Understanding the hidden reactions and the importance of profile—part 3
5a. 5b.
Figure 5. No wetting or poor wetting of solder.
tine. One of the mitigation measures will component terminations and exposes clean
Figure 4. Sn-Pb-Bi ternary system temperature
be baking the packages either in vacuum surfaces to facilitate the interconnection.
composition phase diagram.
or in an inert atmosphere such as dry An additional role of flux is to facilitate
nitrogen or argon. Repackaging and reseal- the wetting of the solder. The flux consists
ing of the packages until they are ready to of solvents, activators, and rosin. The risks
of a system or subsystem. Thus, risk
be assembled is recommended. This can associated with this step are:
mitigation comprises alleviating the
impose a cost constraint.
exposures to damaging and degrading
1. Very low temperature leads to risk
conditions.
of incomplete removal of oxidizing
Interconnection process
The ensuing paragraphs describe the
products, resulting in no-wetting/poor
In the surface mount assembly process,
possible risks associated with individual
wetting (Figure 5).
the reflow is a critical process consisting
thermal exposures in the product cycle and
2. Very high temperature leads to risks
of preheat, soak, reflow and cool down.
also some possible mitigation measures.
of a) damage to temperature sensitive
The circuit board with its components
packages (capacitor crack) b) non-
experiences thermal exposures during this
Packages
uniform differential heating of larger
process. It is important to be cognizant of
Most plastic encapsulated microcircuits are
components (surface temperature
the risks associated with this process.
sensitive to moisture permeation to differ-
higher than internal temperature) and
ent degrees, depending on the moisture
hence thermal stress damage and c)
Pre-heat
sensitivity/resistance level (MSL), with
premature volatilization of solvents,
It is in this step that the boards and the
level 1 being the most moisture resistant
(solder balling) d) Multi-axial stresses in
components are slowly brought up to a
and level 6 being the most prone. The
multilayer boards leading to delamina-
temperature where the flux activation
risk of any significant moisture absorption
tion and field failures (Figure 6).
is initiated to react with the component
in a package is the propensity of pack-
3. Too high a ramp rate will result in risks
termination and board pad surface metal-
age popcorning during the assembly and
due to solder splatter, solder balling,
lurgies. The preheat step is to be care-
rework operations. This damage varies
premature decomposition and degrada-
fully controlled in terms of the ramp rate.
from observable package cracks, bulging, to
tion of flux prior to soldering (charred
Unusually high ramp rates present several
internal latent damage depending on the
flux).
risks including the possibilities of compo-
package design, materials, and construc-
4. Long soak times may also result in
nent damage, board warpage, solder paste
tion.
degradation in wettability, in addition
splattering, premature paste dry out, solder
There are a number of mitigation steps
to affecting throughput.
balling, etc….
to minimize the risk.
5. Too short soak times may not bring all
The mitigation involves careful control
the constituent parts of the assembly to
1. Use the package within the allowed ex-
of the ramp rate. The optimum ramp
the optimum temperature for adequate
posure limits as per the given moisture
rate depends on the component mix and
flux activation under all packages.
resistance level.
board combination. It should be noted
2. If all the components in the package
that smaller components like thin small The mitigation involves the pre-assembly
are not used in a given run, it is impor-
outline packages (TSOPs), owing to their thermal profiling of the board with
tant to track the exposure and ensure
small heat capacity, attain the temperature thermocouples at various critical locations
the total exposure time is still within
faster than the more massive components on the board to ensure that appropriate
the allowed limits.
like ceramic packages. Thus, there can be a temperatures are attained, and controlling
3. If the package environmental exposure
temperature lag among the various compo- the conveyor speed for the appropriate
history is unknown then the packages
nents. Also, thinner boards attain tempera- soak time.
have to be baked at the recommended
ture much faster than thicker boards. Thus
conditions such as at 125°C for 24
the ramp rates need to be optimized for Reflow
hours prior to assembly.
any given board thickness, structure and As has been discussed earlier, this is a criti-
the bill of materials involved. cal step for joint formation. As a general
It is important to recognize the risks as-
rule 25-30°C above the melting point of
sociated with the baking operation. One
Soak temperature and time the alloy, and 30-120 seconds dwell are
of the major risks is the oxidation of the
This process step, as described earlier, used. One has to be very careful in using
lead metallurgy, making it less solderable.
involves the activation of the flux so that these numbers. The risks associated with
The oxidative degradation of solderability
the flux dissolves any oxidizing products this step are numerous and can be catego-
depends on the time and temperature rou-
on the pad surface on the board and the rized as three types.
36 – Global SMT & Packaging – September 2008
www.globalsmt.net
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