ntroduction
Each year the military and aerospace sector demands higher-speed systems with better accuracy at lower cost. Increasingly stringent size, weight and power constraints, together with the demanding power supply requirements of large FPGAs, have led to a decisive move towards POL (point of load) power supply architectures.
One solution to address these needs is Linear Technology’s µModule technology, providing a system-in-package solution that is claimed to simplify design and minimize external components.
Figure 1: µModule Product Construction
The original packaging technology selected for µModule switching regulators was LGA (land grid array) and this has served the broad marketplace well. However, some applications subject to very harsh environments prefer BGA (ball grid array) interconnect, and Linear Technology has developed packaging to meet these requirements.
Figure 2: LGA and BGA package interconnect
In this article, we will take a more detailed look at the comparative performance of LGA and BGA packaging and discuss the merits of gold or tin-lead (SnPb) alloy and lead-free (Pb-free) component finishes.
Component terminal finishes
The European military and aerospace market presents quite a fragmented demand picture when considering component finishes with some companies having adopted Pb-free finishes entirely whilst others have policies to completely avoid them, more common is a mix dictated by the specific needs of individual projects.
A major factor is the continued exclusion of military and aerospace equipment from the RoHS II (Restriction of Hazardous Substances) directive that permits the indefinite use of SnPb component finishes. Of foremost concern is the impact on reliability from the formation of tin whiskers on pure tin plating, leading to potential equipment failures from short-circuits of adjacent fine-pitch conductors. The addition of lead (Pb) to the tin plating remains the industry-standard approach for mitigation of whisker formation.
Weighed against the use of SnPb finishes are the practicalities of component availability, lack of distribution inventory and extended delivery times. By mandating use of SnPb finish companies are also sometimes foregoing access to a wider range of new, Pb-free only components. Whilst this can be overcome by third-party companies offering strip and re-plating or re-balling processes the additional heat cycles and difficultly in re-testing together with the associated costs make this a less desirable approach.
When Pb-free components are used another form of tin whisker mitigation frequently employed is the use of polymer conformal coatings such as Parylene or Arathane which have been shown to resist the penetration of tin whiskers for many years.
Interconnect considerations
Given the mix of needs discussed in the previous section, Linear Technology µModule products in LGA packages offer a universal solution as gold plated pads have been in long-standing use within military and aerospace systems and they have the advantage of also being RoHS compliant.
However, one concern of using gold plated components is gold embrittlement and this is especially true of large BTCs (bottom termination components) subjected to harsh environmental conditions. During reflow gold is dissolved into the solder joint creating weak interfaces in the crystalline structure, the CTE (coefficient of thermal expansion) differences between the gold-tin IMC (intermetallic compound) platelets and the surrounding volume of solder can result in fracturing of the solder joint and an eventual open-circuit when the assembly is repeatedly temperature cycled. A long-standing industry rule of thumb recommends not exceeding a threshold of ~3 wt% gold within the joint, and Linear Technology LGA µModule products meet this requirement for both SnPb and SAC305 paste. Some companies conduct their own trials on BTCs with representative PCBs and environmental conditions using special daisy-chain interconnect samples for this purpose.
Increasing the volume of solder in a joint and the standoff to the PCB improves interconnect reliability by making the joint more compliant. Tests in the military and aerospace industry have concluded that BGA packages are generally preferred over LGA where harsh environmental conditions are likely to be encountered, particularly in airborne systems. An added benefit is that cleaning becomes easier thereby reducing concerns over contaminants.
For these reasons µModule regulators are now offered in BGA packaging in addition to LGA. The downside of the BGA package is slightly reduced thermal efficiency of around 0.5ºC/Watt and an overall increase in component height of 0.6mm. Linear Technology BGA µModule regulators are offered as standard in SAC305 Pb-free configurations and on specific products according to demand with tin-lead Sn63Pb37 balls.
The European military and aerospace market presents quite a fragmented demand picture when considering component finishes with some companies having adopted Pb-free finishes entirely whilst others have policies to completely avoid them, more common is a mix dictated by the specific needs of individual projects.
A major factor is the continued exclusion of military and aerospace equipment from the RoHS II (Restriction of Hazardous Substances) directive that permits the indefinite use of SnPb component finishes. Of foremost concern is the impact on reliability from the formation of tin whiskers on pure tin plating, leading to potential equipment failures from short-circuits of adjacent fine-pitch conductors. The addition of lead (Pb) to the tin plating remains the industry-standard approach for mitigation of whisker formation.
