Solder joint reliability prediction for chip components, MELFs, TSOPs, SOTs, etc.
Werner Engelmaier
“What is difficult for these components is the determination of the loaded solder joint geometries.”
Solder joint reliability
prediction for chip
components, MELFs, TSOPs,
SOTs, etc.
I have written about estimating solder For the global thermal expansion significantly, a convention defining
joint reliability in my columns numerous mismatch between the component and h=1/2 the stencil thickness is
times, most recently in References 1 and 2. the printed circuit board (PCB), the solder frequently adopted; 133 psi = 919
This topic has of course received extensive cyclic creep-fatigue damage, ΔD, for solder kPa.
treatment in industry documents such as joints that are either leadless or have non-
Compliant lead structures are those where
IPC-SM-785, IPC-D-279, and IPC-9701
3-5
. compliant leads is given by
a flexible intermediary lead between the
The reliability of the solder attachments
component body and the solder joint
of a component can be estimated with the
prevents the solder from reaching its
Engelmaier creep-fatigue model in terms of
yield strength. This curtailment of the
the mean cyclic life
Equation 3
cyclic stress range reduces the size of the
hysteresis loop and thus increases the
where
creep-fatigue life of the solder joints.
F = engineering factor,
~
1.2 to 0.7
Components with compliant leads include
for filleted SJs,
~
1.5 to 1.0 for SJs
QFPs; components with non-compliant
without fillets;
Equation 1
leads include thin TSOPs, SOTs,
DNP = distance from the neutral point/
essentially all components with Alloy 42
plane;
leads and essentially all surface mount
for which the creep-fatigue ductility ΔCTE = CTE-mismatch;
connectors. Chip carriers with J-leads and
exponent, m, is given by ΔT = cyclic temperature excursion;
gullwing leads can be either, depending on
h = solder joint height; leadless and
the design details.
non-compliant-leaded solder joints;
It is the h-parameter in Equations 3
and for solder joints with compliant lead and 4 that cannot be determined for some
Equation 2 structures is given by components by a geometric measurement
without at least some ambiguity. This is
and where
already reflected to some degree in the
= fatigue ductility coefficient;
definition of h for Equation 4; however, the
= mean cyclic solder joint
reasoning and the underlying assumptions
temperature;
Equation 4
have not been discussed before. An
= half-cycle dwell time in minutes.
additional issue is what value to put into
where Equation 3 for h when the solder joints are
The parameters in Equations 1 and 2 are
very thin.
given for different solder alloys in Table 1;
F = engineering factor ≈ 1.0;
This problem was the issue in a
as can be seen, these parameters have been
K = lead stiffness in the direction of
number of recent queries I received either
established only for near-eutectic SnPb
the neutral point/plane;
directly or through the IPC-Technet
solders (based on the classical experimental
A = load-bearing solder joint area;
Forum.
results of Roger Wild, IBM
6
and SAC
h = solder joint height, since for
The underlying issues are illustrated in
405/305
7
).
many leads this height may vary
Figures 1 through 5.
30 – Global SMT & Packaging – June 2009 www.globalsmt.net
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