Underfilms have material properties similar to underfills but are easier to process.

A common failure mode for portable electronics is mechanical failure as a result of being dropped from sufficient height to cause damage. Most portable electronics producers qualify assemblies by performing some type of system-level drop test, while component suppliers typically utilize industry standard drop tests such as JEDEC JESD22-B111.¹ It is well known that Pb-free solders do not perform as well in drop or shock testing as the Pb-free solders they replaced.

Much work has been carried out to optimize Pb-free alloys to improve drop test performance.^2-6 Many companies have also implemented epoxy underfill to further increase robustness of the interconnection between the BGA and printed circuit board. These underfills are typically dispensed around the BGA following SMT assembly and electrical testing. Capillary action then slowly draws the underfill under the BGA. The underfill may be dispensed in sufficient volume to cover the entire BGA underside, the corner areas only or just the package sides. Finally, the underfill is cured in an offline batch oven. While excellent drop test results have been demonstrated with underfill, the process adds cycle time and may involve processes not normally performed during assembly. Another option for providing greater BGA to PCB mechanical robustness that may provide a more seamless integration into the SMT manufacturing process is the use of a preform underfilm material.7 The material can typically be provided in reels similar to discretes and is placed around the perimeter of the BGA following solder paste printing and prior to BGA placement. In reflow, the underfilm softens and initially expands such that it contacts both the BGA and the PCB. By the conclusion of reflow, the material fully cures, while remaining adhered to both the BGA and PCB.

For our study, the preform underfilm material was a fully cured, low flow polyurethane polymer. It comes in a variety of shapes and sizes, and for this study it was specified in 4.0 x 0.76mm preforms with a thickness of 0.165mm. The preforms were provided in EIA-481-compatible 12mm wide carrier tape with a cover tape similar to how discretes would be supplied. The underfilm material is a thermoplastic designed to soften, flow and expand during reflow, thereby filling in the gap between package and PCB. Upon cooling in the reflow oven, the material resolidifies and bonds the package to the PCB. For this study, it was decided to use two of the preforms per corner, which effectively formed an L-shaped pattern. It should be noted that L-shaped preforms could also have been used.

The test vehicle (Figure 1) was a 338 sphere 11 x 11mm body size PBGA with spheres at a staggered 0.5mm pitch (Figure 2). It is a package typical of portable electronics applications. The composition of the Pb-free spheres was SAC 105 (98.5%Sn, 1.0%Ag, 0.5%Cu by weight). The PBGA was molded all the way to the substrate edge in what is called a Mold Array Process (MAP).

The dummy die size inside the daisy-chain package was 5.3 x 5.4 x 0.28mm; the two-layer substrate thickness was 0.21mm, and the total package thickness excluding spheres was 0.92mm. The pads on the package were SMD with a specified 0.38mm copper diameter and a 0.30mm soldermask opening.

The test board used was based on the JEDEC drop testing specification for devices with less than or equal to a 15 x 15mm body size. The board (Figure 3) was 132 x 77mm and 1.0mm thick with eight layers of copper. The surface finish was OSP. SMD pads on the PCB were designed to exactly match the package, with a copper diameter of 0.38mm and a soldermask diameter of 0.30mm. The only modification made to the test board to accommodate the underfilm preforms was the addition of four additional pads in each corner of the package footprint. Once printed with solder paste, these tacky pads served as a way to keep the underfilm in place prior to reflow.

Assembly was carried out using a high-volume production SMT line with automated pick-and-place equipment and automated solder paste volume inspection. PBGAs were supplied in JEDEC trays. The Pb-free solder paste used was a halogen-free, no-clean with a SAC 305 composition (96.5%Sn, 3.0%Ag, 0.5%Cu). The reflow profile (Figure 4) had a peak temperature of 244.5º to 246ºC, depending on thermocouple location. Total reflow time was approximately six minutes. Each underfilm preform was placed on the two tacky pads near each corner of the part following solder paste printing. BGAs were then placed normally. Figures 5 to 10 show photos taken at various steps in the assembly process from printing and underfilm preform placement through BGA placement and reflow.

As shown in Table 1, a total of 14 boards containing 130 total components were assembled. On the boards for drop test, only five sites were populated (Figure 3). This is permitted by the JEDEC standard and was done to provide the most consistent drop test results by not populating parts close to corner standoffs. For the boards for thermal cycling, all 15 sites were assembled. Two additional boards were assembled for cross-sectioning. One hundred percent yield was achieved in assembly, and all underfilmed BGAs appeared to have the material adhered as expected between the BGA and the PCB in all four corners.

