Troubleshooting Application Note AN839

VISHAY SILICONIX
Power MOSFETs
Application Note 839
Guidelines for Handling Failed Power MOSFETs
on PCB Assemblies
By Kandarp Pandya
INTRODUCTION
When a power MOSFET fails, the challenge is to determine
the root cause of failure and implement a containment plan.
However, this is more easily said than done. The complexity
of semiconductor technologies and the advanced
techniques required to analyze them means that failed
components must be handled with extreme care in order to
retain all possible failure signatures. Unfortunately the
urgencies of the manufacturing environment often lead to
short-cuts as a failed component is rushed back to its
manufacturer for failure analysis. If this is not carefully done,
vital failure signatures can be lost, which adds further
difficulties to the already complex task of failure analysis.
The end result is further delays or loss of opportunities to
correctly identify the root cause of failure.
Typically, failures are reported as In-Circuit Tester (ICT),
Functional Test, or Field Returns. The following steps help to
ensure that the source of MOSFET failure will be identified
while avoiding any further damage or loss of failure
signatures:
heating up each solder joint. This approach is more likely
to alter the failure signature. Do not use standard
soldering stations. An advanced soldering station like the
Metcal APR5000 is needed to generate a de-soldering
temperature profile that is close to the reflow profile for
safe removal of the part. Alternatively, a heat gun such as
the Steinel model HG3002LCD, which enables air flow
with controlled temperature for uniform heating, can be
used for safe removal of the part from the PC board
assembly.
By way of an example, here are step-by-step instructions
for removing a PolarPAK® power MOSFET from a PCB.
Engineering Lab Set-up:
1. Test PCB used for this experiment (figure 1). See
Appendix A for details.
1. Follow ESD protection procedures throughout handling
of the suspect MOSFET.
Fig. 1 - PC Board assembly with Soldered Part
2. Steinel Heat Gun with Digital Display, Model HG3002LCD
(figures 2, 3a, 3b, 3c, and 3d). See Appendix B for details.
3. De-soldering: After determining that the MOSFET has
failed, the next step is to remove the suspect MOSFET
from the PB board assembly. The objective here is to
remove the part from the PC board assembly without
causing any further physical damage and/or loss of
evidence. Only uniform, controlled heating should be
used for removal of the MOSFET from the PC board
assembly. Using standard soldering stations can easily
subject the part to non-uniform heating by individually
Document Number: 71436
Revision: 15-Jun-10
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APPLICATION NOTE
2. Resistive Test: Use a low-voltage laboratory-grade digital
multimeter such as Keithley or HP to measure the
resistance between gate and source, gate and drain, and
drain and source. Do not use a portable DMM. The 9 V
battery supply can easily increase the extent of the
damage or even destroy the evidence. It can also lead to
conflicting findings with repetitive tests. Thus a MOSFET
with damage only in the gate-source region and
exhibiting low gate-to-source resistance on the first
measurement can later exhibit high resistance and
healthy functionality. The result will be no fault found on
FA report.
Application Note 839
Vishay Siliconix
Guidelines for Handling Failed Power MOSFETs
on PCB Assemblies
Fig. 3c - Air Flow Control
3. Electronic
lab
tools,
measuring
instruments,
thermo-couples with temperature indicator, and
soldering material.
Fig. 2 - Steinel Heat Gun
APPLICATION NOTE
Fig. 3a - Temperature Setting Wheel
4. The Steinel Heat Gun, Model HG3002LCD, shown in
figure 2, is a calibrated heat gun which has all the features
required to work with a SMD part in the laboratory
environment. The temperature setting wheel (figure 3a)
enables setting the temperature of the air blown by the
heat gun. The set temperature is displayed on an LCD
panel (figure 3b), which also displays vertical bars to
indicate airflow. Airflow can be set with the airflow control
knob as shown in figure 3c. The on/off controls switch in
figure 3d enables heat gun operation with controlled
airflow at a preset temperature.
5. The heat gun can be held upright over the PCB assembly
at a distance of 1" to 1-1/2". This can be done manually,
but it is more conveniently done with a suitable vertical
clamp, if available. The latter can be used as a
mechanical fixture. figure 4 shows our lab arrangement,
which uses a fixture capable of providing X-Y-Z
movement to hold the heat gun in its upper head. The air
nozzle of the gun can be lowered over the PCB and kept
at a fixed distance.
