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Benefit By Reversing Electronics Manufacturing
These basic process steps illustrate the reverse manufacturing of a double-sided aluminum circuit assembled and interconnected without solder. From the top left, aluminum material is provisioned with cavities by milling, etching, or embossing, wherein components are placed and then coated with an insulating material. Holes are drilled, then filled with insulating material, then redrilled. At the same time, viaholes are formed to access component terminations. The circuit pattern is then plated and circuits sealed after the last layer is completed leaving open features required for interconnection and power (open features are not illustrated). The metal core can serve both as a heat spreader and a power or ground layer.
 

Many of the problems which have confronted the electronics manufacturing industry in recent years have been related to the solder assembly process. While lead-free solders were touted as drop-in replacements for traditional lead-containing solders, field experience has shown this not to be the case. More expensive, lead-free and tin-rich alloys with higher melting temperatures have caused the electronics industry to scramble for solutions to a number of new problems (e.g., "head-on-pillow" and pad cratering issues). The use of lead-free solder has also had deleterious spillover effects on printed-circuit-board (PCB) laminate materials due to the higher soldering temperatures required.

As a result, some efforts have focused on developing circuit fabrication techniques without solder, by reversing the manufacturing process. Instead of placing components on circuit boards, circuits are built on "component boards" with all component leads/terminations exposed and interconnected by an additive or semi additive plating process.

The new approach is a subset of solderless assembly for electronics or solder alloy free electronics (SAFE) manufacturing methods. SAFE approaches are cost effective by means of fewer manufacturing steps, but require discipline and attention to detail in component selection and layout. While engineered organic resins can be used alone or with other materials, aluminum is an attractive choice as a circuit substrate due to its various positive properties. It features a coefficient of thermal expansion (CTE) very close to that of copper, with dimensional stability far exceeding that of FR4 circuit materials, low density, good thermal dissipation capabilities, and low cost (about $1.00/lb.). It also comprises 8.3 percent of the earth's crust and is highly recyclable.

To fabricate an aluminum circuit, a sheet of aluminum is prepared and components placed on it. Sites can be prepared by chemical machining, mechanical machining, laser cutting, punching, or other methods. The aluminum substrate can also be embossed or cast with cavities if required. Aluminum can be anodized, s that the surface is nonconductive aluminum oxide, or it can be coated with insulating material electrophoretically or electrostatically to make its surfaces nonconductive. Cavities can be formed to match the heights of electronic components so that the lead terminations of those components are flush with the surface of the aluminum board to facilitate processing. With aluminum, the overall number of processing steps is significantly reduced from manufacturing approaches based on conventional PCB materials.

Capital Equipment List for Traditional Electronics Manufacture (Abbreviated list)

PCB Fabrication:{/SB}
  • Shearing Equipment
  • Drilling Equipment (mechanical and laser)
  • Surface Preparation Equipment (chemical and mechanical)
  • Metallization and Plating Equipment
  • Photoresist Application Equipment (includes solder mask)
  • Photoimaging Equipment (contact and laser direct print)
  • Image Development Equipment
  • Lamination Equipment
  • Routing Equipment
  • Cleaning Equipment
  • Testing Equipment (electrical and X-ray)
  • Packaging Equipment

PCB Assembly:
  • Baking Ovens
  • Solder Paste Application Equipment (stencil printer)
  • Solder Paste Inspection Equipment (optical or x-ray)
  • Pick-and-Place Equipment
  • Component Placement Inspection (optical)
  • Reflow System (convection ovens, vapor phase, others)
  • Specialty Cleaning Equipment
  • Inspection and Test Equipment (optical x-ray and electrical)
  • Solder Rework and Repair Equipment
  • Depanelization Equipment
  • Packaging Equipment

Capital Equipment List for SAFE Assembly (Abbreviated list)
  • Pick-and-Place Equipment
  • Component Placement Inspection (optical)
  • Shearing and Punching Equipment
  • Encapsulation Equipment
  • Via Formation Technology (Laser or photoimage)
  • Surface Preparation Equipment (chemical and mechanical)
  • Metallization and Plating Equipment
  • Coating Equipment (for photoimage materials)
  • Image Development Equipment
  • Routing / Depanelizing Equipment
  • Cleaning Equipment
  • Packaging Equipment

Although these lists are not exhaustive (many minor process steps are not represented in both equipment lists), they provide a reasonable approximation of the differences in capital equipment requirements of traditional versus SAFE assembly approaches.

Numerous Benefits
Reverse manufacturing of electronic circuits can provide a number of advantages compared to traditional PCB manufacturing, especially for circuit assemblies made with aluminum. These include economic benefits, electrical benefits, thermal benefits, and mechanical and reliability improvements. For example, manufacturing cost is a constant concern. The use of aluminum, the third-most abundant material on Earth, which is sold by weight regardless of thickness, can help control electronic manufacturing costs. Aluminum is less expensive per unit volume than FR-4 circuit material. In contrast, the price of polymer materials varies with the price of oil.

