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Method for the use of thermo-chromic materials in serial number labels for determining the type of solder used in printed circuit assemblies

IP.com Disclosure Number: IPCOM000006610D
Publication Date: 2002-Jan-16
Document File: 5 page(s) / 44K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for the use of thermo-chromic materials in serial number labels for determining the type of solder used in printed circuit assemblies. Benefits include an improved, foolproof method for indicating if a printed circuit board contains lead solder or lead-free solder.

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Method for the use of thermo-chromic materials in serial number labels for determining the type of solder used in printed circuit assemblies

Disclosed is a method for the use of thermo-chromic materials in serial number labels for determining the type of solder used in printed circuit assemblies. Benefits include an improved, foolproof method for indicating if a printed circuit board contains lead solder or lead-free solder.

Background

              Lead-free boards are required to be identified at various times during the life cycle of a printed circuit board. Environmental laws are expected to require a board manufacturer to implement an easy and foolproof method for determining if a printed circuit assembly (PCA) has been manufactured using lead free technology. Detecting lead-free solder is not easy during or after assembly. Lead-free solder looks visually the same in color and texture as tin/lead solder, making conventional visual inspection inadequate. Lead-free boards must be identified at the following points of its life cycle:

•             Following surface-mount technology (SMT) assembly during repair

•             During repair at an original equipment manufacturer (OEM) location

•             During repair at an end-customer location

•             At the end of its useful life when the board is recycled or put into a landfill

              Because the same printed circuit board (PCB) can be manufactured with tin/lead or lead-free solder, simply writing lead free on the PCB using silkscreen or PCB copper layers is not adequate. A foolproof witness mark is required that is visible to the end user clearly indicating if a board was manufactured using lead or lead-free solder.

              The conventional process to distinguish boards soldered with lead-free solder from those soldered with tin/lead solder is by labeling the boards (see Figure 1). However, the labeling process is not standardized in the industry. This solution also lends itself to incorrect labeling. The label is added before or after the fabrication process using the same labeling processes used to place serial number labels on PCBs.

              Thermo-chromic materials are quite common. They are used in consumer items such as beverage mugs, frying pans, dishwashers, toys, hair curling irons, and laminators to indicate the temperature of certain elements of these products. For instance, a frying pan may have a dot of a thermo-chromic material applied to its center to indicate to the cook when the temperature for proper frying of an egg (for example) has been reached.

              The physical principles on which thermo-chromic materials work are mostly related to the electronic band structure in the materials. Molecular structure changes also cause color changes. Reversible thermo-chromic materials (those that change their color above a certain temperature and revert back to their original color when cooled to room temperature) are more common. However, irreversible thermo-chromic materials (those materials, which do not change back to their original color) also exist. They gene...