Fig. 3b - LCD Display for Temperature and Air Flow
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Document Number: 71436
Revision: 15-Jun-10
Application Note 839
Vishay Siliconix
Guidelines for Handling Failed Power MOSFETs
on PCB Assemblies
Temperature (°C )
Reflow Solder Profile (°C)
260
240
220
200
180
160
140
120
100
80
60
40
20
0
0
50
100
150
200
Time (s)
250
300
350
Fig. 5a - Recommended Reflow Profile in Celsius
(Red = Maximum Values, Blue = Minimum Values)
Temperature (°F)
Reflow Solder Profile (°F)
Fig. 4 - A Representative Fixture Setup
Reflow Profile Definition and Development
1. The Vishay reliability manual, available online at
www.vishay.com/docs/80126/80126.pdf, covers the
recommended basic reflow profile definitions. Graphical
representations of the recommended reflow profile in
degrees Celsius and in Fahrenheit are shown in figure 5a
and 5b respectively. The temperature values indicated by
the red line show the maximum values and those
indicated by the blue line show the minimum values.
500
450
400
350
300
250
200
150
100
50
0
0
50
100
150
200
250
300
350
Time (s)
Fig. 5b - Recommended Reflow Profile (°F).
(Red = Maximum Values, Blue = Minimum Values)
3. Observe the temperature reflow profile in figure 6, which
matches the recommendations in figure 5a and 5b.
Document Number: 71436
Revision: 15-Jun-10
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APPLICATION NOTE
2. Following the profile is a task in itself, requiring a high
degree of experience and skill. There are four
interdependent parameters, two of which are the settings
of the calibrated heat gun: the temperature and airflow
settings. The third parameter is the distance of the air
nozzle from the component/PCB assembly. The fourth
parameter is the time duration for a given temperature
setting. Temperature settings and time durations are
controlled dynamically by the operator using a stop
watch.
Application Note 839
Vishay Siliconix
Guidelines for Handling Failed Power MOSFETs
on PCB Assemblies
Fig. 6 - Reflow Profile generated by using Heat Gun
Thermo-couple placement:
TC1 - Green Trace - Top of Component
TC2 - Blue Green - Close to lead frame on PCB
TC3 - Pink Trace - Bottom of PCB
De-soldering procedure:
1. The test sample is shown in figure 1 the PCB assembly
2. Air nozzle distance from component: 1" (25.4 mm)
3. Air flow setting: medium
4. Temperature settings and time steps operations:
(a) 370 °F (180 °C) for 200 s - (pre-heat period)
(b) 700 °F (240 °C) for 25 to 40 s - (reflow period)
6. It should be noted that higher temperature settings are
required for the heat gun to get the desired temperature
on the component and PCB surfaces. This is due to the
distance between the air nozzle and the component. The
airflow setting also contributes to this difference in the
temperature settings.
7. Also take note that this profile is developed for
soldering/de-soldering a part on a 1" x 1"
(25.4 mm x 25.4 mm) PCB as defined in Appendix A.
Different temperature settings and air speeds may be
required for different PCBs. However, such changes are
easily made by an experienced laboratory technician.
APPLICATION NOTE
5. At the end of step 2 the solder under the part needs to be
checked to see if it is soft enough so that the part can be
lifted away. Figure 7 shows a removed part with the PCB.
Fig. 7 - Removed Part and PCB
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Document Number: 71436
Revision: 15-Jun-10
Application Note 839
Vishay Siliconix
Guidelines for Handling Failed Power MOSFETs
on PCB Assemblies
Appendix A
Standard PCB used for MOSFET Soldering/re-work:
a) Dimensions: 1" x 1" x 0.062" (25.4 mm x 25.4 mm x 1.575 mm)
b) Material: FR4
c) Number of copper layers: 2
d) Copper thickness on both sides: 2 oz. (0.076 mm)
e) Area covered by copper on top side: 20 %
f) Area covered by copper on bottom side: 100 %
Appendix B
Steinel HG3002LCD Heat Gun Specifications
Technical Specifications:
Voltage: 120 VAC
Output: 1500 V
Temperature, Step 1: 120 °F
Temperature, Step 2: 120 °F to 1100 °F
Indicator Range Display: 120 °F to 1100 °F
Blower: Continuously variable
Air Flow: Min. 7 cfm, Max. 17.6 cfm
Weight: 31 oz. with cord and plug
More information at URL: www.steinel.net/products/heatguns/electronic_heatguns.cfm
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APPLICATION NOTE
Document Number: 71436
Revision: 15-Jun-10