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Some urban legends or myths have been developed over time to where they almost become “true” without any real basis in fact. For example, urban legend has it that Walt Disney wanted so much to finish the projects prior to his death that he had himself (or some believe just his head) cryogenically frozen so that his cancer could be healed at some future date. The infamous “Nessy” of Loch Ness monster fame was photographed showing a head and neck in the “physician’s photograph” purporting that a sea monster or previously thought to be extinct dinosaur lives in a deep inland lake in Scotland.
The electronics manufacturing industry has its own myth in polyimide tape being the most effective way to shield neighboring components from heat exposure. Legend has it that this thin, adhesive-backed “protector” will shield components from damage.
Within the last year, we undertook an experiment to determine how effective different materials including polyimide tape would be in protecting neighboring devices during localized heating. This unwanted heating of components is driven by a desire to reduce neighboring component damage, to eliminate the need for removing components near the device being reworked which could be reflowed increasing the IMC layer thereby reducing the reliability of the interconnection. Historically, non-semiconductor components have been able to meet the heat withstand requirements of board assembly temperature conditions. For lead-free reflow, wave soldering components are qualified for 275°C for 10 seconds. Similarly, for SMT reflow soldering, components are typically qualified at 260°C for 40 seconds (though component body temperatures can reach 260°C during reflow).
This work examined the current state of shielding options that rework technicians have in protecting neighboring devices. The purpose of this study was to determine the shielding effectiveness by measuring the temperature at select neighboring devices based on a set distance from a rework source location. The options included polyimide tape, 304 stainless steel, and copper tape with an adhesive backing as common rework shielding materials. Non-traditional materials such as a ceramic non-woven fiber and an organic water/clay gel, both of which are utilized outside of the electronics industry, were also part of this study.
While protection against conductive heat transfer will have to be left up to the design engineers, shielding against radiation and convective heating sources is a function of the shield material properties.
There are a variety of materials, namely stainless steel, copper tape, polyimide tape, clay/water gel or a ceramic non-woven material which can shield the aforementioned heat energy from being transferred from the rework area to other areas of the PCB. Stainless steel metal shields are designed to shield a component from absorbing excessive heat either by dissipating, reflecting or simply absorbing the heat. The physical properties of stainless steel’s reflectivity and emissivity, thermal conductivity and specific heat capacity make it the ideal material for the fabrication of heat shields. The copper tape allows the copper shield to be flexible and be easily applied to a PCB in the rework area. The adhesive side allows it to adhere to a board surface and stay tacked down. Polyimide tape is the most commonly used (and misused) methodology for masking areas on a PCB.
The ability of polyimide tape to maintain excellent physical, electrical, and mechanical properties over a wide temperature range makes it an ideal “duct tape” for the PCB assembly industry. The one drawback with this material is its relatively poor thermal insulation properties. The clay/water gel shielding material is a product which has been used in welding applications due to its extreme resistance to heat. The gel contains deionized water and clay. The product is easily applied and, due to its consistency, able to work its way into small spaces on a PCB. The gel residue can simply be “washed off” using water.
Ceramic fiber non-woven shields, previously relegated to aerospace, nuclear energy and high temperature processing industrial environments, offer users several properties which makes it well-suited for PCB rework shielding. This ceramic fiber material offers several advantages including the properties of high-temperature stability, low thermal conductivity, high heat reflectance and the ability to be easily wrapped and cut to shape. These final (2) materials are not commonly used in the PCB assembly area but are new materials requiring further testing of their impact.
Editor's Note: This article originally appeared in the April issue of SMT Magazine.
