Design changes can mitigate skyrocketing metals prices.

The Holy Grail of photovoltaic production (and, therefore, adoption) is reducing cost so that the price tag for solar-generated electricity is comparable to that of traditional grid power. The industry has been on good pace to deliver on this requirement, and still is, except for a small speed bump. The bump’s name is silver, and its meteoric price escalation has thrown a little rod into the PV price model wheel. After silicon, printed metallization inks are the second-most expensive component in silicon solar cell manufacture. And, over the past 12 months, the price of silver has more than doubled (see "Market Watch," pg. 16). (As of this writing, the price was over $35 per troy ounce.) In fact, the cost of the silicon cell front-side silver paste is now in the region of $0.20 per wafer, whereas a year ago it was less than $0.10. But take heart. By employing some novel technologies and in the future perhaps some different materials, silver consumption can be significantly reduced.

The accuracy and repeatability of modern screen technology and metallization print platforms have already helped reduce silver consumption significantly by condensing the front-side conductor width to 70 µm and lower. Indeed, three years ago when the average width was in the range of 120 to 140 µm, many thought this unachievable. And, while sophisticated printing technology has vastly improved on required paste volume and cell efficiency, even more can be done to lower requisite silver use.

The first approach for silver volume reduction is achieved by disassociating the main use of front-side silver – the busbars – from the printing of the collector fingers by doing the printing in two stages. This technique, known as Print-on-Print (PoP), enables manufacturers to control the amount of silver used in the busbars independently from the volume used in the rest of the grid. The PoP process utilizes a two-stage print approach that yields higher, narrower collector fingers for reduced shadowing and greater conversion efficiency. By design, this technique then also permits independent control of the busbar height, as the busbars are generally printed with the second material print, where a thinner layer of material can be applied, thereby reducing the silver volume used for the busbars. By my calculations, for a standard two-busbar design at 2.0mm wide each on a 156mm wafer, simply reducing the busbar height from 30 µm to 20 µm will yield a reduction in silver paste consumption of approximately 28mg. This represents a significant cost savings of more than 10%.

Another strategy for silver reduction is a process originally developed by the Energy Research Centre of the Netherlands (ECN) and commonly referred to as dual printing. This method, which has seen a growing level of interest in the solar industry, uses a standard mesh screen to print the busbars in the first operation and a single layer electroformed nickel stencil to print high-aspect-ratio collector fingers in the second print pass. Like PoP, this enables the cell manufacturer to control the volume of the silver in the busbars, reducing it to the bare minimum required to achieve ample conductivity and provide a connection point for the tabbing ribbons. Another advantage to the dual print technique is that the fingers only need to be printed once to achieve a high aspect ratio, which eliminates the requirement for precise matching of the stencil and screen. There is a disadvantage to dual print at the moment, which is that there are currently very few pastes on the market optimized for stencil printing, which requires a minimum of post-print paste slump to achieve the best possible aspect ratios. The paste should shear thin when the squeegee moves across the stencil, pour into the apertures and then recover instantly to rebuild its structure. Development of materials that can accomplish this is certainly possible. It just hasn’t been perfected yet and, therefore, makes current adoption of dual print a bit challenging.

Last, many solar scientists are evaluating a complete change of material – either to nickel or to copper – eliminating silver entirely. The challenge with copper is that it leaches into the silicon, so a passivation layer or a diffusion barrier is required, and nickel has to be plated prior to copper plating. And, because copper is subject to various corrosive elements, it then has to be passivated and made solderable, which generally involves a top layer plating of tin. So, now, instead of one screen printing process for silver, there are three electroplating processes with various chemistries and cleaning steps inbetween. At least for the near term, the additional process steps of this approach may negate any savings potentially achieved by moving to copper. According to the International Technology Roadmap for Photovoltaics (ITRPV), however, there are implications that copper plating may start hitting the mainstream solar market by 2015.

Though many predict that silver consumption will be dramatically reduced (perhaps halved) come 2014, implementing some of the aforementioned conservation measures now will go a long way toward total solar power cost reductions today.

Tom Falcon is a senior process development specialist at DEK Solar (dek.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column runs bimonthly.

So says Flextronics’ top engineer, Dr. Dongkai Shangguan, who argues that regardless of cost, electronics must be at the forefront.

