MILITARY & DEFENCE SUPPLEMENT COVER STORY
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 its µModule products with both LGA and BGA configurations. By way of background, it is important to
distinguish between component tests used in the initial qualification and subsequent reliability monitoring and interconnect tests that are really a characterisation 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) however in the available space this article focuses 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-
Note (3) - All BGA packages used SAC305 solder balls
Figure 1: µModule Product Construction
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. Both SnPb and SAC305 (Pb-free) solder gave good results, passing at 20.71g RMS.
Figure 2: LGA and BGA Package Interconnect
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. Note (1) : When the 5 failures in table 1 are analysed with a Weibull distribution plot, it predicts a 1% 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 other µModule products.
Test Condition
Temp.Cycle 0°C -100°C with SnPb solder paste
Temp.Cycle 0°C -100°C with SnAgCu solder paste
Temp.Cycle 0°C -100°C with SnPb solder paste
Temp.Cycle 0°C -100°C with SAC305 solder paste PCB with round pads
Temp.Cycle 0°C -100°C with SAC305 solder paste PCB with square pads
/ ELECTRONICS
CONCLUSION 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-
Table 1: LTM4601A Daisy Chain Test Results of LGA & BGA
Product and Package
LTM4601A 15x15mm LGA-133
LTM4601A 15x15mm LGA-133
LTM4601A 15x15mm BGA-133 Note(3)
LTM4601A 15x15mm BGA-133 Note(3)
LTM4601A 15x15mm BGA-133 Note(3)
Sample Size
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. Linear Technology (UK) Ltd.
www.linear.com 01628 477 066
Number of Cycles
40 6,000 40 3,500 39 6,000
Device Cycles
240,000 140,000 234,000
Continuity Rejects
5 Note (1) 0 Note(2) 0
40 6,000
240,000
0
40 6,000
240,000
0
ELECTRONICS | JULY/AUGUST 2015
S7
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