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Johanson Dielectrics Solder Recommendations for Lead Free Assembly
Christopher England Applications Engineer HVS products
June 21, 2006 |
General:
Soldering temperature profiles used must provide adequate temperature rise time and
cool-down time to prevent damage due to thermal shock. These guidelines are
emphasized because cracking or other damage caused by handling or thermal shock is not
necessarily apparent under ordinary visual inspection techniques. The damage can be
very small (micro-cracks) and can occur under the terminations where even high
magnification cannot detect them. The problem is further complicated by the fact that
these micro-cracks may not be initially detectable by standard electrical testing. Once
initiated, the cracks can grow with time and cause latent failures. Attention to these
details will aid in the successful use of the inherently reliable multilayer ceramic
capacitor.
Ceramic capacitors larger then EIA size 1812 are known to be very susceptible to thermal
shock damage due to their large ceramic mass. These large parts require more care during
installation than smaller surface mount devices. Higher temperatures are now required for
“Lead Free” solder profiles. The attached diagram from J-STD-020C shows both
standard and lead free profiles. |
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Solder Pre-Heat Cycle:
Proper preheating is essential to prevent thermal shock cracking of the capacitor. The
circuit assembly should be preheated as shown in the recommended profiles at a rate of
1.0 to 3.0°C per second to within 75 to 125°C of the maximum soldering temperature.
Temperature change should be distributed as evenly as possible throughout large
capacitor bodies as applying heat or cold to a localized spot within the device may result
in thermal gradients great enough to cause cracking.
SMT Soldering Temperature:
Solders typically utilized in SMT Reflow Solders have melting points between 179°C
and 217°C. Activation of rosin fluxes occurs at about 200°C. Based on these facts reflow
temperatures between 210 to 260°C should be adequate in most circumstances. Use of
thermal profiling is advised for accurate characterization of circuit heat absorption and
maximum component temperature conditions that occur during the soldering process.
Solder Reflow:
Recommended temperature profiles for reflow soldering are shown in Table 1 and Figure
1 from J-STD-020C.
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Solder Wave:
Solders typically utilized in the solder wavehave melting points between 179°C and
227°C. Wave soldering can be utilized for lead free assembly, but the preheat
requirements generally make this process very difficult to accomplish as peak
temperatures may reach 270°C . Wave soldering is not recommended for ceramic
MLCCs larger then 1210 and thicker than .050” size due to the incompatibility of the
chip's mass with the steep temperature gradient typically present in this process. Cool
down after solder wave requires rate control <2°C/sec. Land patterns should be 2/3
narrower than the chip width to control solder fillet volume and minimize local
Coefficient of Thermal Expansion (CTE) mismatch between the capacitor, solder and
board.
Soldering Iron:
Ceramic capacitor attachment with a soldering iron is discouraged due to the inherent
process control limitations. In the event that a soldering iron must be employed the
following precautions are recommended.
- Preheat circuit and capacitors to within 100°C of soldering temperature
- Never contact the capacitor with the iron tip
- 20 watt iron output (max)
- 350°C tip temperature (max)
- 1.0 mm tip diameter (max)
- Limit soldering time to 3 sec...
Cool Down Cycle:
After the solder reflows properly the assembly should be allowed to cool gradually, again
maintaining the thermal gradient of 3°C/sec. maximum at room ambient conditions.
Attempts to speed this cooling process or immediate exposure of the circuit to cold
cleaning solutions increase the possibility of thermal shock cracking of the ceramic
capacitor.
Ceramic capacitor attachment with a soldering iron is discouraged due to the inherent
process control limitations. In the event that a soldering iron must be employed the
following precautions are recommended.
IPC 7351 Land
IPC 7351 Land Pattern Guidelines:
Appropriate pad design, solder application, and component orientation are all ingredients
of a quality, defect-free soldering process. The Institute for Interconnecting and
Packaging Electronic Circuits (IPC) has developed and published IPC 7351 "Surface
Mount Design and Land Pattern Standard ". This standard presents industry consensus on
optimum dimensions based on empirical knowledge of fabricated land patterns. The
standard also contains an excellent analysis of solder joints and their relation to
component, PCB, and placement tolerances. A summary of the IPC land pattern design
recommendations for solder reflow and solder wave processes are listed in table below. It
is highly recommended that the PCB designer/SMT process engineer obtain the complete
IPC 7351 standard (http://www.ipc.org) |
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Tomb Stoning / Chip Movement:
Tomb-stoning or draw bridging is illustrated in figure 2. Tomb-stoning or other
undesirable chip movements may result if unequal surface tension forces exist as the
molten solder wets the MLCC terminations and mounting pads. This tendency can be
minimized by insuring that all factors at both solder joints are equal, namely; pad size,
solder mass, termination size, component position and heating. Tomb-stoning is easily
avoided through proper design, material selection, control of solder mass and proofing of
the process. |
Figure 2
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