In his engineering career, Dr. Dongkai Shangguan has migrated from the automotive industry to the upper tier of the world’s second-largest EMS/ODM. One might say that during that time, he’s also moved from the floors of a leading source of air pollution to a company now positioning itself to help customers alter that course of history. As a director of multiple trade associations involved in alternative energy, including SEMA, IPC and the Sustainable Electronics Manufacturing Working Group Board, Dr. Shangguan is also in position to influence not just Flextronics’ approach, but that of the entire industry. In an exclusive interview, Dr. Shangguan elaborated on the industry’s place in this new green world, and how Flextronics is working to shape it.

CA: Flextronics has customers all around the world, in markets with an innumerable amount of regulations and rules to understand. How do you stay on top of it all?
DS: Flextronics has a very logistical problem: We have environmental and sustainability initiatives throughout the organization. We have a global Design for the Environment (DfE) program for all of our design centers worldwide. Within all these business models, DfE is an interconnected piece. We have a global DfE program with processes, methodologies, checklists, restricted materials lists and several tools to enable our global design team to meet and exceed the environmental requirements of regulations and our customers. Our customer base is very diverse, and each has its own requirements, some that are beyond [exceed external] regulations. That’s our challenge, and we try to enable our global design teams to meet those requirements and go beyond in terms of designing products for energy efficiency and design for recycling.

When you go beyond that to address needs for recycling and energy efficiency, then you go to the manufacturing phase, and we are a leader in lead-free technology. We were first, I think, to get into volume production of lead-free. We published and promoted lead-free and environmentally friendly technologies throughout the industry. After lead-free implementation, we got involved in halogen-free and other initiatives in electronics manufacturing.

CA: Can you give some examples?
DS: Of course. We have implemented plastic molding technology that eliminates the need to paint certain plastic encoders. The painting process has fumes, and it takes energy. Our SSKP process gives us such a good molding surface, we do not have to paint. That’s an example from the plastics area.

Then you consider clean technology. We have a comprehensive involvement in the clean tech business. Our solar PV module assembly, inverters, and so on – we are very involved in the clean tech business. Electronics, plastics, clean tech and others.

Then end-of-life: We are involved in logistics, rework, recovery. In logistics, we have a tool that can simulate to help us optimize logistics. Our tool enables us to optimize logistics and minimize environmental impact. This helps us and our customers optimize logistics. We are deeply involved in repair and recovery to minimize impact on waste stream.

CA: Often recovery is driven by cost – metals costs go up, recovery becomes more attractive. Have we broken that cycle?
DS: In our facilities, recovery has always been part of our practice, regardless of fluctuations in the metals market. Of course when the prices go up, there is more interest from the people who work with us, but from our point of view, material recovery has been and will always be part of our standard factory processes.

CA: Is Flextronics converting any of its factories to greener use?
DS: From treatment, to facilities, to processes. We have a team in the company, and we look at every aspect of operations that has an impact on energy. If you look at injection molding, heat is very important. There’s a lot of heat involved. So how can we optimize it to minimize energy consumption in the process? We look at this in a comprehensive way.

CA: With respect to green initiatives, should industry proactively lobby governments to ensure regulations don’t exceed manufacturers’ ability to compete?
DS: Industry should be more proactive in leading toward sustainability.

CA: Tell me about the Sustainable Electronics Manufacturing Working Group.
DS: We want to come up with an action format for industry sustainability. Initially we are working on a definition for sustainability. There are so many interested parties: OEMs, customers, NGOs, governments. We first are trying to define sustainability from the industry’s perspective, define the scope, where the industry can impact and influence, and from there, come up with an action roadmap, best practices and guidelines that the entire industry can leverage.

I gave a keynote for the May summit in Nuremberg, Germany, to help this group define the scope and hopefully come up with an action roadmap. Electronics has a holistic program, but it’s a global problem. Electronics is here to do our share, to provide our leadership and knowledge, to contribute to sustainability.

CA: Can you give us a glimpse of what the global sustainability roadmap will cover?
DS: Right now, it’s too early.

CA: Within the industry, is the emphasis on recycling and reuse, or designing with lower-impact resources? Or both?
DS: Many customers have DfE expectations, and these are one of the parameters we have as part of our design specifications. I would say maybe recyclability is a very important element of our plan. It’s becoming very common now in the product development process.

CA: The Conflict Metals situation in the DRC has shown that efforts to stop the exploitation of certain local workers might inadvertently end up hurting those same workers, who rely on the mines for their only income. What do you think of the so-called law of unintended consequences, that perhaps in trying to solve one environmental issue, we could inadvertently create another?
DS: [Pauses.] That’s a very complex question. I would agree in general that we should have a very holistic review or status before we launch an industry-wide or worldwide initiative that could have a lasting impact. We will not be able to solve all the conflicts in the world. [Pauses.] I would say that Flextronics is very aware and sensitive to the conflict metals issue and has processes in place to be sure we source our materials in a responsible and ethical way. We are a member of the EICC and make sure we do things, including material sourcing, in a very ethical way.

