Selecting DC/DC regulators for reliability

Central topics:

• Reliability Testing of DC/DC uModule Regulators
• Reliability test report analysis

Let’s look at DC/DC regulators: basically, a complete DC/DC regulator circuit sealed in a package similar to an IC. We assume that you have already researched the options available from several vendors, and all of them provide the power conversion function to a greater or lesser degree and meet most of your needs. How do you then decide which is the best product for your design? What do your customers value in the end product you develop and produce? Which DC/DC regulator parameters determine performance?

As you make your selection, keep another parameter in mind: Study the reliability reports of each DC/DC regulator. Pay attention to products that have been rigorously tested and are manufactured by companies with a reputation for performance and reliability.

Since we are discussing sealed DC/DC systems, this How-to will take a closer look at the in-depth and detailed reliability testing that Linear Technology performs on its DC/DC µModule regulator products (Figure 1). You can see the complete reliability test report at the following URL:


Figure 1: The DC/DC micromodule regulator is a sealed complete circuit containing an inductor and high reliability



Figure 2a: Reliability test report of DC/DC micromodule products

Figure 2b: An example of a detailed test in a DC/DC micromodule

product

reliability

test report

...

The data accumulated to date demonstrates the robustness of the packaging, assembly and electrical performance of the micromodule products.

The report contains the following tests:
1. Operating Life
2. Preconditioning
3. Temperature, Humidity Bias
4. Power Cycle
5. Temperature Cycle (Various Temperature Ranges)
6. Thermal Shock (Various Temperature Ranges)
7. Soldering Shock
8. High Temperature Bakeout (Various Temperature Ranges)
9. Mechanical Shock
10. Variable Vibration Frequency
11. Board Mounting/Temperature Cycle (Various Temperature Ranges)

Linear Technology still monitors these test results and updates the data quarterly, then provides new test reports online.

The following is a brief description of each reliability test performed on the DC/DC micromodule product:

1. Operating Life 1,635,385 device hours with zero failures
a. Tested after biasing with the absolute maximum voltage rating of the product in the test environment
b. Increased ambient temperature to 125oC to accelerate testing

2. Preconditioning 2,500,608 device hours with zero failures
a. 192 hours of 30ºC/60% relative humidity + 3 IR reflows per JEDEC245ºC standard (J-STD-020 humidity level 3)
b. Simulated circuit board assembly conditions
c. With a more humid environment
d. Checked for package integrity and delamination

3. Temperature, humidity bias 446,400 device hours with zero failures
a. Checked for dendrite growth and corrosion
b. Biased with the maximum rated voltage of the product under test in a humid environment

4. Power Cycle 18,575,540 device hours with zero failures
a. Tested by turning the product on and off
b. Junction temperature range of 50ºC to 100ºC

5. Temperature Cycle - 40ºC to 125ºC 1,078,000 device hours with zero failures

6. Temperature Cycle - 55ºC to 125ºC 4,406,436 device hours with zero failures

7. Temperature Cycle - 65ºC to 125ºC 7,402,790 device hours with zero failures
a. Performed on multiple package sizes
b. Air to air or chamber
c. Devices tested for electrical integrity at 500 and 1,000 readout points

8. Thermal Shock - 55ºC to 125ºC 4,648,000 device hours with zero failures

9. Thermal Shock - 65ºC to 150ºC 5,818,300 device hours with zero failures

10. Thermal Shock - 40ºC to 125ºC 733,000 device hours with zero failures
a. Hot liquid to cold liquid bath test (Figure 3)
b. Complies with J-STD-020 LEVEL3 standard
c. Electrically tested after two endpoints (500 and 1,000 cycles)


Figure 3: Thermal shock bath system

11. Solder shock 245ºC 2,278 units sample size, zero failures
a. Immersed in molten solder;
b. Immersed for 10 seconds.

12. High temperature baking at 175ºC 154,000 device hours, zero failures

13. High temperature baking at 150ºC 3,202,874 device hours, zero failures
a. Verify intermetallic growth on package connection
wires b. Package connection wire pull test after 1,000 hours
c. Different package connection wire thickness and length
d. Pull to record fracture mode
e. Monitor 500 and 1,000 hours

14. Mechanical shock
a. 1,500g peak shock test
b. Per JESD22B104 condition B

15. Variable vibration frequency
a. 20g peak acceleration
b. Per JESD22B103 condition A

16. Daisy chain board installation/temperature cycle 211,500 device cycles, zero failures
a. Test solder joint integrity (Figure 4)
b. 0ºC to 100ºC, daisy chain unit with real-time monitoring
c.


Test Figure 4: Daisy-chain monitor test unit

17. Board installation/Temperature cycle -40ºC to 125ºC 1,689,000 device cycles with zero failures

18. Board installation/Temperature cycle -55ºC to 125ºC 211,500 device cycles with zero failures
a. Solder joint integrity test
b. Tested in ATE (Figure 4) or on the bench

Not every system designer chooses or needs a high-end, high-reliability DC/DC regulator. The extra cost associated with the high performance of these products often puts designers off. Then one thing that is sometimes forgotten is that designers spend a lot of time and resources perfecting the complex digital components of a system, only to then not know how to choose the right DC/DC regulator for that design. It is imperative to carefully study the manufacturer's reliability test methods to avoid the need to modify the design later and spend time debugging the power management portion of the board, which can be very costly. It is normal to trust the manufacturer, but it is wise to verify and compare reliability data beyond the FIT rate and surface numbers. You must ask for detailed information.

Linear Technology provides extremely detailed reliability test results for its DC/DC uModule regulator products. The reliability report is updated quarterly and includes test data for different package sizes, batches, and different products. This reliability test report is very easy to understand: it is available online, available to everyone who wants to see it, and no permission is required to read it. For system designers who strive to provide competitive products, choosing a high-end DC/DC regulator backed by a rigorous and systematic reliability test program will ensure good system-level reliability and good product reputation with end users.