire and solder joint, fillet lifting between fillet and land and fillet lifting between land and board. The responsible committee is K 682 "Montageverfahren für elektronische Baugruppen" ("Electronics assembly technology") of the DKE (German Commission for Electrical, Electronic and Information Technologies) at DIN and VDE.

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Responsible national committee

DKE/K 682 - Aufbau- und Verbindungstechnik für elektronische Baugruppen