But overall, these are very, very complex issues. We should probably have some industry forums and come up with some industry-
wide approach.

CA: The electronics industry has a mixed history of environmental leadership. We were clearly at the forefront with CFC elimination in line with the Montreal Protocol. However, we’ve been lagging in keeping up with certain water and toxic chemical best practices, particularly on the bare board side. Do you think electronics companies should be a driver toward environmental best practices?
DS: First, we should be more proactive. We should be leading the trend of work and giving information. A lot of people don’t like legislation. That’s a further incentive for us to be proactive to legislation. We are going to energy-efficient product design, more energy-efficient processes. We have an energy-efficiency program for our factories. We must make sure we minimize our carbon footprint and work toward energy savings in every one of our factories.

CA: Are customers willing to pay for best practices?
DS: Customers can mean our OEM/ODM customers and the consumer. And they can have different perspectives. Some are consumer products; some are industrial types of products, and the answers can vary. Any responsible company has to be very serious about sustainability.

This is a megatrend. It’s coming. It’s not something you can avoid, put your head in the sand and hope it goes away. This is also an opportunity. So, regardless of any cost impact on a specific product, we are taking the position that electronics must be at the forefront of sustainability. Second, we need to leverage technology and innovation to enable design and sustainability. To leverage technology and innovation, I believe we can achieve sustainability and minimize the environmental impact. Take no-clean. After we made the investment and the transition, the no-clean process should be lower-cost than the original process. So fundamentally this should be lower cost. I don’t think this applies to every technology, but in general, we should leverage innovation to come up with better technology while minimizing cost.

CA: Do you monitor end-technologies to help Flextronics pick which markets to go after?
DS: No. I support certain technology groups from a corporate level. Our objective is to develop and employ technologies to certain business groups. Also, I chair the Engineering Leadership Council. This council works with all our business groups to help integrate their capabilities across the company. Sustainability is part of this function. We enable sustainability through technology innovation. We focus on technologies that facilitate system ability.

Overall I have a responsibility for technology and deployment. We have a roadmapping process. We work with our key customers. We leverage industry and university technologies and our own technology, and we capture it on our technology roadmap. We have corporate milestones in anticipation of the future needs of our factories. Whether it’s mechanical or assembly, or plastic injection molding, this technology deployment is worldwide across our business groups. Of course, when it comes time  for unique requirements for a particular factory, we work with that group and address that group.

That also includes process optimization, equipment evaluation and selection, reliability, even test technology. It’s a comprehensive development program.

Green by Association

Several trade groups and nongovernmental organizations are vying for influence in the AE market. A short list:

iNEMI (inemi.org): Consortium of electronics OEMs and suppliers. AE efforts currently concentrate on wind system reliability.
IPC (ipc.org): Electronics supply chain trade group. Is working on assembly workmanship standards for solar PV modules.
PV Group (pvgroup.org): A group of SEMI members involved in solar manufacturing supply chain. Mission is to advance industry growth, support efficiency improvements and promote sustainable business practices through standards development, events, public policy, EHS, and market data.
Solar Engineering & Manufacturing Association (SEMA) (solar-ema.org): Association focused on education, training, advocacy and collaboration within solar manufacturing industries.
Sustainable Electronics Manufacturing (SEM) (sustainability2015.com): Industry-wide group of leading electronics technologists whose mission is to promote a sustainable global electronics environment.

Powered by Plexus

Most of the Tier 1 and 2 EMS companies have to some degree targeted the alternative energy sector. Plexus, for example, is concentrating on alternative power conversion and power generation; those areas are among the contributors to a $32 million year-over-year jump in the EMS firm’s quarterly revenues in its Industrial/Commercial business segment. CIRCUITS ASSEMBLY spoke with sector vice president Todd Logue in August.

CA: Which alternative energy markets seem most appealing to Plexus’ customers at this stage?
TL: Plexus has been focusing and investing in the alternative energy market for nearly five years. We have current customers in both solar and wind, and are actively targeting growth in this marketplace. We have steered away from photovoltaic assembly and have concentrated on complex printed circuit manufacturing and higher level assembly. We have targeted effort on the products after the photovoltaic arrays that convert, manage and measure the power.