X-ray analysis. Following assembly, all boards underwent x-ray, with close attention paid to corner joints that may be in contact with the underfilm material. No misshapen or anomalous solder joints, such as those with opens, shorts, or excessive voiding, were observed (Figure 11). Additionally, the tacky pad locations were x-rayed, and all the printed solder paste was seen to be soldered to the pad, as opposed to being dispersed in the underfilm (Figure 12).

Cross-sectional analysis. Underfilmed and non-underfilmed BGAs were both cross-sectioned to the outer row to assess the solder joint formation and standoff (Figure 13). The underfilm was seen to be enveloping the three to four solder joints closest to the corner. The solder joint standoff between the underfilmed and non-underfilmed BGAs was found to be similar. A cross-section through the tacky pads on an underfilmed sample further confirmed that the solder paste reflowed and remained on the tacky pads (Figure 14).

Underfilm rework evaluation. Using an extra board that was assembled with underfilm, a small study was carried out to ascertain the reworkability of underfilmed BGAs. Since the material softens at elevated temperatures, parts were removed using a hot air rework station using a standard rework cycle. A slight upward force was used to remove the BGAs once the Pb-free solder spheres melted. Excess solder was then wicked from the BGA pads and the entire area cleaned up with IPA and an acid brush. The resulting sites appeared to be underfilm-free and ready to accept another BGA (Figure 15).

Board-level thermal cycling. Boards populated with 15 parts per board were placed into thermal cycling, for a sample size of 30 parts with underfilm and 30 control parts without. The single chamber cycle went from 0º to 100ºC with 10 min. ramps and dwells for a cycle time of 40 min. or 36 cycles per day. As of this writing, the boards had gone 3,000 cycles with no failures in either group. This would far exceed most requirements for handheld portable electronics.

JEDEC drop testing. Drop testing was carried out using a JEDEC-compatible drop tester (Figure 16). During the drop, continuity to the daisy-chain parts was continually measured using a high-speed event detector. Additionally, triaxial strain gauges were affixed to the PCB on the bottomside of the BGA footprint to measure strain during the drop (Figure 17). Boards were attached to the base plate on the drop tester using four steel standoffs in the board corners. The JEDEC standard calls for boards to be dropped to Condition B, which is 1,500 Gs, as measured on the base plate with a 0.5ms duration and half-sine pulse.

Two of the underfilmed boards and two of the non-underfilmed boards populated with five parts each were initially dropped 100 times at 1,500 Gs. No electrical failures were recorded on any of the four boards. It was decided to drop the remaining four boards to the maximum 120cm height on the drop tester to achieve failures in a reasonable amount of drops to enable assessment of the performance of the underfilm material. Dropping from this maximum height resulted in approximately 3,500 Gs being recorded on the accelerometer on the base plate. See Figure 18 for a typical accelerometer output during a 3,500 G drop. The 3,500 G drop also resulted in a maximum of approximately 1,500µm strain being recorded on the strain gauge under the mounted BGA. One non-underfilmed board was dropped 118 times and the other 140 times, resulting in four of the five parts failing on each board. The first of these failures was recorded at 20 drops. The number of 3,500 G drops to failure for these two non-underfilmed boards is plotted on Weibull axes (Figure 19).

One underfilmed board was then dropped 300 times at 3,500 Gs with no failures recorded on any of the five parts. The intention was to drop the other underfilmed board 300 times, but following 163 drops, a failure was recorded on one of the underfilmed BGAs. The test board was inspected and was found to have broken in the corner where it attached to a standoff. The part that recorded the failure was located in the broken corner. This failure was discounted, and drop testing was terminated at this point. If a true failure had occurred on this board at this point, the underfilm material would have increased the drops to first failure by at least eight times (20 versus 160+ drops) versus non-underfilmed parts. If this second board had not experienced a PCB fracture and experienced no failures out to 300 drops as the other underfilmed board did, the underfilm material would have provided a 15 times improvement in drops to first failure. All 3,500 G drop test results are summarized in Table 2.

Drop testing failure analysis. A non-underfilmed part that had a failure at 104 drops was subjected to dye penetrant analysis following 140 drops.⁸ Typical solder joint fracturing was observed in the BGA corner (Figure 20). Since no true failures were recorded on any underfilmed parts, no analysis was performed on these.

