A study of multiple reball processes looks at copper dissolution and functionality.

There are many differences in the alignment and placement of balls for reattachment. Yet to ensure a good metallurgical and mechanical attachment, the common element for all reballing methods is the need to reflow solder balls while aligned on the BGA lands.

Most questions concerning BGA reballing process reliability focus on two specific areas: First, the physical or mechanical strength and reliability of the ball attach to the device. (Of course, it is desired that the physical characteristics of the replaced solder ball be the same or very similar to the original solder ball.) Second, the effect on additional heat cycles (necessary for the reballing process) on the device itself. It is generally understood that exposure to reflow temperatures can degrade many materials used in various steps of electronics manufacturing. While BGAs come in many different configurations, many are packaged in some type of plastic. These plastics can be affected by increased exposure to thermal heat cycles or excessive temperatures. Many vendors specifically warn against reballing, even informing users that using a reball process will void the device warranty. One other area of concern is the effect reballing – solder ball removal and device land preparation, in particular – will have on the device lands. Thanks to Pb-free soldering, users are now aware of the aggressive nature of high-tin-content solders on other metals. If dissolution of the copper device lands is occurring, what effect will that have on the overall reliability of the device and ball attach?

Testing Process

The testing process includes reballing PBGAs (plastic ball grid arrays) and subjecting them to various tests. The BGAs were received from the vendor with Pb-free solder balls and then reballed using Pb-free solder balls. Cross-sections were performed to evaluate the grain structure and intermetallics formation of the devices before and after reballing, as well as the potential effect of copper dissolution of the device lands. Functional tests were performed on the devices before and after processing to determine if any failures could be attributed to the rework process, and in particular, the additional thermal cycles.

Process step 1: ball removal. Two common methods are used for removing solder balls from BGA devices. One uses a handheld soldering iron, a “wicking” tip, and wicking braid. The other method uses a flowing solder pot to remove the solder balls.

The manual method usually starts with the application of a high viscosity flux (paste flux or “sticky” flux) to the solder balls or residual solder (if removed from a PCB) of the device. The technician can then quickly move the wicking tip across the device, permitting the bulk of the solder to be wicked onto the tip for removal (Figure 1).

After the bulk solder removal has been completed, the technician can then reapply paste flux and use wicking braid with the soldering iron to remove all remaining solder residue. The advantage of this method is that because it can be accomplished relatively quickly, there is little time for the heat to transfer to the die and cause thermal degradation. The disadvantage is that it is considered a contact desoldering method, and depending on the integrity of the device materials and technician’s skill, may result in damage to the solder resist or lands on the device.

The dynamic or automated method uses a flowing solder pot to “melt” and flow away the solder balls (Figure 2). This is considered a noncontact desoldering method. Nothing, besides molten solder, touches the bottom of the device. This means there is virtually no chance of lifting a land on the device or damaging the solder mask. One disadvantage is solder remaining on the device lands will now be “crowned” as a result of the surface tension of the solder (Figures 3 and 4). This may affect the reballing process because of the additional variability of the height of solder on the lands. There are also questions about the effect the flowing solder bath may have on the device lands because of copper dissolution of the device lands.

Process step 2: reballing. BGAs reballing was performed using EZReball preforms. After all residual solder was removed from the device lands, a thin layer of water-soluble paste flux was applied. The preforms were aligned with the BGAs and placed on a piece of ceramic substrate. This is then processed through a reflow oven using an appropriate thermal profile for the solder ball alloy. After the BGA is removed from the oven and cooled, the polyimide portion of the preform is peeled from the device, leaving the solder balls soldered in place on the BGA. In most cases when using solder preforms, there are some inconsistencies in ball shape and location. This usually requires an application of flux to the BGA and an additional reflow cycle through the reflow oven. After this secondary reflow, the BGAs are then cleaned and inspected.

Process step 3: placement. The reballed BGAs are then placed and reflowed on the test platforms. These test platforms permit the BGA to undergo full functional test (Figure 5).

