• Rework of BGA/CSP Components using Lead Free Solders

Reworking BGAs/CSPs Using Lead-Free Solders
Time. Cost. Quality. Repeatability. These are the primary concerns of the repair and
rework cycle. Too much time means excess cost which is not acceptable. Poor quality has
never been an option with electronics manufacturing. Repeatability requires process
control, of course, but being able to quickly duplicate precise heating profiles – from
operator to operator, facility to facility – underlies the entire repair and rework process,
feeding back to time, cost and quality.
Oh for the days of through-hole components and tin-lead solder. Just heat the iron and go.
Component profiles were not complex, operators could see all connections and the
properties of solder were well understood. You didn’t have to be a rocket scientist to
work the repair and rework bench.
Those days, however, are long gone. Today it’s small, sensitive array packages with
complex profiles and hundreds of connections that can only be seen with sophisticated
vision systems. Operator turnover is great; yet, operator experience with array packages
is essential if time and quality goals are to be achieved.
Let’s now add another variable: lead-free solders. Reflow temperatures are higher, time
above the higher reflow temperatures are different, appearance of the joint is
considerably different, and the need for process control is even greater than it was for
eutectic solders.
Can we hope to reach time, cost and quality goals with array packages and lead-free
solder?
With the right thermal profiles, better equipment and a bit of knowledge, the answer is
yes.
The Basic Steps
Like leaded components with eutectic solder, the basic steps to proper BGA/CSP rework
using lead-free solder are the same – well, at least in theory:
1. Establish thermal profile
2. Remove failed component
3. Clean and prepare site
4. Replace component with flux or solder paste
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5. Reflow
6. Inspect
So far so good. Now the caveats.
Forget the soldering iron. Convection not radiation is the heating method of choice.
Convection allows for greater process control, of course, and without process control
repeatability is impossible.
The Thermal Profile
Better than conduction, convection also makes it easier to establish a good, repeatable
thermal profile, one that won’t over heat the component, or hold for too long above
reflow.
Establishing the correct, ideal profile takes experience, patience and knowledge of lead-
free. And while standard reflow consists of three zones (pre-heat, soak and reflow) plus
cool down, lead-free demands an extra ramp zone and more precise heating control.
A standard thermal profile using eutectic solder is shown in Figure 1. The parameters of
each zone are well understood and easily monitored.
Figure 1: Standard Reflow
Zone Time Duration Target Temperature °C
(seconds)
Pre-heat 60 to 90 100 to 120
Soak 60 to 90 155 to 175
Reflow 30 to 60 200 to 220
Lead-Free Solder
The higher temperatures needed for lead-free (up to 235°C), coupled with the thermal
sensitivity of BGA/CSP’s demands precise temperature and the addition of a ramp stage
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where temperatures rise at a rate that will not harm packages. That’s why today’s more
sophisticated rework systems employ four heating zones and one cooling zone. Without
this extra step, lead-free rework is doomed. Higher temperature requirements, coupled
with the thermal sensitivity of BGA/CSP, can be problematic without the ability to ramp
temperatures at a rate that will not harm packages.
The addition of a controllable pre-heater allows for efficient, controlled pre-heating and
avoids the thermal damage risked when working with expensive, but sensitive, packages
unsuitable for heating above 240°C with quick reflow times.
Tightening Lead-Free Temperatures
The temperatures used in lead-free are being tightened by both the suppliers and solder
manufacturers. The maximum solder temperature has a peak of 235°C and a low of
2I7°C.
But, component suppliers' maximum temperature, at the component lid, is 265°C, with
the most common temperatures ranging from 240° to 250°C. These temperatures are very
close to the 225°C - 233°C solder temperature. In addition, the time above reflow has
gone from 60 - 90 seconds (for eutectic solder) to 15 - 30 seconds for lead-free.
To meet this demand, rework systems must be capable of ramping up very fast, and then
down again to achieve this small peak temperature.
New Delta Considerations
Another factor to consider when moving to lead-free is the delta across the surface of the
component. Usually, a delta of 10°C is considered acceptable.
Delta Measurements across
BGA the surface of the
components used to be 10°C
but lead-free requires 5 °C.
The new delta is critical for thermal strength, but it is difficult to achieve as it is
measured from top to bottom. From tc/1, tc/2, tc/3, all have to be within 10°C (from the
lid to the ball and under the surface of the bottom of the PCB as shown below).
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 TC/1
TC/3
TC/2
Different lead-free compositions exist and these will be fine tuned as time and processes
mature. The most common are listed below in Figure 2.