Weighed against the use of SnPb finishes are the practicalities of component availability, lack of distribution inventory and extended delivery times. By mandating use of SnPb finish companies are also sometimes foregoing access to a wider range of new, Pb-free only components. Whilst this can be overcome by third-party companies offering strip and re-plating or re-balling processes the additional heat cycles and difficultly in re-testing together with the associated costs make this a less desirable approach.
When Pb-free components are used another form of tin whisker mitigation frequently employed is the use of polymer conformal coatings such as Parylene or Arathane which have been shown to resist the penetration of tin whiskers for many years.
Interconnect considerations
Given the mix of needs discussed in the previous section, Linear Technology µModule products in LGA packages offer a universal solution as gold plated pads have been in long-standing use within military and aerospace systems and they have the advantage of also being RoHS compliant.
However, one concern of using gold plated components is gold embrittlement and this is especially true of large BTCs (bottom termination components) subjected to harsh environmental conditions. During reflow gold is dissolved into the solder joint creating weak interfaces in the crystalline structure, the CTE (coefficient of thermal expansion) differences between the gold-tin IMC (intermetallic compound) platelets and the surrounding volume of solder can result in fracturing of the solder joint and an eventual open-circuit when the assembly is repeatedly temperature cycled. A long-standing industry rule of thumb recommends not exceeding a threshold of ~3 wt% gold within the joint, and Linear Technology LGA µModule products meet this requirement for both SnPb and SAC305 paste. Some companies conduct their own trials on BTCs with representative PCBs and environmental conditions using special daisy-chain interconnect samples for this purpose.
Increasing the volume of solder in a joint and the standoff to the PCB improves interconnect reliability by making the joint more compliant. Tests in the military and aerospace industry have concluded that BGA packages are generally preferred over LGA where harsh environmental conditions are likely to be encountered, particularly in airborne systems. An added benefit is that cleaning becomes easier thereby reducing concerns over contaminants.
For these reasons µModule regulators are now offered in BGA packaging in addition to LGA. The downside of the BGA package is slightly reduced thermal efficiency of around 0.5ºC/Watt and an overall increase in component height of 0.6mm. Linear Technology BGA µModule regulators are offered as standard in SAC305 Pb-free configurations and on specific products according to demand with tin-lead Sn63Pb37 balls.
Reliability testing and characteristics
It is important to keep the concerns in perspective, and Linear Technology has conducted extensive trials on interconnect reliability of µModule products with both LGA and BGA configurations.
By way of background, we should distinguish between component tests used in the initial qualification and subsequent reliability monitoring and interconnect tests that are really a characterization of performance with selected PCB configurations, assembly process and temperature cycle parameters.
Extensive µModule product testing at component level indicates an exceptionally high level of reliability with 0.72 FITS (1 FIT = 1 failure in one billion device hours). This article will then focus on interconnect level testing.
Three types of interconnect testing have been performed:
1) Daisy chain testing: where each pad in the µModule regulator is connected to the next to form a complete circuit, monitored in real time during temperature cycling in accordance with IPC-9701 and JESD22-A104. This approach ensures that each pad is tested and since many pads are connected in parallel in the application this is generally preferred over functional testing.
Table 1: LTM4601A daisy chain test results of LGA & BGA
Click on image to enlarge
Note (1): When the 5 failures in table 1 are analyzed with a Weibull distribution plot, it predicts a 1-percent failure point at 1780 cycles.
Note (2): Subsequent further testing of LGA products with SAC305 (Pb-free) paste to 6000 cycles shows zero failures out of a total of 240 samples of various other µModule products.
Note (3): All BGA packages used SAC305 solder balls
2) Functional testing: where the µModule regulator is temperature cycled and tested for correct operation on an evaluation board. Much testing was performed on LGA µModule regulators comparing Pb-free SnAgCu paste with SnPb paste. However, testing was limited to a maximum of 2000 cycles, which did not produce any failures of either solder.
3) Random vibration testing: conducted in accordance with MIL-STD-202G, method 214A over 50Hz to 2000Hz frequency range and represents a very severe test. Testing of the LTM4610A in LGA-133 passed test letter C at 9.26g RMS but failed in the Z-axis with test letter F at 20.71g RMS. Trials on the LTM4601A in the BGA were conducted and the results are shown below in table 2. Both SnPb and SAC305 (Pb-free) solder gave good results, passing at 20.71g RMS.