Process Steps – Thermal Summary

BGAs that were removed, reballed and replaced had been exposed to the following thermal excursions:

• Full reflow for initial placement.
• Full reflow for removal.
• Limited thermal exposure during solder removal.
• Full reflow for ball attach.
• Full reflow for ball irregularities.
• Full reflow for rework placement.

This reveals that, in most cases, a reballed device will have seen five complete thermal profiles. Four additional reflow cycles would be encountered for any subsequent reball attempts (Figure 6).

Copper dissolution evaluation. Ten specimens were prepared for evaluation of copper dissolution during the ball removal process. One specimen was used as a control sample. This control sample was exactly as received from the vendor. Group A consisted of three specimens prepared with a flowing SnPb solder pot. Group B was three specimens prepared using a Pb-free flowing solder pot. For both Group A and Group B, one BGA was subjected to the solder bath once; one BGA was subjected to the solder bath twice, and one BGA was subjected to the solder bath three times. It was thought that any differences would be more pronounced after multiple cycles in the molten solder. Group C was three specimens prepared using a soldering iron with a blade tip and wicking braid.
For the ball removal evaluation, cross-section analysis was performed on the test specimens. The area of focus was the copper lands of the BGA. The control sample revealed a copper thickness of approximately 13 µm and a nickel thickness of 7 µm. Groups A, B and C revealed no significant changes in the metal thickness measurements, regardless of the number of solder bath exposures and wicking operations. It was hypothesized the nickel layer acted as an effective barrier against the aggressive nature of the Pb-free solder (Figures 7 and 8).

Ball removal thermal effects. In addition to the cross-section review of copper dissolution, a thermocouple was embedded in the die area of the BGA to observe the internal temperatures during wicking or exposure to the solder baths. As expected, a manual operation using wicking braid and a soldering iron provided reduced thermal exposure to the device when compared to a flowing solder bath. Using an iron and tip, the tip is quickly moved across the surface of the device limiting the time and contact area. Of course, a solder bath has a much greater capacity for heating and in higher temperatures for longer durations (Figures 9 and 10).

 

Device functionality. After the process steps were completed, the test specimens were sent to the vendor for full functional testing. Five control samples were not reballed; 10 samples were reballed once (five heat cycles), and 10 samples were reballed twice (nine heat cycles). Electrical testing included base loopback, top loopback, memory, flash, script and SRAM. The 10 single reball samples had one failure, and the double reball samples had one failure. The failed samples were removed, reballed and replaced, and then passed all tests. One of these samples was subjected to 17 thermal cycles and still passed.

Conclusions

During the solder ball removal process, the manual method using wicking braid resulted in the BGA being subjected to fewer thermal stresses than would be encountered with a flowing solder bath method.
It was expected there would be significant copper dissolution when using a Pb-free flowing solder bath to remove the solder balls. However, the robust nickel layer on these particular devices appeared a very effective barrier, preventing solder from leaching the copper from the device lands.

All devices passed all electrical tests after numerous thermal cycles.

While the results of this testing shine a positive light on the reballing process, it must be remembered this was a test of one particular device from one particular vendor. Should these results drive requests of BGA suppliers for waivers to the three-reflow cycle limitation on its warranties? Doubtful. It should, however, provide the confidence to proceed with reballing plans expecting the physical attributes of the device to be unaffected.

Acknowledgments

Thanks to Terry Munson and Paco Solis of Foresite Inc. for analytical services, and Ryan Malcolm and Daniel Beeker of Freescale Semiconductor for parts and testing.

Ray Cirimele is operations manager at BEST, Inc. (solder.net); rcirimele@solder.net.

The city-state’s focus on business growth and addressing niche market needs creates a robust supply chain.

