The EMPF Helpline received a call from a control and telemetry electronics systems integrator involving a component failure in a radio transceiver. The component was a 24 pin 2.4 GHz Wireless LAN device in a Land Grid Array (LGA) package. The EMPF was tasked with performing electrical failure analysis to determine the failure mode and to identify any evidence as to whether the cause was related to packaging, handling, or other sources.

Isolation of the failing pin or area is critical to the success of electrical failure analysis, and certain key pieces of information were requested from the customer prior to the start of analysis:

• Component part number
• A determination whether the part needs to be removed from the board
• Conditions of the component during failure – temperature, voltage applied, state of operation, etc.
• Length of time in field application or under test
• Pin or area of component suspected of failure
• Manufacturer’s data sheet with test specification and package layout information
• A known good component available for comparative testing purposes
• Available electrical test information or data logs showing which pin measurements are out of specification
• Photos of the condition of the part as submitted

With the above information supplied by the customer, the EMPF performed electrical failure using the following methods:

• De-solder of the component from the board
• Optical microscope analysis
• X-ray analysis
• Electrical power up testing
• Probe station microprobing

The component was first removed from the board by de-soldering using hand tools with a spaded tip and flux. The remaining solder was also removed using solder wick to leave the attachment land pads clean and flat. The following optical visual inspection of the external package did not reveal any defects and an initial X-ray analysis did not show any anomalies.

A manual Suss Microtec SM5 probe station with DC/RF probes and video imaging was set up for three DC probes to connect to three land pads for a full powered up electrical test of the device as shown in Figure 3-1.

A thin wire was also soldered to the larger center ground pad to provide a solid connection that did not need to be changed during testing. A good unit was also configured the same way for comparison purposes. Figures 3-2 and 3-3 show the test set-up with 3 probes connected to the pads on the underside of the device and the center ground pad wire.

Table 3-1 shows the pin and voltage applied for testing as well as current draw for the pins under test for both the good and failed devices.

Pins 1, 4, and ground were connected first, followed by a separate consecutive probing of pins 7, 20, and 22 for current readings at 3.3 volts. Since all three Vcc pins were found to draw excessive current on the failed device, the enable pin was tested and confirmed to also have excessive current flow. The enable pin affects all three Vcc pins so it was tested
separately with a curve tracer and found to have a shunt of 500 ohms to ground.

X-ray re-analysis was performed on the failed device for
in-depth views of the internal package connections to pin1-Enable and ground. Top down and high angle views did not show any defects or cause for the pin1 trace to be shunted to ground in the path from the external land pad to the bond wire connection on the die.

The customer advised the EMPF that the above failure mode isolation of the pin 1-Enable to ground shunt to the internal die was sufficient information for them to confirm that Electro-Static Discharge (ESD) issues during their assembly were the most likely cause of failure.

Conclusions
Analysis during electrical microprobe station isolation analysis revealed over-current conditions on Vcc pins
leading to a result of finding a shunt between pin1-Enable and ground. The isolation of the failure mode to the
internal die was key information for the customer to decide that mounting of the device to the board should be done at their board manufacturer who is fully equipped with ESD protective surfaces and procedures.

If you have any questions regarding the diagnosis of failure modes, mechanisms and the use of DC/RF microprobing for component and die circuitry fault isolation analysis, please contact the EMPF Helpline at (610) 362-1320. A manufacturing expert will be able to offer technical insight and appropriate advice regarding your concerns.