A study on a new electronics assembly process suited to the manufacture of high-density, high-performance, high-reliability and environmentally compliant next-generation solutions
by Joseph Fjelstad, Verdant Electronics
New solder-free electronics assembly process, known as Occam, is being developed for products, built this way, to be more reliable than previous solder-free strategies (e.g., using conductive adhesive as a solder substitute) as well as traditionally manufactured soldered assemblies. The process is a reverse order interconnection solution that employs mature, low-risk, familiar core processing technologies in a novel sequence. Components are interconnected by means of copper plating after they are assembled into their final positions in an encapsulated module, thus doing away with conventional printed circuit boards. Prototype assemblies using this new technology are currently being characterized.
The process is eminently suited to the manufacture of high-density, high-performance, high-reliability and environmentally (i.e., RoHS) compliant next-generation solutions for products ranging from consumer to military and aerospace applications. This process inherently eliminates high-temperature exposure, tin whisker risk, and vulnerability to mechanical shock and thermal cycle fatigue failure. Other anticipated benefits include simplified design (tightened geometries for higher density form factors), fewer processes (including elimination of all solder processing and associated issues), and diminished material costs and supply infrastructure.
The technology addresses three serious challenges to producing electronic assemblies:
- the regulatory imperatives (e.g., RoHS) to produce lead-free electronics require
- subjecting them to very high temperatures associated with lead-free solder, and involves
- reliability risks associated with the extensive use of tin plating as a termination finish
- the relentless drive to reduce size and cost results in increasing challenges for reliable component placement and attachment
- global sourcing and supply-chain expansion means more distant PCB suppliers, reducing the resources and support for domestic technology development.
An assembly designed and built using a proposed reverse order interconnection process solution contains no solder. It, thus, completely bypasses the issues related to:
- the high temperatures required for lead-free soldering
- flux removal from tiny spaces where it is not easily removed
- mechanical shock and thermal cycling reliability of solder connections.
Encapsulation
The assembly’s fully tested and burned-in packaged IC components are pre-encapsulated. Encapsulation completely bypasses the issue of whisker risk due to the presence of tin plating on component terminations. It also eliminates any concern about complete coverage by conformal coating (for which there is now no need).
Though some embodiments of the technology can be used with traditional circuit boards, the more attractive assembly embodiments do not require a separately fabricated printed circuit board. It thus completely bypasses many of the issues related to supply chain management, inventory management, and design modification. Instead, the connections to the component terminations are created by copper plating to the exposed surfaces of the terminations arrayed across the encapsulated module’s surface, with circuit layers created by either co-laminating or as has been done in the production of some bare die multi-chip and microprocessor modules in past years, the building up successive layers of circuitry. The interconnections can be redesigned as needed up to the moment they are made. Connections between layers are made during either the build-up or co-lamination, so there is no high-aspect-ratio via drilling. These assemblies are expected to be low cost and amenable to in-place thermal enhancements, EMI shielding, embedded electrical and optical components, and more.
Interconnection process
The Occam process is a reverse order interconnection process that employs mature, low-risk, familiar core processing technologies in a proven sequence. Components are interconnected by copper plating after they are assembled into their final positions. As illustrated in Figure 1, a conventional circuit board is not required, nor is solder.
In the build-up approach the conventional sequence of creating the interconnecting pattern between termination points (board fabrication), assembly and connection is reversed so that the sequence is simply assembly then interconnection.
For purposes of comparison, in conventional electronic assembly, components are placed on board lands and temporarily immobilized by the contact between the terminations and pre-deposited solder paste until the solder is reflowed to provide the permanent immobilization. In the present process, components are placed on a removable tacky film on a temporary or permanent base. The film and base temporarily immobilize them until the structure is encapsulated. The entire array of tested and burned-in components thereby becomes a monolithic assembly, with each component now permanently immobilized by every part of it. The bottoms of these terminations can be exposed by removing the temporary base and film or by making holes in a permanent one by such means as mechanical abrasion, water-jet material removal, or laser ablation.
The assembly is now ready to be metallized with copper using standard printed circuit additive (build-up) processing methods, with circuit patterns created to make the required interconnections between leads of all of the components. In most cases, more than one layer will be required, so an insulation layer is placed over it and the process is repeated until all required interconnections are made. See Appendix C for views of prototypes.
The final circuit layer can be connected to whatever user interfaces, displays and power connections are required for operation, and then coated with a conformal or rigid protective insulator layer.
Where two layers of components are required they can be stacked and joined back-to-back with a central support, which could include various heat spreader constructions.