In March, I toured seven facilities in Singapore and visited with several Singapore government agencies. Perhaps the most interesting impression I came away with was the optimism encountered in virtually every management team. To be sure, every company has been impacted by the global recession, but the perception was that this was a cyclical bump in the road to be dealt with. To those familiar with the region, this attitude is not a surprise. Over the past three decades, Singapore and its supply base have evolved in response to continually changing market conditions and industry focus. The result is a resilient management attitude that sees change as a door to opportunity and focuses on building strong businesses that cater to under-addressed market needs. At a time when many US companies are re-evaluating their market and supply-chain strategies, I thought it interesting to explore that attitude and the business models it generates in more detail.

Robust government support. One of the reasons the supply base is optimistic is because they have a robust government support infrastructure behind them. Government agencies listen to Singapore-based suppliers and multinational companies investing or sourcing in the region and develop programs tailored to their needs. These public/private partnerships pave the way for larger R&D investments, acquisition of new capabilities, internal process improvements and expanded marketing reach. But an interesting dynamic is that participation comes with a cost. Companies participating in agency-sponsored programs pay a portion of the cost of shared infrastructure and must list expected results from the activity as part of the proposal submission. At the end of the project, results are measured. While the model is applied on a relatively small geographic scale, Singapore remains a strong player economically at both the regional and global level. According to International Enterprise (IE) Singapore, the country’s GDP was $178.8 billion (S$257.4 billion) in 2008, up 1.1% in terms of real GDP growth, despite the recession. Total trade increased 9.6% in 2008 to $644.2 billion (S$927.7 billion).

Key agencies and programs include:

IE Singapore (iesingapore.com), an agency under the Ministry of Trade and Industry. It assists Singapore-based suppliers wishing to expand to new markets and foreign MNCs wishing to find new suppliers. IE Singapore’s supply base search and matchmaking services make sourcing hard-to-find commodities much less of a challenge. MNCs can submit specifications for supplier capabilities to an IE Singapore representative, which will develop a list of compatible suppliers. With offices in over 30 cities worldwide, including London, Frankfurt, New York and Los Angeles, IE Singapore officers will also set up meetings with the short list of suppliers chosen by the MNC’s sourcing team, so that the team can visit and/or audit the selected companies during a single visit. In some cases, that visit may include side trips to satellite facilities in other lower-cost-labor markets. They also organize trade missions to various countries, which can provide a localized “first look” to sourcing teams wishing to explore options prior to traveling to a supplier facility. Programs such as this help lower costs of supplier identification and qualification on both the supplier and customer side of the equation.

The Singapore Economic Development Board (sedb.com), the lead government agency for planning and executing strategies to enhance Singapore’s position as a global business center and grow the Singapore economy. The EDB dreams, designs and delivers solutions that create value for investors and companies in Singapore.

The Agency for Science, Technology and Research (a-star.edu.sg), Singapore’s lead agency for fostering world-class scientific research and talent for a knowledge-based Singapore. A*STAR actively nurtures public sector research and development in biomedical sciences, physical sciences and engineering, with a particular focus on fields essential to Singapore’s manufacturing industry and new growth industries. It oversees 22 research institutes, consortia and centers, and supports extramural research with the universities, hospital research centers and other local and international partners. At the heart of this knowledge-intensive work is human capital. Top local and international scientific talent drive knowledge creation at A*STAR research institutes. The Agency also sends scholars for undergraduate, graduate and post-doctoral training to the best universities, a reflection of the high priority A*STAR places on nurturing the next generation of scientific talent.

One of its research institutes is the Singapore Institute of Manufacturing Technology (simtech.a-star.edu.sg). SIMTech develops high-value manufacturing technology and human capital for use by Singapore industry. It collaborates with MNCs and local companies in precision engineering, electronics, semiconductor, medical technology, aerospace, automotive, marine, logistics and other sectors. Its technology competencies include research groups, research programs and innovation and commercialization projects. Current research groups are focused on manufacturing processes, manufacturing automation and manufacturing systems. Innovation and commercialization foci include product innovation and development; equipment innovation and development; and sustainability and technology assessment. This platform permits participating companies to access collaborative research, participate in consortia or access expertise for a specific internal project.

