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Surface Treatment Enabling Low-Temperature Soldering to AluminumJuly 15, 2019 | Divyakant Kadiwala, Averatek Corporation
Estimated reading time: 4 minutes
The majority of flexible circuits are made by patterning copper metal that is laminated to a flexible substrate, which is usually polyimide film of varying thicknesses. An increasingly popular method to meet the need for lower cost circuitry is the use of aluminium on polyester (Al-PET) substrates. This material is gaining popularity and has found wide use in RFID tags, low-cost LED lighting, and other single-layer circuits. However, both aluminium and PET have their own constraints and require special processing to make finished circuits. Aluminum is not easy to solder components to at low temperatures, and PET cannot withstand high temperatures. Soldering to these materials requires either an additional surface treatment or the use of conductive epoxy to attach components. Surface treatment of aluminum includes the likes of electroless nickel immersion gold plating (ENIG), which is extensive wet chemistry and cost-prohibitive for mass adoption.
Conductive adhesives, including anisotropic conductive paste (ACP), are another alternative to soldering components. These result in component-substrate interfaces that are inferior to conventional solders in terms of performance and reliability. An advanced surface treatment technology will be presented that addresses all these constraints. Once applied on aluminum surfaces using conventional printing techniques such as screen, stencil, etc., it is thermally cured in a convection oven at low temperatures. This surface treatment is non-conductive. To attach a component, a solder bump on the component or solder printed on the treated pad is needed before placing the component. The aluminum circuit will pass through a reflow oven, as is commonly done in PCB assembly. This allows for the formation of a true metal-to-metal bond between the solder and the aluminum on the pads. This process paves the way for large-scale, low-cost manufacturing of Al-PET circuits.
Aluminum is the most abundant metal in the earth’s crust. Its alloys have found wide use as a building material in the construction of automobiles, aircraft, bicycles, building frames, etc. Other uses range from electrical connectors, packaging cans and foils, and household utensils. While it is a material of choice in the above fields, it is second to copper in the field of flexible circuits.
This is despite the various advantages that aluminum has over copper. Aluminum is more than three times lighter than copper. The density of aluminum is 2.7 gm/cm3 while that of copper is 8.92 gm/cm3. Its electrical resistivity is 26.5 nΩ·m (at 20°C) while that of copper is 16.78 nΩ·m (at 20°C). Also, its thermal conductivity is 237 W/(m·K) while that of copper is 401 W/(m·K).
Although it is not as good an electrical and thermal conductor as copper, it can radiate heat better than copper due to its lower density. Overall, aluminum has 68% of the conductivity of copper but has only 30% of the weight of copper. This means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance. This will be similar for aluminium traces in the case of flexible circuits.
Also, aluminum is generally less expensive when compared to copper conductors. A recent check indicated the price of aluminium was 35% less than that of copper. It is three times less expensive than copper on an equal weight basis and six times less expensive on an actual usage basis. This is the biggest advantage that aluminum has over copper. Table 1 lists the comparative properties of the two metals relevant to flexible circuits.
Flexible Circuits and Al-PET Substrates
The majority of flexible circuits are made using copper on polyimide (Cu-PI) substrates. These consist of copper foil laminated onto polyimide film. Varying the thickness of copper and polyimide gives rise to various combinations of thicknesses of Cu-PI to suit the conductivity and dielectric requirements of the end applications. Traces are formed using photolithography followed by a print-and-etch process. Components are soldered on to make the finished circuits. A reasonable selection of solders is available that can easily bond to copper traces without the need for any special surface treatment.
Table 1: Properties of aluminum and copper.
An increasingly popular method to make flexible circuits is by using aluminum on PET (polyethylene terephthalate) or Al-PET substrates. These are available in varying thickness of aluminum foil laminated onto PET film (Figure 1).
While aluminum is less expensive than copper, PET is also significantly cheaper than polyimide film. Hence, lower material cost is a major driver for the increasing use of Al- PET substrates, but their use has been limited because of processing challenges.
Figure 1: Typical laminated construction of Al-PET substrates.
The process to generate the traces on aluminium substrates is similar to that of copper. A dry-film or liquid resist is used for photolithography, which is then followed by chemical print-and-etch to form aluminum traces. But attaching the components onto aluminium is a challenge. Unlike copper, it is not easy to solder to aluminum. Soldering to aluminum is difficult because of the presence of a thin layer of aluminum oxide. This layer forms naturally when the bare metal is exposed to air. Since most flexible circuit manufacturing is done under atmospheric conditions, all aluminum surfaces will have an oxide layer. While the formation of this natural oxide is self-limiting, its presence cannot be overcome by the flux used in existing solder pastes. If harsher fluxes are used within solder pastes to address the aluminum oxide problem, they will cause corrosion of the very thin aluminum layers and thus reliability problems.
There are two methods currently used to attach components to Al-PET substrates: one is the zincate and plating process while the second is using conductive epoxy.
To read the full article, which appeared in the July 2019 issue of SMT007 Magazine, click here.
Absolute EMS, Inc., an award-winning EMS provider of turnkey contract manufacturing services, continues to harness the power of nitrogen to enhance the quality, reliability and efficiency of their manufacturing processes. The company’s state-of-the-art facilities are equipped with nitrogen reflow ovens and selective soldering machines, ensuring consistent, high-quality results for every project.
KYZEN, the global leader in innovative environmentally friendly cleaning chemistries, will exhibit at the SMTA Monterrey Expo & Tech Forum, scheduled to take place Thursday, March 14, 2024 at the Cintermex Convention Center, Hall 2A in Monterrey, Nuevo León. KYZEN will showcase the new aqueous chemistry AQUANOX A4618 in addition to highlighting stencil cleaning chemistries KYZEN E5631J and CYBERSOLV C8882.
The challenges of the Corona crisis have further accelerated this trend. Many companies now offer only online events, whether due to time or cost reasons. However, at Rehm Thermal Systems, you have a choice: the provider of thermal systems for various industries offers a variety of short, concise online webinars and complements them with in-depth in-person seminars on various topics.
Hentec Industries/RPS Automation, a leading manufacturer of selective soldering, lead tinning and solderability test equipment, is pleased to announce that Dynamic Grid has ordered a Hentec Industries/RPS Valence 3508 selective soldering system.
AIM Solder, a leading global manufacturer of solder assembly materials for the electronics industry, is pleased to announce its participation in the upcoming SMTA Capital Expo & Tech Forum taking place on March 7 at Sweeney Barn in Manassas, Virginia.