Conclusions
It is likely that the military and aerospace sector will continue to require a mix of component finishes for the foreseeable future and that manufacturers committed to supporting this market must maintain support for SnPb component finishes.
BTCs such as Linear Technology µModule products with gold-plated LGA packaging have proven reliability and wide industry acceptance. Now BGA versions are providing an alternative to LGA for particularly harsh environment applications.
BGAs with RoHS compliant SAC305 (Pb-free) balls using both SAC305 and SnPb solder paste have been shown to be very reliable and capable of greater than 6000 temperature cycles without failure. For applications where SnPb is mandated, there is now a factory built option of SnPb BGA.
In the limited space available, only a small selection of the reliability data has been shown, for more details please refer to the Linear Technology website.
About the author
Steve Munns is Mil-Aero marketing manager at Linear Technology Corporation
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. DARPA to optimize bandwidth access for military, commercial use
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It is important to keep the concerns in perspective, and Linear Technology has conducted extensive trials on interconnect reliability of µModule products with both LGA and BGA configurations.
By way of background, we should distinguish between component tests used in the initial qualification and subsequent reliability monitoring and interconnect tests that are really a characterization of performance with selected PCB configurations, assembly process and temperature cycle parameters.
Extensive µModule product testing at component level indicates an exceptionally high level of reliability with 0.72 FITS (1 FIT = 1 failure in one billion device hours). This article will then focus on interconnect level testing.
Three types of interconnect testing have been performed:
1) Daisy chain testing: where each pad in the µModule regulator is connected to the next to form a complete circuit, monitored in real time during temperature cycling in accordance with IPC-9701 and JESD22-A104. This approach ensures that each pad is tested and since many pads are connected in parallel in the application this is generally preferred over functional testing.
Table 1: LTM4601A daisy chain test results of LGA & BGA
Click on image to enlarge
Note (1): When the 5 failures in table 1 are analyzed with a Weibull distribution plot, it predicts a 1-percent failure point at 1780 cycles.
Note (2): Subsequent further testing of LGA products with SAC305 (Pb-free) paste to 6000 cycles shows zero failures out of a total of 240 samples of various other µModule products.
Note (3): All BGA packages used SAC305 solder balls
2) Functional testing: where the µModule regulator is temperature cycled and tested for correct operation on an evaluation board. Much testing was performed on LGA µModule regulators comparing Pb-free SnAgCu paste with SnPb paste. However, testing was limited to a maximum of 2000 cycles, which did not produce any failures of either solder.
3) Random vibration testing: conducted in accordance with MIL-STD-202G, method 214A over 50Hz to 2000Hz frequency range and represents a very severe test. Testing of the LTM4610A in LGA-133 passed test letter C at 9.26g RMS but failed in the Z-axis with test letter F at 20.71g RMS. Trials on the LTM4601A in the BGA were conducted and the results are shown below in table 2. Both SnPb and SAC305 (Pb-free) solder gave good results, passing at 20.71g RMS.
Table 2: Random vibration test of BGA-133 package
Conclusions
It is likely that the military and aerospace sector will continue to require a mix of component finishes for the foreseeable future and that manufacturers committed to supporting this market must maintain support for SnPb component finishes.
BTCs such as Linear Technology µModule products with gold-plated LGA packaging have proven reliability and wide industry acceptance. Now BGA versions are providing an alternative to LGA for particularly harsh environment applications.
BGAs with RoHS compliant SAC305 (Pb-free) balls using both SAC305 and SnPb solder paste have been shown to be very reliable and capable of greater than 6000 temperature cycles without failure. For applications where SnPb is mandated, there is now a factory built option of SnPb BGA.
In the limited space available, only a small selection of the reliability data has been shown, for more details please refer to the Linear Technology website.
About the author
Steve Munns is Mil-Aero marketing manager at Linear Technology Corporation
Related posts
. FPGA testing for DO-254 compliance
. Automatic C-to-VHDL testbench generation shortens FPGA development time
. Simulation enhances military ground vehicle instrument testing
. Designing low-power video image stabilization IP for FPGAs
. DARPA to optimize bandwidth access for military, commercial use
. Implementing analog functions in rugged, rad-hard FPGAs
. Using stable, reliable systems for mission-critical video walls
. Mil-Aero top 10 'How-To' articles for 2012
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