SPRING Singapore (spring.gov.sg), the enterprise development agency for growing innovative companies and fostering a competitive small- and mid-sized enterprise (SME) sector. The agency works with partners to help enterprises in financing, capabilities and management development; technology and innovation, and access to new markets. As the national standards and accreditation body, SPRING also develops and promotes internationally recognized standards and quality assurance.

Programs Translated to Results

The Growing Enterprises with Technology Upgrade (GET-Up) program is one example of cross-agency cooperation with local industry. GET-Up is a joint initiative by A*STAR, EDB, IE Singapore and SPRING Singapore to partner with SMEs to boost their efforts to grow their businesses and create future industries. Assistance can include loaned research personnel, strategic planning/roadmapping assistance, loaned consultants and access to A*STAR laboratories and facilities for specific R&D efforts.

A recent survey conducted by the NUS Entrepreneurship Centre comparing over 100 companies that had participated in GET-Up projects, and 100 companies of similar size that had not, found:

GET-Up participants projected revenue and employment growth of 15% and 18%, respectively, compared to 6% and 7% estimated by companies not participating in GET-Up.

Companies in GET-Up also reported a higher proportion of new and improved products in annual sales, and noted that 16 to 20% of sales were derived from new and improved products, compared with 11 to 15% for non-participating companies.

On average, GET-Up companies spent 3 to 4.9% of their sales on R&D, compared with 1 to 2.9% for non-participating companies.

The real strength of the Singapore model is breadth and scope of the supply, which focuses on higher-mix, lower-volume, complex projects for higher reliability product niches, such as medical, laboratory equipment, instrumentation and aerospace. There is a strong electronics manufacturing services sector, but there is an equally strong supply chain for precision metal forming and machining, custom plastics, rubber and polymer formulation, optics, and mechanical and electromechanical assembly. Many of these companies have multinational operations in lower cost countries, such as China, Indonesia, Malaysia, India and Vietnam. This offers customers a choice of build site locations, while still working through the Singapore headquarters in a country with strong IP protection and a legal system based on British Common Law. Continuous improvement initiatives, industry-specific quality certifications, engineering expertise and robust program management are also standard practice, even at the component supplier level.

The companies toured were:

AMS Biomedical (Pte.) Ltd. (ams-biomedical.com), a Singapore-based contract manufacturer of complex electro-mechanical equipment and devices for the medical industry. Its capabilities include engineering design, as well as electronic and electromechanical assembly. The company is registered to ISO 9001:2000 and ISO 13485:2003. Its manufacturing operations are located in Singapore.

In terms of developing a unique niche, AMS Biomedical and its parent company, Manufacturing Integration Technology Ltd. (mit.com.sg), focus on low-volume, technically complex electronics and electromechanical equipment in the medical and semiconductor industries, respectively. DfM/DfA are key disciplines in the product development process. Precision machining, welding, casting and secondary processes, such as surface treatments, are vertically integrated capabilities. There is also a robust supply chain management process with strong traceability, documentation control and IP protection elements.

Beyonics Technology (beyonics.com), a top 15 EMS company headquartered in Singapore, with manufacturing facilities in Singapore, China, Indonesia, Malaysia and Thailand. It is publicly traded on the Singapore Stock Exchange and had revenues of S$1.41 billion in fiscal 2008. Its industry focus includes automotive, consumer electronics, data storage, electronics/electrical, electronics equipment, medical and healthcare, communication and networking, and security surveillance applications. Certifications include Six Sigma, ISO 13485, ISO 14001, ISO 9001, OHSAS 18001 and ISO/TS 16949. Manufacturing capabilities include PCBA and system-level electronic assembly/test; medical disposables assembly and packaging; metal stamping; aluminum die-casting and precision machining; plastics injection molding, and surface finishing. It has Class 100, Class 1,000 and Class 100,000 clean rooms. In recognition of its strategic use of Singapore’s advantages to achieve manufacturing excellence, Beyonics received the Singapore Advantage Award, MAXA 2007.