Using aluminum cuts manufacturing costs by reducing the number of manufacturing steps required, eliminating conventional processing steps associated with solder. The final cost of aluminum-based circuit assemblies can be 25 to 35 percent less than circuit assemblies produced by conventional means (exclusive of component costs). In addition, with aluminum, electronic components need not be prepared with special finishes to maintain solderability, and they do not require special treatment to keep out moisture since they will not be subjected to the high temperatures required to process lead-free solders.
At any given lead pitch, solderless assembly methods can significantly reduce circuit layer count by freeing up routing space as well as assembly height, since solder often comprises one-half of the overall height of a board-mounted package or device.
 


Reverse manufactured assemblies offer several electrical/electronic benefits. For example, where connections are made to terminations on component lands, the point of interconnection can be made without benefit of a large pad, which can reduce parasitic capacitance. This also frees routing space, allowing for a potential reduction in total layer count (further reducing cost). Judicious component selection and use of a common grid pitch (e.g., 0.5mm) has been shown to reduce layer count while improving signal integrity. The aluminum core can also serve as a power, ground, or integral shield. Moreover, the completed assembly can also be relatively easily provided with metal plating after the assembly is complete. This makes the entire assembly metal jacketed and immune to electromagnetic interference (EMI) and electrostatic-discharge (ESD) effects, and nearly hermetic, exclusive of areas left open for external input/output (I/O) connection.

At any given lead pitch, solderless assembly methods can significantly reduce circuit layer count by freeing up routing space as well as assembly height, since solder often comprises one-half of the overall height of a board-mounted package or device.

When used as a carrier, aluminum is by default a heat spreader as an integral part of the assembly, supporting thermal design and long-term reliability. Integration within the aluminum assembly also protects the encapsulated components from the effects of shock and vibration. Since the CTE values of aluminum and copper are close (22ppm/°C vs. 18ppm/°C), thermal stress is low on aluminum-board interconnections. Thin aluminum base material can be used for forming a product into a desired shape.

The use of aluminum boards provides some measure of design security, making the reverse engineering of a product more difficult. The benefit extends to a range of electronic markets, from consumer to military electronic products. The assembly process with aluminum boards can also make it more difficult to extract and reuse components, injecting them into the supply chain as counterfeit devices.

The electrical, mechanical, and thermal benefits of aluminum boards can boost electronic product reliability, in concert with avoiding the thermal effects from the high-temperature lead-free soldering process. Also, eliminating the solder joints eliminates a major source of failures in electronic circuits. Aluminum boards can also remove concern for tin whiskers, which are conductive anodic filaments (CAFs) that form between adjacent viaholes in certain circuit-board materials.

The RoHS Mandate
Elimination of lead from electronic solder by European Union (EU) RoHS mandate has been costly. However, reverse manufacturing processes, which eliminate solder, also make compliance with the RoHS mandate almost automatic. An aluminum circuit assembly has an all-copper and polymer interconnection system on an aluminum base. Both RoHS and REACH concerns should be obviated. Additionally, the material declaration process is greatly simplified. These same benefits hold true relative to the use of conflict materials which is of growing concern among increasing numbers of both governmental and non-governmental organizations (NGOs) as the structures completed as described are completely devoid of any proscribed or sanctioned materials and will more easily pass regulatory scrutiny.

Environmental concern has grown over the last few decades, making most electronics manufacturers aware of the need to produce "environmentally friendly" products. This can be done with a board material that is easily recycled, such as aluminum. Also, by not using solder and the energy required in soldering processes, energy is saved in the electronics manufacturing process through the use of aluminum boards.

A recurring concern associated with reverse manufacturing has been how to test and rework these new electronic assemblies. Of course, if they are manufactured properly, there will be no need to test and rework the boards. Eliminating solder and solder joints can eliminate a leading source of failures in many circuit boards, especially under conditions of shock and vibration. Moreover, below 0.5mm component lead pitch, which is where the component roadmap trends are headed, assembly yields drop off appreciably, even with multiple preassembly inspection steps implemented and/or applied.

High yield should be a goal for all electronics manufacturers. With aluminum boards and reverse manufacturing approaches, if the components are properly burned-in and fully tested, and manufacturing processes are properly controlled, a final product should be achieved with high yield. In this way, the reliability limits of future electronic products will be established by IC and component reliability rather than by the reliability of the circuits and plated viaholes used to interconnect the ICs and components.

 

Contact: Verdant Electronics, 500 Yale Ave. North, Seattle, WA 98109 206-351-8943 E-mail: joe@verdantelectronics.com Web:
http://www.verdantelectronics.com

 
 
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