The mantra of the recent InterSolar Conference in San Francisco was: control costs, improve yield, and assure reliability. There were also calls for new, disruptive technology. Photovoltaic (PV) manufacturers could benefit from specifics.
There are at least three technology-related issues associated with PV manufacturing. One is product design. Another is selection of materials. A third is the manufacturing and assembly processes; this is an area where critical cleaning and contamination take center stage. Unfortunately, the general consensus among PV manufacturers runs along the lines of “we do whatever the wafer fab people do, only less.” The main issues are driving the costs down and improving yield. The bulk of costs are in the materials, and process costs are already low.
Might judicious critical cleaning and appropriate contamination control and the use of the correct controlled environments contribute to more productive growth for forward-thinking PV manufacturers? Selective contamination control and value-added critical cleaning have proven important for high-value products in aerospace, engineered coatings, and medical device manufacture.
Ubiquitous contamination
“Everyone has cleaning issues and contamination problems. They may not believe it, but they do,” asserts Cheryl Tulkoff, senior member of the technical staff at DfR Solutions in Austin, Texas. “Controlling particulates in spin rinse driers has been a chronic issue. Just keeping a fab clean of particulates is a continuing challenge. The chemicals (really all materials – people, equipment, gases) have to be vetted, ” she says.
Tulkoff explains that there are also issues with electronic assemblies. Potting compounds and conformal coatings do not adhere well to contaminated surfaces. When bottom termination components (BTCs) are used or simply tight spacing exists, it is extremely difficult to remove flux or achieve low levels of flux residue on surface mount assemblies. Even the weak organic acids left behind by no-clean fluxes cause problems. She adds that on the solar panel itself, if there is contamination on or below the surface, there is loss of efficiency.
Challenges
Several other experts in the field indicated contamination sources in PV manufacture, including particulates and thin films. Patrick McCluskey, associate professor of mechanical engineering at the University of Maryland, explains that, during module assembly, “the cells are placed between two glass plates and sealed using thermoplastics so that they are laminated to the EVA (ethylene vinyl acetate) encapsulant. If there is any surface contamination, be it dust, dirt, fingerprints, or outgassing, there will be a reduction in light adsorption; ionics and particulates are of most concern.” He adds that contamination can also result in delamination, which may be a long-term effect. In addition, the presence of halogens, including flux residue, can eventually result in corrosion of interconnects.
Selective contamination control and value-added critical cleaning can benefit photovolatic manufacturers.
Allen Zielnik, senior consultant at Atlas Material Testing Technology in Chicago, observes that oils, including silicone oils from the glass handling system, may be deposited from rubber suction cups or conveyor belts. Contamination can occur when the wafers are positioned and when the array is interconnected. Zielnik notes that shunts and delaminations are a potential consequence of contamination. In a large array of 2.3 million PV batteries, even a single shunt, a single short-circuit, can result in significant power loss.
John Wohlgemuth, principal scientist in PV Reliability at the National Renewable Energy Laboratory in Golden, Colo., explains that the main issue is how much energy you get out of the systems. “You don’t have to be as fussy in certain areas, but we may have to be very fussy in others. You have to worry about contamination at key steps. For example, if you don’t prepare the surface before applying the anti-reflective coating, it will not adhere, and the system won’t be as efficient. If the coating itself is contaminated, metals may be driven into the assembly, resulting in a lower long-term efficiency. Processes run at higher temperature require careful contamination control.”
PV manufacture is customer-driven, a factor that can impact contamination control and acceptance criteria.1 “We manufacture for OEMs, so we build for an industry where the customer calls out the specifications,” explains Dennis Willie, engineering manager within Flextronics’ Advanced Engineering Group in Milpitas, Calif. Some customers assert that minor contamination (small conductive or non-conductive) particles outside critical functional areas are not critical to long term quality or reliability. Others have a semiconductor background, where high emphasis is placed on cleanliness that perhaps drives acceptance yield, scrap, and operational efficiency challenges.
The future
In a competitive, demanding environment, PV may take a cue from lessons learned in the electronics industry. Jon Custer-Topai, vice president, Custer Consulting Group, The Sea Ranch, Calif., explains that, “On the market side, the situation is similar to where the electronics industry was 10 to 15 years ago. PV has moved from R&D to being a commodity item. The PV business is going to China. We need someone to emulate Apple, to build a following; no one has separated themselves from the pack.”
Starting in the mid- to late-1990s, electronics assemblers were part of a golden age, where low residue (no-clean) fluxes obviated the need for cleaning or defluxing. Contamination control was not a major issue. By 2008, technology, economics, and customer requirements changed the picture. Miniaturization, high components population, environmental constraints, and cost pressures were impacting the industry. Commodity product is now assembled in geographical locations where labor is less expensive and where environmental constraints might be less extreme. Successful, high technology assemblers found a niche by addressing the higher cleanliness and cost requirements of demanding customers. Guidance documents for cleaning were revised.2 By analogy, PV manufacturers who want to separate themselves from the pack might do well to develop a simpler, more reliable product with more favorable cleanliness and surface quality properties.
A companion article with additional discussion of contamination control issues and standards for reliability in PV manufacturing can be found in Reference 1.
References
1. Kanegsberg B. Future Solar. Clean Source Newsletter, November/December, 2012. http://bfksolutions.com/index.php/newsletter.
2. IPC CH-65B. Guidelines for Cleaning of Printed Boards and Assemblies. 2011; ipc.org.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, Second Ed., CRC Press. Contact: [email protected]
This article appeared in the November/December 2012 issue of Controlled Environments.
