Assembly process Accelerated testing

Reliability of electronic assemblies is a complex subject.This chapter has touched on only one aspect of the problem understanding the primary failure mechanisms of printed circuit boards and the interconnects between these boards and the electronic components mounted on them. This approach provides the basis for analyzing the impact of design and materials choices and manufacturing processes on printed circuit assembly reliabihty. It also provides the foundation for developing accelerated testing schemes to determine reliability. It is hoped that the fundamental approach will enable the reader to apply this methodology to new problems not yet addressed in mainstream literature. 

During PCB assembly and transportation, forces imposed on a component can cause the device to move from its original position on a PCB. Solder paste and/or flux is used to retain components in place during the assembly and transportation of the PCBs. The tackiness required of a flux or paste depends on the assembly process utilized. In-line automated assembly requires less retention force than manual batch assembly. Additionally, placement equipment utilizing table movement rather than head movement requires much greater retention forces due to high accelerations imposed on the PCBs and components. The retention force can be calculated by simply multiplying the acceleration imposed on a component by its mass. A tack test can be performed on the solder paste or flux as described in section 2.3.5 of this chapter to determine its retention capability. 

Self-assembly is a massively parallel process, and can normally involve very large numbers of components (a large crystallization might involve 1027molecules). Robotic pick-and-place methods for placement are limited by the fact that they are serial. Although they can be accelerated by using a number of robotic devices in parallel (for example, the multiple scanning probe heads of the IBM millipede 131), they cannot approach the number of molecules in a test tube, for example. 

Obviously, a sensoTs function can be tested at the end of the fabrication process after the sensor is completely assembled and fully packaged. At this time a primary input signal (pressure, acceleration, yaw rate, mass flow, etc.) can be applied directly, and a comprehensive test of the specified performance is possible. Unfortunately, by the time a defective sensor component is packaged, loss of time and capital is maximum, since a fully packaged sensor needs to be discarded. 

This economical test exposes die climatically unstable points of electronic components. Due to the nature of the test, the entire board is evaluated. This test accelerates the mechanisms of electrochemical migration. Consequently, faults that previously would appear after months or even years can be detected during the development process. To identify potential weak points, the assembly is operated in standby mode and immersed in deionized water. Testing while the assembly is in full operation is even more effective. The sensitivity of the circuit to moisture exposure is assessed on the basis of flie recorded test current, combined with a subsequent examination of the assembly. Through weak point analysis, a Yes/No decision can be determined concerning the expected service life, of the assembly. 

Underfill. An underfill is then injected into the gap between the chip and chip carrier and then cured to complete the flip chip process. The function of the underfill or encapsulation as it is sometimes referred to is to provide mechanical integrity and environmental protection to a flip chip assembly. Studies have demonstrated that both thermoset and thermoplastic ICAs can offer low initial joint resistances of less than 5 mS2 and stable joint resistances (Au-to-Au flip chip bonding) during all the accelerated reliability testing listed in Table 1. The reliability results have indicated that there is no substantial difference in the performance of thermoset and thermoplastic bumps and both types of polymers apparently offer reliable flip chip electrical interconnections (53). 

The reliability of die attach adhesive has been extensively investigated for the past three decades. A complete survey of these studies has been previously published [4]. The following sections underline the critical factors that have an impact on the electrical, mechanical, and thermal properties of the adhesive bonds. For process engineers, the main concern is the evaluation of die attach integrity in relation with the assembly technologies and after a series of accelerated ageing tests. 

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