Printed Circuit Boards
Written by a world-renowned electronics expert, this reader-friendly guide helps engineers and technicians solve issues in PCB layout, fabrication, assembly, and testing.
TABLE OF CONTENTS Printed Circuit Boards
13.11: Adhesive for Mixed Technology Assembly
13.11 Adhesive for Mixed Technology Assembly
In many of today's circuit board designs, some surface mount components are attached to the bottom side of the board. Through-hole components as well as large surface mount components are attached to the top side of the board. When surface mount and through hole components are combined, the board forms a mixed technology assembly.
With mixed technology assemblies, bottom side surface mount components require adhesive application so that they do not fall off during the subsequent component placement and wave soldering operations.
13.11.1 Requirements of Adhesive
The adhesive should be selected to meet the following basic requirements:
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Hold the component in the given place during the cure process
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Maintain that orientation through the wave soldering operations
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Must have adequate adhesion to different surfaces
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Must not be affected by exposure to the environment of solder flux and wave soldering.
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It must be chemically inert throughout the life of assembly.
Epoxies and Acrylics are commonly used as adhesives for SMT assembly. Adhesives must be stored in a cool, dry and dark location.
Epoxies provide good insulation resistance, high bond strength and low curing temperature. A disadvantage of epoxies is that defective components are difficult to remove during repair. They are typically single-component heat-curing systems and are available in a range of formulations for special requirements.
Acrylic adhesives have fast curing time, high peak profile of dispensed dots and good temperature stability. They are usually cured by applying UV and IR energy. The main disadvantage...
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