CA: What is Plexus doing in those markets?
TL: Plexus is currently building inverters and power conversion devices for multiple customers. We supply products for both the commercial and residential markets. We have developed proven expertise in multiple facets of design-for-manufacturability, hardware, software, mechanicals and power electronics. On the manufacturing side, we build alternative energy products from multiple manufacturing sites and continue to expand system integration and supply chain models.

CA: Are EMS companies expected to perform R&D in alternative energy today, or is that onus still on the OEM?
TL: From our experience, we have seen a trend in the outsourcing of alternative energy R&D efforts. We have alternative energy companies today that engage not just with our product services group for new product introduction, but also our product development teams. We see this especially where custom solutions and IP ownership are most valued. By developing custom solutions, we are able to provide the highest level efficiency and maximize power budgets when those are the critical criteria that define success.

Saying this, the trend has been gradual, and the majority of development work is still completed at the alternative energy OEM. Projecting forward, we anticipate the trend to continue, as the competitive drivers in the market accelerate, highlighting time-to-market requirements. However, standardization and integration will lead to a higher value on the efficiency needs and overall system performance. This is where we expect our design and product introduction experience will add the most value. The other immediate market we are working in includes a wide variety of custom design development, including GUI for grid systems.

CA: Who should drive environmental best practices?
TL: The rapid growth of electronics in today’s society highlights the need for electronics companies to drive toward environmental best practices. Plexus’ goal is to have environmentally sustainable operations. Plexus is focused on reduction of waste and improvement in efficiencies to increase output through lean principles. Consumers drive OEM supply chain decisions with their buying decisions, and consumer environmental awareness continues to grow. We anticipate that the combination of raised governmental standards and consumer awareness will be a major factor in buying decisions in the future.

Mike Buetow is editor in chief of Circuits Assembly (circuitsassembly.com); This email address is being protected from spambots. You need JavaScript enabled to view it..

Weighing in at some 1800 pages, the 2011 iNEMI Roadmap is the consortium’s most comprehensive to date. Editor in chief Mike Buetow spoke with iNEMI CEO Bill Bader last week about the roadmap’s latest points of emphasis, and about how an upsurge in membership is setting the pace for the future.

A design for quantitatively determining annular ring and breakout angle on PCB innerlayers.

If plated through-holes in multilayer printed circuit boards are not suitably registered, board reliability is threatened. Specifically, the annular ring on boards may be insufficient to ensure a good layer-to-layer bond and concomitant PTH integrity.

Useful data on PTH registration may be obtained using the test systems described by Paur in his two patents.^1,2 These systems provide more than a simple go/no-go measurement, but do not provide continuously variable data. The misregistration data are “binned,” as the system indicates misregistration within some interval, rather than being continuous. The systems also consume some space on a board panel and entail a slight increase in drilling time and tool use.

Acceptance standards IPC-6012 and IPC-6013 define three classes of PCB reliability, in order of increasing reliability requirements, denoted as Class 1, 2 and 3. Classes 1 and 2 permit some missing annular ring on internal layers, expressed as “breakout angle.” (Figure 2 demonstrates the concept of breakout angle. As of this writing, Class 1 boards may have internal annular ring with 180˚ breakout, and Class 2 boards may have internal annular ring with 90˚.)

The IPC the acceptance standards call for vertical microsections of coupons on PCBs, which do not allow measurement of breakout in internal annular ring. As Clifford3 demonstrated, even when vertical microsections meet Class 3 acceptability requirements, the PTHs themselves in reality may not meet breakout requirements. Horizontal microsections could reveal the actual breakout, but preparing such microsections is time-consuming in comparison to vertical ones. Another serious disadvantage of horizontal microsections is loss of a retainable record, as each section is destroyed as one proceeds into layers deeper and deeper in a PCB. Fabricators and users typically want microsection records and samples retained for some time after boards are made and shipped.

This unappealing situation led us to search for an exact way to measure internal annular ring breakout.

Geometry Review and Basis

Before going into the details of the new design, it may be worthwhile to review a tool from trigonometry, the Law of Cosines. We will use the law later in this article.

Many engineers and scientists are comfortable with use of the Law of Sines, which permits calculation of the length of all sides and magnitude of all angles in a triangle if the length of one side and magnitude of two angles are known. Per Figure 1, if the length of side A and angles b and c are known, the Law of Sines allows the length of sides B and C to be calculated:


Eq. 1

Less frequently seen is the Law of Cosines, which permits calculation of the length of all sides and magnitude of all angles in a triangle, if the length of two sides and magnitude of the angle between them are known.

