AN1902 Freescale Semiconductor / Motorola, AN1902 Datasheet - Page 23

• Manufacturer Part Number: AN1902
• Description: Quad Flat Pack No-Lead (QFN)
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
AN1902/D
Samples of QFN wirebonded in daisy chain format were used to study the solder joint
reliability. Odd leads were wirebonded to the even leads (ie: 1-2, 3-4, 5-6, etc.) in
sequence. A complementary pattern was designed on the test PCB to provide one
electrical circuit (net) through the package when the package is attached to the test
PCB.
Figure 27 (L) Test PCB, (R) Daisy Chain QFN Package (Not to Scale)
The image on the left is an actual photograph from a 44ld 9x9mm QFN PCB pad con-
figuration. The package (X-ray photo shown) fits inside the white outlined area. The
double wirebonds in the package were enhanced to show the daisy chain pattern. Test
points exterior to the white outline allow one to isolate which pair of solder joints failed
during testing.
Assembled PCBs can be temperature cycled at a variety of temperature ranges. The
most common test conditions for Motorola are 0°C to 100°C and –40°C to 125°C tem-
perature ranges. The 0°C to 100°C condition has 10 minute ramps between tempera-
ture extremes with 5 minute dwells at the temperature extremes. Dwell time and ramp
time are 15 minutes each for the -40°C to 125°C condition.
The test process is air-to-air temperature cycling, where daisy chained components
are monitored in a chamber with its air exchanged to cause the packages to reach the
desired temperatures. This contrasts with air-to-air temperature shock, where the
components are shuttled back and forth between two chambers at the temperature
extremes. Motorola has the capability of continuously monitoring the resistance
through a daisy chain package and it’s complementary test PCB. Failure is defined as
resistance through the daisy chain net of 300 Ohms or greater. Daisy chain nets are
tested (time zero testing) prior to temperature cycling. Most nets start with initial resis-
tance below 10 Ohms.
7.2 Solder Joint Reliability (SJR) Results
Motorola [6] continues to work on understanding and improving the solder joint reli-
ability of QFN packages. From the various experiments, the best “time to first failure”
values are provided in
Table 7
and
Table 8
for the different package size, lead count
and pitch. These results were obtained after several iterations. Contact local Product
Package Engineer to obtain specifics on PCB and stencil design layouts. All experi-
ments were performed using similar size test boards.
MOTOROLA
Quad Flat Pack No-Lead (QFN)
23
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