The figure above shows an example of a more advanced structure. Stacking and interconnection of assemblies from one side to the other can be accomplished by interconnection pins inside the assembly either individually as shown or in arrays. Interconnection between sides can be augmented by various connector structures or flex circuits as well. Note that these graphics do not show any of the thermal-management, optical interconnect, EMI, connector, 2nd-side, and odd-form enhancements that are enabled by this process.
Elimination of solder
This non-traditional concept involves the elimination of solder from the assembly process. The notion of eliminating solder is not new, but the previously proposed means to do so (e.g., replace with conductive adhesive) have not been accepted by the market, presumably because of difficulties in operation or lack of reliability.
While the solution is likely to be viewed early on by some as impractical at first because solder has always been the de facto standard for electronic assembly, those skilled in the technology of electronic assembly will recognize that is both possible and practical in view of the prior work in building much more complex and challenging multichip modules and IC packages using bare die. All of the materials, equipment and processes required to implement the process are available and operational today. The biggest change to the assembly factory will be simply the importing of a mature process of additive board fabrication.
Because the process does not entail exposure of the assembly to the high temperature needed for reflow soldering, component moisture sensitivity level (MSL)–which is a measure of the risk of component damage due to explosive outgassing of absorbed moisture in the package during soldering–ceases to be a concern.
All components can be treated as if they were MSL-1, which means that they do not require dry storage, special handling or recordkeeping. The process also allows use of components that are not capable of withstanding lead-free soldering temperatures (aluminum electrolytic capacitors, optoelectronic devices, connectors, etc.).
As with a standard printed wiring board, the interconnect structure still must be designed and fabricated, but some of the constraints are relaxed. There is no need for large component pads or lands for soldering, simplifying routing. This allows a higher circuit density and hence a reduction in the number of layers needed. There is no need for drilling high-aspect-ratio vias all the way through the assembly as special structures have been anticipated to address the need when faced.
Also, eliminating the printed circuit board avoids many of the less obvious costs such as supply-chain management (vendor qualification, lead-times, incoming QC, etc.), testing, inventory management, protective storage, bake-out, handling, etc. Because the interconnection is not created until after assembly, the design can be modified as needed with no need for drills, fills and jumper wires. It is anticipated that the finished product will be totally encapsulated with a tough and properly CTE-matched epoxy or other material and, hence, be quite rugged.
In sum, the improved approach to assembly offers the OEM a new choice for producing products that should prove a highly reliable and cost effective approach to electronic assembly, while, as an added benefit, the products will not only comply with RoHS requirements but will actually be far more environmentally friendly than solder-assembled products.
Development, qualification and adoption
Work is currently underway to develop the Occam Process from end to end. For example, several technical process and materials issues, workmanship attributes, inspection protocols, etc, associated with optimized and tailored interconnection options, need to be resolved, and are currently under development and investigation.
These include:
- refined definition of the encapsulant and its properties, particularly its shrinkage and CTE
- refinement of the manufacturing preparation steps: planarization, metallization, pretreatments, cleaning, etc
- refinement of the placement/positioning process, exploring the use of fiducials and true-position feedback
- refinement of the process to expose the terminations after encapsulation
- refinement of the build-up interconnection steps, including tailoring of the materials and processes.
Full qualification of the process and product will obviously be required but it is anticipated that there will be a number of willing participants among military product developers. Military suppliers and customers have been profoundly impacted by the wholesale move to lead-free electronics. Though they themselves are exempt from legislated lead-free requirements, military product developers are finding themselves increasingly unable to procure components with the tin-lead solder finishes that they know are reliable.
Because solder joints are known as one of the “Achilles’ heels” of electronic assemblies, their elimination–enabled by the improved manufacturing technology–will cause high-reliability electronics product developers to want to evaluate it promptly. If it surpasses traditional assembly methods in terms of reliability alone, they should rapidly adopt the approach.
Future implications
The technology has far-reaching implications. Instead of three different sectors for electronic manufacturing–PCB fabrication, components manufacturing, and assembly–there are just two (see Figs. 1 and 4). That is because printed circuit manufacture and assembly are essentially fused into one continuous manufacturing operation.
Such a solution will greatly increase control, and harness the revenue presently lost in supply chain management. The technology greatly simplifies supply chain management by reducing to a minimum the number of items that must be managed. The technology could have a significant impact in the area of military electronics as well.
In addiiton, the environment will benefit and so will the suppliers and consumers of high functionality electronics, particularly those seeking to make and buy products and applications that are mechanically robust, highly reliable and environmentally friendly.