Referring again to Figure 1, if the length of sides A and B and the angle c between them are known, the Law of Cosines allows the length of side C to be calculated:

Eq. 2

Refer now to Figure 2, representative of a hole drilled in a circular copper pad of a board. Using Figure 2, we show below that, knowing the displacement d of the center of the drilled hole of diameter r from the center of the circular pad of radius R, one can calculate the breakout angle 2τ by using the Law of Cosines.
The displacement d of the center of the drilled hole (red dot) and the center of the copper pad (blue dot) is, in terms of x and y,

Eq. 3



In the following, the notation (a,b) indicates the angle between a and b.
Using the Law of Cosines,


Eq. 4

Hence,

Eq. 5
and, with d calculated from ,



Eq. 6

From Figure 2, notice that


Eq. 7

So


Eq. 8

How does one determine x and y when the pad is on an internal layer? The next section has the answer. It also includes some comments on measurement of R and r. The latter two parameters are known, at least approximately from the design, but users of this new system can measure true values with relative ease.

Registration System and Measurements

Consider Figure 3, which shows the circular pad and drilled hole from Figure 2 and adds a chevron and a square pad to the right and left, respectively. The chevron and square pad are the two elements of the novel registration measurement structure. The chevron and square pad are formed at the same time, from the same material, and by the same process as the circular pad. Typically, the chevron and pads would be copper foil remaining after printing and developing of copper-clad laminate.

Notice that the length of the sides of the square pad is designed to be the same as the diameter of the circular pad, R. However, any layer-to-layer variations in printing, developing and etching the internal circular pads will also affect the square pad, so the true circular pad diameter can be determined by measuring the length of a side of the square pad.

The length of the chevron is equal to the diagonal of the square, Eq. 9.

E

Eq. 9

The diameter of the drilled hole, r, can be measured directly from the microsection, as the technique and equipment are designed to take the section through the center of the drilled hole.

Figure 4 contains the same elements as Figure 3, and adds the line S representing the plane of a vertical cross-section through the center of a hole drilled in a circuit board.

The user finds the value of x to be used in Eq. 3 by measuring the distance x’ between the observed center of the drilled hole and the observed center of the square, then subtracting from x’ the distance (already known as D from the design of the pads and their locations) between the center of the circular pad and the center of the square pad. Notice that the user makes one measurement, x’, to find the value of x from Eq. 10.


Eq. 10

The value of y to be used in Eq. 3 is found by measuring the distance y’ between the center of that portion of the chevron visible in the cross-section and the observed center of the square, then subtracting 2D from the measured distance y’. Notice, once again, that just one measurement, y’, is made to find the value of y from Eq. 11.


Eq. 11

The results from Eq. 10 and 11 allow calculation of breakout angle by utilizing the Law of Cosines. Note that, by definition of internal annular ring per IPC-T-50, the radius of the drilled hole, r, must be used, not the radius of the plated hole.

Advantages and Applications

This new method has some attractive features:

• Minimal space consumption on panel may often be incorporated into an A/B coupon.
• Precision of measurement of breakout angle and misregistration is limited by uncertainty in measurements of linear dimensions in coupons.
• Sample preparation takes place concurrent with standard microsection of multilayer panels.
• Allows preservation, as part of quality records, of breakout and misregistration information of all layers.
• If space consumption on a panel is extremely critical, the chevron could be incorporated as a “negative”; i.e, by removal of copper in the square pad.

A number of applications of the new method come to mind:

• Measurement of breakout angle for conformance to IPC Class 1 or 2 requirements, from vertical microsection.
• Variables data can be used in statistical process control of PCB lamination, for displacements of innerlayers.
• Variables data can be used in designed experiments for process optimization.
• Design rules can be formulated that incorporate statistics of drilling and lamination misregistrations to determine pad size on internal layers required to ensure a desired yield in fabrication.

References

1. Tom R. Paur, US Patent 4,894,606, System for Measuring Misregistration of Printed Circuit Board Layers, January 1990.
2. Tom R. Paur, US Patent 4,918,380, System for Measuring Misregistration, April 1990.
3. Tom Clifford, “Round the Clock,” Printed Circuit Design & Manufacture, June 2004.

Ed.: This article was first published at IPC Apex Expo in April 2011 and is republished here with the authors’ permission.

Russell Dudek is advanced technology manager and Louis Hart is quality assurance manager at Compunetics (compunetics.com); This email address is being protected from spambots. You need JavaScript enabled to view it..