Conclusions

A preform underfilm material designed to be a drop-in to the SMT process was evaluated for assembly characteristics, yield, drop test performance and thermal cycling reliability. The following conclusions were drawn from the study:

• The preform underfilm material was seen to be a drop-in to a conventional, high-volume SMT process and was handled and pick-and-placed very similar to discretes.
• Only a small design change was carried to the test PCB to accommodate the underfilm material. Tacky pads were added to the footprint to prevent any movement of the preforms prior to reflow.
• One hundred percent SMT assembly yield was achieved with 21,970 0.5mm pitch joints formed robustly on 65 BGAs containing the underfilm material. Following reflow, all BGAs with underfilm appeared to have the material adhered as expected between the BGA and the PCB in all four corners.
• The underfilm material had no observable effect on solder joint shape or standoff on the corner joints it came in contact with.
• Using a JEDEC-based drop test at a severe 3,500 Gs, drops to first failure was increased by a minimum of eight times and probably much greater (15 times or more) with the use of the underfilm material. An exact improvement factor could not be determined since no actual drop test failures were recorded with the underfilm material.
• Initial results suggest the underfilm material results in no degradation in thermal cycling performance. Thermal cycling to at least 50% failure continues.

Acknowledgments

The authors wish to acknowledge David Mendez and Kirk Kringel at Flextronics for technical and logistical assistance, Randy Temple and Dave Skupien at Alltemated for supplying the underfilm material along with applications assistance, Terry Burnette at Freescale for carrying out the rework evaluation, Scott Stoller at Freescale for performing the drop testing, Roy Arldt at Freescale for cross-sectional analysis and Ed Hall at Freescale for management support of this project.

References

1. JEDEC JEDS22-B111, “Board Level Drop Test Method of Components for Handheld Electronic Products”, July 2003, www.jedec.org.
2. C. Chiu, K. Zeng, R. Stierman, D. Edwards, and K.Ano, “Effect of Thermal Aging on Board Level Drop Reliability for Pb-free BGA Packages”, ECTC, p.1256, June, 2004.
3. D. Henderson, “On the question of SAC solder alloy – Cu pad solder joint fragility”, Webcast Meeting on SAC Solder Joint Fragility, Binghamton, NY, September, 2004.
4. S. K. Saha, S. Mathew and S. Canumalla, “Effect of Intermetallic Phases on Performance in a Mechanical Drop Environment: 96.5Sn3.5Ag Solder on Cu and Ni/Au Pad Finishes”, ECTC, S29p5, June, 2004.
5. M. Amagai, Y. Toyoda, T. Ohnishi, S. Akita, “High Drop Test Reliability: Lead-free Solders”, ECTC, S29p7, June 2004.
6. Alltemated Place-N-Bond Pick-and-Place Underfilm product data sheet, http://www.alltemated.com/img/Place-N-Bond-Underfilm-data-sheet.pdf?vsid=ypxrplzu.
7. W. Liu and N.-C. Lee, “Novel SACX Solders with Superior Drop Test Performance,” Proceedings of 2006 SMTA International, September 2006.
8. T. Burnette and T. Koschmieder, “Solder-Joint Failure Analysis”, Advanced Packaging, January 2003, pp. 25-27.

Ed: This article was first published at SMTA International in October 2011 and is reprinted here with permission of the authors.

The authors are with Freescale Semiconductor (freescale.com); andrew.mawer@freescale.com.

Mid-market providers are responding to OEM desires for more regional approach.

As wages continue to rise in China and fuel costs escalate worldwide, some North American manufacturers are reevaluating their global operations to determine if production locations are optimal. This is reinvigorating discussion among EMS providers over whether China or Mexico is the best site for lower-cost production.

Both locations have inherent pros and cons, but unlike a decade ago when the World Trade Organization welcomed China as a new member, a comparison of China with Mexico can no longer be based solely on input costs, i.e., labor and transportation. Global electronics supply chains have evolved to be much more complex, and OEMs are demanding faster and more solutions-focused services from EMS suppliers. At the same time, the economies of North America and Asia have changed dramatically since 2000, with growth slowing in the West just as thousands of poor Chinese people are joining the middle class.

The decision for EMS providers, then, turns on cost strategy and customers; the best location, under the best circumstances, is the one that satisfies both.

