The monitoring of electronic assemblies for fractured solder joints is not a straight-forward task. Fully fractured solder joints, regardless of whether they occur during accelerated reliability testing of prototype test vehicles or operational use of product in the field, do NOT result in steady-state electrical opens. Typically, solder joint fracture occurs initially on only one solder joint in the whole electronic assembly; while the other solder joints have been subjected to the same cyclic environment and have accumulated fatigue damage, they will fail following a statistical failure distribution. The fractured solder joint will cause intermittent failures or circuit malfunctions. Also typically, the failures occur in a corner-most solder joint of a component having the highest, or near highest, probability of failure, perhaps due to a large component size, a large CTE-mismatch between component and substrate, etc.
The electrical indications of failure are characteristically intermittent because the fractured solder joint surfaces never physically separate as long as the component is still attached to the substrate by some unfractured solder joints. The solder joints neighboring a fractured solder joint keep the solder joint fracture surfaces in compressively loaded contact. The displacements during thermal cycling, and even during board bending are predominantly in the x- and y- (shear) directions, NOT the z- (tension) direction. Thus, electrical indications of solder joint failures occur only during dynamic loading of the solder joints. To get a measurable manifestation of failure requires that the solder joints be monitored either during the transient periods in thermal cycling or during a mechanical excitation (vibration or mechanical shock.)
In practical terms this means that electrical continuity daisy-chained nets of test vehicles for accelerated reliability tests need to be monitored continuously during cycling. Multiplexed monitoring looks at given continuity test net always during the same time slot, which may or, more likely, may not, fail during the critical transient transition periods. It clearly is not adequate to look for failure during a halt in the cyclic test program. Neither is it adequate to look at equipment with a reported failure indication on some test fixture that does not in some form dynamically excite the assembly under test; the likely result of such an investigation is "NTF - No Trouble Found."
Given the need to test a statistically adequate sample size under a variety of parametric variations, the monitoring equipment requirements can become formidable rather quickly. For pure research purposes, it might be of interest to determine when a partial crack in a solder joint results in a specific increase in electrical resistance or a specific decrease in solder joint strength. However, from a product reliability point of view, these criteria are not particularly relevant. Partial fractures need to be almost complete with the fracture surfaces physically separated before even a 10% increase in resistance can be measured. Similarly, the reduction in joint strength due to micro cracks or partial fractures has no significant reliability impact, except for uses with significant vibrational loading. Solder joints in operating electronic equipment are not subjected to stresses large enough to cause overstress failure in solder joints even with substantially reduced strength. Further, equipment to record high-speed, transient, electrical resistance data is expensive; a fact that becomes very important with increasing numbers of test nets that need to be monitored.
From pure functional considerations, monitoring for short duration high resistance spikes is most meaningful, since this closely resembles the actual failure indication in product. From practical considerations, this allows the most cost-effective use of resources, since monitoring equipment to meet this test requirement is relatively inexpensive.
Engelmaier Associates, L.C. is a consulting company that provides consulting services on reliability, manufacturing and processing aspects of electronic packaging.
Engelmaier Associates, L.C. provides services to OEMs, printed circuit board manufacturers, PWB subcontract assembly companies and end users in applications such as telecommunications, computers, military, aerospace, and automotive. These services can be in the areas of design for reliability, accelerated reliability testing, reliability prediction, design for manufacturability and quality, processing effects, environmental stress screening, and failure cause analysis, for solder joints, plated-through holes/vias, and flexible circuitry. Workshops in theses areas are offered as well.
Werner Engelmaier, President of Engelmaier Associates, L.C., has been instrumental in the development of many industry documents, such as IPC-SM-785 "Guidelines for Accelerated Reliability Testing of Surface Mount Solder Attachments", IPC-9701 "Performance Test Methods and Qualification Requirements for Surface Mount Solder Attachments", IPC-D-279 "Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies", ANSI/IPC J-STD-012 "Implementation of Flip Chip and Chip Scale Technology", and ANSI/IPC J-STD-013 "Implementation of Ball Grid Array and Other High Density Technology".