Beyonics’ business model also has some unique aspects. Its manufacturing footprint is entirely in Asia, and part of its value proposition is its size and range of services combined with a single region approach, providing economies-of-scale at much lower overhead than competitors fielding a mix of factories in high- and low-cost regions. This can be particularly attractive to companies that wish to manufacture in a single location. Another element is its level of vertical integration. In the medical industry, the company manufactures both electronics and disposable products. At a systems level, it can manufacture custom metal and plastics components, as well as electronics.

CEI Contract Manufacturing Ltd. (cei.com.sg), headquartered in Singapore and with manufacturing facilities there, China, Indonesia and Vietnam. It is publicly traded on the SSE, had revenues of S$89.5 million in FY 2008 and is profitable. It concentrates on the analytical equipment, medical devices, oil and gas, avionics, electroluminescent displays, photonics and metrology instruments, and semiconductor equipment industries. Certifications include ISO 13485:2008, ISO 14000, ISO 9001:2000, ISO/TS 16949, AS 9100 (LOC), ISO 14001 and UL508. It has Class 10,000 clean rooms. Capabilities include board- and system-level assembly/test, metal stamping and machining.

CEI is an excellent example in terms of use of government agency programs to enhance capabilities. The company’s low-volume, high-mix focus and a market orientation toward projects with customer-specific and traceability requirements drove a need for strong shop floor information technology support. Five years ago, it tapped A*STAR resources in searching, selecting and developing a shop floor optimization software engine for a proprietary manufacturing execution system. Today, this MES is in plant-wide operation in incoming inspection, materials and kitting verification, and production scheduling and shipment traceability. It also provides quality alerts and documentation control, and runs in parallel with an Oracle MRP system. CEI is also a GET-Up participant and used that program to enhance its technology capabilities through the addition of one of A*STAR’s photonics engineers. A cross multi-disciplinary engineering team was formed to work on laser, photonics, optics assembly and testing-related processes, which has enhanced internal capabilities for its metrology business segment.

First Engineering Ltd. (first-engr.com.sg), which provides ultra-precision molds, plastics components and modular manufacturing assembly for high technology engineering applications. It is one of a few companies in the world able to produce ultra-precision optical plastic lenses. Headquartered in Singapore, its capabilities include product design, tooling, production and assembly services, predominately for the hard disk drive, PC peripherals, optical-related products, life science, healthcare, business machinery and automotive industries. Manufacturing facilities are located in Singapore, Malaysia, India and China. Certifications include ISO/TS 16949, ISO 13485 and ISO 14000. Six Sigma programs are also in place.

First Engineering has tapped EDB funding to enhance its plastics optics technology and expand into imaging systems and healthcare applications. Head-worn eyewear personal video viewers use plastics technology to provide the big-screen experience, built using a technology that also has applications in low-cost, wearable augmented information displays and healthcare protective eyewear and diagnostic devices. First Engineering used EDB funding in adding a high-volume, high-cavity production line using cube mold technology. The line produces double the output of conventional multi-shot technologies through increases in yield and reduction in processing cycle time. There are also improvements in energy consumption, equipment footprint size, degree of automation, and range of production techniques.

Forefront Medical Technology (forefront.sg), an integrated medical device contract manufacturer that provides collaborative product design and development, rapid SLS prototyping, tool making, injection molding and extrusion, clean room scalable assembly and packaging, testing, sterilization, and global logistics. Its manufacturing facilities are located in Singapore and China, and the firm is registered to ISO 9001:2000 and ISO 13485:2003 and FDA CFR 21 Part 820.
FMT is both an ODM and a contract manufacturer. Its core business base is airway devices, but it also manufactures drug infusion products and is engaged in the development and investigation of devices for urology and cardiac applications. A key part of its business model is full lifecycle support. Its client base isn’t simply looking for a product development or manufacturing solution. They also want a logistics solution that addresses issues such as sterilization, customs processing and end market fulfillment. The company has both strong supply-chain management and program management functions to support this effort. In growing its business, FMT has tapped EDB resources for tax abatement. The money saved in tax relief has enabled them to put more into R&D. Its R&D investment typically runs 6 to 7% of revenues.

