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Assembly Recommendations

Solder Assembly

Application and Thickness

Solder joints deserve additional consideration in the design ofThermal Clad assemblies. This section covers solder surface finishes, application and thickness, alloy and flux. The typical application technique is metal stencil. Dispensing ofsolder to specific locations is used for secondary operations or special attachment requirements. No other decision will effect the reliability of the solder joint as much as the thickness of the solder to be used. A minimum of 0.004" (100µm) is recommended (after reflow). This thickness dissipates stress build up in the joint. Additional information regarding solder joint reliability is offered in the appendix.

Surface Finishes

The typical surface finishes available for Thermal Clad circuitsare hot air solder leveling (HASL), organic solderability protectant(OSP) and solderable tin. HASLis a 63/37 Pb/Sn coating withexcellent shelf life and solderability. OSPis a thin coating used to protect the copper and has a shelf life of 3-6 months. Immersion tinis a planar coating with a shelf life of 6 to 12 months (depending on storage conditions), and can be used for a planar solderability circuit while protecting the copper baseplate from oxidizing.

Alloy

In Thermal Clad assemblies, we look to the solder alloy to servetwo important functions beyond the obvious mounting needs.1.The solder alloy must be a good heat transfer medium. Thismeans we need good wetting (no voids) as well as good thermal conductivity.2.The solder alloy must withstand thermal cycling.The addition of silver (Ag) to the solder alloy is helpful to bothneeds. We can recommend a 2% Ag alloy (62 tin, 36 lead, 2 silver). The silver serves to increase the tensile strength of the alloy. In its molten state, the surface tension of this composition is helpful in aligning surface mount components.Lead-free solder can be used with Thermal Clad providingmaximum process temperatures are not exceeded.
Note: For copper-based Thermal Clad, the soldering process should notexceed 260°C for one minute, and for aluminum base Thermal Clad should not exceed 300°C for one minute. See graph below which references a typical oven reflow temperature profile.

Flux

An RMAflux (mildly activated rosin) is suggested since it caneasily be cleaned. Cleaning will not damage the dielectric under nor- mal process conditions. Long term service durability is dependent on complete removal of flux residuals.“No clean” fluxes are used in many successful Thermal Cladapplications. It is important to follow the process temperature recommendation when using these materials. Solder processes below minimum temperatures do not allow conversion or volatilization of corrosive flux components. It is particularly difficult to assure adequate process temperature exposure in rework processes.
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