11.4.3 PCB Surface Finish Requirements
A uniform PCB plating thickness is key for high assembly yield. For an electroless nickel immersion gold finish,
the gold thickness should range from 0.05 µm to 0.20 µm to avoid solder joint embrittlement. Using a PCB with
Organic Solderability Preservative (OSP) coating finish is also recommended as an alternative to Ni-Au.
11.4.4 Solder Stencil
Solder paste deposition using a stencil-printing process involves the transfer of the solder paste through pre-
defined apertures with the application of pressure. Stencil parameters such as aperture area ratio and the
fabrication process have a significant impact on paste deposition. Inspection of the stencil prior to placement of
package is highly recommended to improve board assembly yields.
11.4.5 Package Placement
Packages can be placed using standard pick and place equipment with an accuracy of ±0.05 mm. Component
pick and place systems are composed of a vision system that recognizes and positions the component and a
mechanical system that physically performs the pick and place operation. Two commonly used types of vision
systems are:
• A vision system that locates a package silhouette
• A vision system that locates individual pads on the interconnect pattern
The second type renders more accurate placements but tends to be more expensive and time consuming. Both
methods are acceptable since the parts align due to a self-centering features of the solder joint during solder
reflow. It is recommended to avoid solder bridging to 2 mils into the solder paste or with minimum force to avoid
causing any possible damage to the thinner packages.
11.4.6 Solder Joint Inspection
After surface mount assembly, transmission X-ray should be used for sample monitoring of the solder
attachment process. This identifies defects such as solder bridging, shorts, opens, and voids. It is also
recommended to use side view inspection in addition to X-rays to determine if there are "Hour Glass" shaped
solder and package tilting existing. The "Hour Glass" solder shape is not a reliable joint. 90° mirror projection can
be used for side view inspection.
11.4.7 Rework and Replacement
TI recommends removal of modules by rework station applying a profile similar to the mounting process. Using a
heat gun can sometimes cause damage to the module by overheating.
11.4.8 Solder Joint Voiding
TI recommends to control solder joint voiding to be less than 30% (per IPC-7093). Solder joint voids could
be reduced by baking of components and PCB, minimized solder paste exposure duration, and reflow profile
optimization.
11.5 Baking Conditions
Products require baking before mounting if:
• Humidity indicator cards read > 30%
• Temp < 30°C, humidity < 70% RH, over 96 hours
Baking condition: 90°C, 12 to 24 hours
Baking times: 1 time
SWRS263A – FEBRUARY 2021 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
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