Onn Wah Precision Engineering (onnwah.com), headquartered in Singapore and with manufacturing facilities in both Singapore and China. Its products and services include precision CNC-machined parts, production tooling, and jigs and fixtures. Industries served include aerospace, electronics/electrical, medical, oil and gas, and photonics. Its certifications include AS 9100 and ISO 9001:2000.

Onn Wah’s business model uses Lean manufacturing philosophies to both address the challenges of scheduling in a high-mix, low-volume environment, and ensure high levels of quality for products often mission-critical. The company initiated a continuous improvement program in 2001 to drive its Lean manufacturing implementation, and partially funded the use of an outside consultant through a program offered by SPRING Singapore. Today, Onn Wah’s facilities combine 5S, Total Productive Maintenance and Lean manufacturing principles with a low-cost structure environment that is typically half that of Europe or the US.

Rayco Technologies (Pte.) Ltd. (raycotechnologies.com), a precision elastomeric solutions specialist whose components are used in the medical device, lifestyle products, data storage, electronics, automotive and aerospace industries. Capabilities include dispense-in-place gasketing (DIPG), and liquid injection, compression and transfer molding. It can also overmold a variety of elastomers with materials such as plastic, metal and other substrates, as well as provide PTFE coating. Its manufacturing facilities are located in Singapore and China.

One of the byproducts of a rapidly changing environment is the need to adapt to shifting patterns in demand. Singapore’s government agencies and its supply base provide excellent examples in addressing this challenge through collaborative research, continuous improvement initiatives, robust supply-chain strategies and a focus on creating a business-friendly sourcing environment.

Susan Mucha is president of Powell-Mucha Consulting Inc. (powell-muchaconsulting.com); smucha@powell-muchaconsulting.com.

Cleaning can actually exacerbate flux shorts; x-ray first!

This month we feature a recent issue submitted to the database: Is it a BGA solder short or not?

Optical inspection of BGA joints shows a connection between two balls after rework and repair. This is probably not a solder short; it’s more likely a flux short. During inspection, light can be seen through the material between the two balls. This is probably caused by excessive flux or incorrect material used during rework.

The board was built in medium volume using a Pb-free process for a telecommunications product. Being a Pb-free process, the higher process temperature may have contributed to the discoloration/darkening of the flux.

To confirm this is a flux short and nothing else, use x-ray inspection. With this volume of flux, cleaning may make inspection more difficult. If x-ray shows the solder joints to be satisfactory and electrical test is positive, leave the BGA in place. A review of the rework process should be conducted to confirm the procedures, rework training and materials used.

These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk/), available to all Circuits Assembly readers, allows engineers to search and view countless defects and solutions, or to submit defects online.

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); http://defectsdatabase.npl.co.uk/.

SIR must be designed for, especially in high-frequency boards.

High-frequency technology in HDI assemblies, combined with increased use of Pb-free solders, has initiated closer scrutiny on the flux removal process. Because adequate climatic operating conditions cannot always be assumed, system signal integrity is vulnerable to failure through the “parasitic-type capacitance” of hygroscopic activators. In addition, such contamination, particularly with the new Pb-free solder formulations, is no longer detectable by ion-equivalent measurement alone, as contamination levels are typically below 1.5 µg/cm2   (due to the newer paste formulations). This requires modern cleaning procedures, but also demands an appropriate testing technology for effective monitoring of all assembly line processes.

HDI assembly use, particularly in motor vehicles (i.e., for operating-data acquisition systems) is rapidly increasing. Hence, more of these systems are exposed to widely differing climatic influences, including moisture and harmful gases, which threatens their functional reliability, as well as the products and devices in which they are housed. Moreover, the sensitivity of these circuits to environmental interference is compounded by use of components using high-resistance (ohm). High-frequency circuits that range between 30 MHz and 5 GHZ (a requirement in communications electronics) are highly susceptible to environmental impacts. Thus, to maintain signal integrity, the systems not only require adequate insulation resistance, they also must have stable impedance. For this reason, capacitive surface effects must be taken into account in the circuit design.

