Max

When we started our thermal cycling tests we where sure that the J-lead
will be the worst thing on earth concerning reliability. Now, after 4000
cycles -20 deg. C / 100 deg. C dwell time 30min on both extremes and a
temperature ramp of 2 deg. C / min. ( insuring pure GBS ) we found no
evidence of a faster growth of the average crack length than with gull
wings with the same pitch. Pretty disappointing in the first moment.  What
we see is a better reliability of the solder joints with shrinking pitches.
With all we know now we think we can explain the findings:

- At high temperatures the solder is extremely easy deformable. Thus the
stiffness of the lead doesn't matter much. Total relaxation will occur.

- At low temperatures solder is quite stiff. However, the difference in
elasticity of a Gull Wing and a J-lead ( of same material and pitch !!) are
not big enough to generate a considerable variation in the strain induced.

- A good deal of the damage that occurs during thermal cycling in the
solder joints of leaded components is not due to the global mismatch of
component and PCB but due to the local mismatches of pin / PCB; pin /
solder.

-> And due to bending of the heel bend of gull wings. <-

I think that the latter point ( which is the reason why cracks start in the
heel region of Gull Wing solder joints ) compensates an eventually induced
larger strain of J-leads due to their slightly larger stiffness since
J-leads cause more or less pure shear strain into the solder joints where
Gull Wings cause shear strain in the solder gap and normal + shear strain
in the heel.

Best regards

Guenter

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