Figure 2: Lead Free Temperatures
Alloy System Composition Melting Range
Sn-Pb 60Sn-40Pb 183-188
Sn-Cu Sn-0.7Cu 227
Sn-Ag-Bi Sn-3.5Ag-3Bi 206-213
Sn-Ag-Cu Sn-3.8Ag-0.7Cu 217
Sn-Ag Sn-3.5Ag 221
Solder Paste Composition and Temperature
The wetting process and temperature profiles must be controlled to make sure joints are
not brittle. With lead-free, there must be better regulation of heating and faster ramp up
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and down, particularly in the under-board heater. As a result, hot plates are a thing of the
past when lead-free is involved.
In general, temperatures must be high enough to melt and form intermetallic, high
enough to activate flux and optimize wetting, yet low enough to avoid PCB and/or
component damage.
Obviously, thermal profiles for lead-free are different from those of eutectic solder.
Tolerances are tight – making rework difficult without some type of repeatability and
process control.
An example of the standard profiles used for eutectic solders compared to lead-free
solder profiles is shown in Figure 3. The differences are substantial. The key to success is
system control and the ability to ramp up faster and cool down quicker.
Figure 3: Reflow Temperatures /times For Tin Lead Compared to Lead-Free
Zone Tin Lead Lead Free
Temp Time Temp Time
Pre-Heat 100C-120C. 60-90s 130C-140C. 100s
Soak 160C-170C. 90s 140C-170C. 90s
Ramp NONE 170C-225C. 100s
Reflow Max 220C. 60s 225C-235C. 15-30s
Cool 60C. 30-60s 60C. 30-60s
Now compare the rework plot for eutectic solder versus that for lead-free. The primary
temperature differences between the two processes are easy to see.
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Figure 4A: Eutectic Solder Reflow Rework Plot
Peak Temperatures Time above reflow
just over 201°C to 60 seconds is typical
205°C
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Figure 4B: Lead-Free Solder Reflow Rework Plot
Delta across top surface of BGA part to
solder ball, and bottom of PCB should
be as close to 5°C to 10°C as possible Time above
225°C 15 to 30
seconds
The top heat is the
pre-heater and the
lower heat is the
nozzle temperature
Inspection Differences
Lead-free solder joints look grainy when compared to traditional eutectic soldering and
are often erroneously rejected by inexperienced operators for quality reasons. When lead-
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free is implemented, companies must set a new standard and train operators in proper
inspection criteria. (See figure 5)
While traditional X-ray inspection works well, in part because joint appearance is not an
issue, vision systems are growing in popularity. In some larger facilities, vision systems
are used to compliment X-ray. Although in these tough times, many companies have
made vision systems the only mode of inspection in order to avoid the high cost of X-ray
– typically $75K to $120K.)
One major difference between X-ray and a newer vision system is the latter’s ability to
look at the joint, at both the top and bottom of the ball, and to check the formation of the
intermetallic joint. In addition, some vision systems have the ability to look under CSP’s
and BGA’S – a mandatory requirement BGA/CSP inspection.
Advances in technology have resulted in visual inspection systems that can go "low"
enough (below 2 mils), to "see" under BGAs. Such equipment typically incorporates a
metal halide light source with fiber optic light bundles that produce intense white light at
5,500°K. This lighting configuration provides true daylight illumination for color
rendering and color balance without blind spots, qualities that are necessary for accurate
visual inspection and evaluation.
- Insert VPI-1000 Photograph -
Caption: Metcal's VPI-1000 Optical Inspection System floods the underside of the component
with bright-white metal halide light that replicates natural daylight, enhances the color rendering
and produces sharp, clear, crisp images on the system’s hi-resolution flat screen color monitor.
Note: CSPs have a stand off height of 0.007″ to 0.008″ (0.2MM), and BGA 0.0.018″ to
0.020″(.5MM). When choosing a vision inspection system, make sure the system is
capable of easily getting under these components.
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Figure 5: Differences in Appearance
LEAD-FREE QFP JOINT CONVENTIONAL JOINT
Conclusion
Array packages and lead-free processes will continue to require post production soldering/rework.
Rework isn’t going away anytime soon. In fact, with thin profit margins, reducing scrap by
reworking assemblies is more, not less, critical to survival.
And while the basic rework steps are the same across technologies, substantial temperature
differences between eutectic and lead-free solders mandate tighter processes, better temperature
profiles and the use of precise rework systems with closed-loop process control.
With the narrow process windows mandated by lead-free, linked to temperature sensitive
array packages, high quality, low cost rework is challenging but achievable with
intelligence and the right equipment.
In short, the face of rework is changing and manufacturers and vendors must keep pace.
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