Corrosion-induced assembly malfunctions (e.g., electrochemical migration and leakage currents) increasingly are the sources of diminished component reliability and service life. In high-frequency designs, the “parasitic-type capacitance” can distort the “ramp-up” of the signal, thereby disrupting its integrity to the point of causing equipment malfunction. Because guaranteed long-term operational reliability is imperative, an increasing importance is placed on ensuring its respective quality. For high-frequency assemblies, this is primarily determined by circuit surface cleanliness.

Unfortunately, sensitive assemblies are not always stored or operated under specified climatic conditions while being transported or demonstrated. For example, IEC TM 60068-2-48 states that reliability can be impaired significantly by prolonged storage, even at relative humidity rates of less than 80%. Moreover, the diverse climatic conditions under which assemblies may be operated are not always known. Therefore, it has become more common that signal integrity of a circuit design no longer can be ensured in extreme situations. For example, malfunctions in the interconnected assemblies of vehicles may result in responses that are difficult to interpret. Speed sensors for automobile tires are monitored not only by the ABS system, but also by the engine management. A malfunction of a monitored component often causes other assemblies to generate false readings.

Contamination “favors” moisture absorption, and with it comes electrochemical migration and corrosion-induced leakage currents. Studies indicate a higher malfunctioning rate among some Pb-free alloys because of the presence of dendrites, particularly “edge-triggered” circuits. Moreover, the intrinsic conductivity and electro-diffusion effect of most contamination lowers the surface resistance. This is because of the increased surface conductivity resulting from hygroscopic-induced moisture absorption, which is intensified by “hydronium” ions dissociated from the activators; malfunctions and assembly failures are the result. In extreme cases, as the board material becomes overheated along the creepage paths, smoldering or even fires may occur, especially in antenna and power-controlling circuits. Similarly, activator residues can change the impedance of connecting surfaces and through-holes, causing statistically fluctuating virtual enlargements of the pad geometries.

With frequencies higher than 1 GHz, the circuit designer must calculate even the low (but limited) resistance of conductive lines. If residues enlarge pad areas, the electrical layout may be changed, and might lead to malfunctions by causing, for example, a time delay at the air-traffic controller. Additionally, surface insulation resistance might be diminished locally and cause a similar effect by crossing leakage currents. Finally, as well as the prior static effects described, dynamic effects also can be present: Parasitic capacitors will distort the ramp slope. Edge-triggered active components might not recognize the signal if the ramp slope is too flat, and the signal integrity of highly integrated, high-speed or high-frequency circuits primarily is affected.

Reductions in SIR and the capacitive potential that can be built up by activator residues can be shown qualitatively under a scanning electron microscope (SEM). The viewing is possible via a test that responds selectively to carbon acid-based activators of fluxes by a corresponding color reaction. The test not only detects the activator residue from fluxes, but also makes their distribution visible.

Impedance spectroscopy promises to be a direct way to measure electrical values. For example, the “ohmic”-shunt quota under chip capacitors can be determined by this method. In conjunction with a corresponding board-storage climate and temperature, it now is possible to check the aging behavior of assemblies.

The intensified use of high-frequency technology, HDI assemblies and Pb-free solders is giving rise to new aspects in flux removal. As a result, any decision concerning cleaning or no-clean manufacture must be discussed intensively with respect to the needs of quality. In spite of the diversity of efforts to circumvent cleaning as a critical step via new joining techniques, it has become quite clear that cleaning is inseparably associated with electronics manufacturing. Accordingly, the creation of qualified cleaning processes that meet ISO 9001 guidelines also requires provision for optimal testing and monitoring procedures. Cost-optimized solutions that guarantee the highest possible long-term reliability of assemblies only can be realized through a close cooperation between the manufacturers, designers and suppliers of cleaning processes.

Harald Wack, Ph.D., is president of Zestron (zestron.com); h.wack@zestronusa.com.