According to Sonny Newman, president of Reno, NV-based Electronic Evolution Technologies (EE Tech), the EMS company’s Empalme, Sonora Region, Mexico, location gives it a unique strategic advantage over competitors without a Mexico manufacturing location.

“The strategic advantage that our customers enjoy with us in Empalme versus Reno is a lower labor and overhead cost out of our Mexican production facility,” Newman says. “By taking a few more weeks to have the product built in Empalme versus Reno, a customer can save a substantial amount. Unfortunately, the material cost is the same, and that represents a large percentage of the builds, but when you are producing thousands or even hundreds of thousands of units, that savings adds up very quickly.”

To deliver this savings advantage, EE Tech works collaboratively with The Offshore Group to run its Mexico operations. The Offshore Group is what is known in Mexico as a “shelter,” or outsourced manufacturing support company.

“The strategic advantage that we have operating in Mexico is made possible by our partnership with The Offshore Group. They handle the functions of our operations that, although not related to actual manufacturing, are critical to our being able to be and remain up and running profitably. Because they handle things like managing our payroll and worker benefits, our import and export operations and worker transportation, as well as maintaining our manufacturing facility, we can focus on our manufacturing mission, which is where we add lasting value for customers.”

Doug Besse, executive vice president, global supply chain and Asian operation manager for Creation Technologies, a global EMS provider based in Canada, also says his company takes a customer-centric approach to making plant-location decisions.

“Creation Technologies looks at it and says that we really need a Mexican manufacturing location for different reasons than we need an Asian location,” Besse says. “It’s different customers looking to do different things. Large North American OEMS are looking at Asia and saying, ‘We want to be in Asia, so we should manufacture in that location.’ Then there are OEMS in North America who say, ‘I have a high labor-cost concentration in my product, but my market is in North America, and I want to be a market leader in North America’ – they tend to look at Mexico as a manufacturing alternative.”

Beyond the ‘China hype.’ According to a 2011 report based on a research project by electronics supply chain consulting firm Charlie Barnhart & Associates, “China and Mexico both possess many world-class manufacturing facilities capable of producing any electronic product being designed today. The question, therefore, is not one of capability but fit. Which country best fits the manufacturing needs of a particular OEM? For OEMs selling high-volume products in the 3Cs (consumer, computer, communications) sector, the answer is undoubtedly still China. For mid-market USA-based OEMs selling lesser-volume products, the answer has become less clear. Many of these companies have been caught up in the China hype only to have become frustrated.”

That frustration stems from multiple factors: steadily rising wages, reduced export tax credits, longer lead times due to greater distance and more shipping requirements; language and time-zone barriers; and the uncertainty of doing business in a country that –although becoming more Westernized economically – remains a Communist country with an authoritarian government that keeps a heavy hand in commerce.

“The composition of the EMS markets in China and Mexico are quite different,” according to the CBA report, China vs. Mexico: An Objective Comparison for Midmarket Electronics OEMs. “While the Chinese market is comprised of both indigenous and global companies, Mexico is comprised almost exclusively of non-Mexican service providers primarily from the United States.” Table 1 shows the top 10 EMS companies from 2011 and whether they have a facility in China or Mexico.

For mid-market EMS providers, though, it’s expected that EMS production in Mexico is increasing and will continue to do so, according to CBA. The past decade saw a dramatic shift in electronics manufacturing to China in pursuit of lower assembly costs, including for LV/HM (low-volume/high-mix) assemblies. But CBA and others have documented a growing return to a regional approach by some North American-based OEMs that is benefiting Mexico.

“Our research has identified a trend of electronic OEMs returning to a regional approach to their manufacturing,” states the CBA report. “Regionalization is defined as when OEMs build their product in the region into which their product will be sold. In other words, they choose to build in region for region.”

The number of EMS companies operating in China, both indigenous and foreign owned, is still too vast to list, according to CBA. The list of EMS in Mexico in comparison is fairly short (Table 2). (This is not an all-inclusive list, but does reflect what CBA believes to be the vast majority of EMS companies currently operating a facility in Mexico.)

EE Tech produces two-thirds of its sales volume at Empalme and about one-third of it in Reno. This is about 90% of unit volume in Mexico – more than 6 million printed circuit boards per year.

Newman says EE Tech chose Mexico because it was “looking for a solution that was going to be a price leader in North America not competing with the 10,000-plus shops already set up in Asia.”

“Our customers usually do not compare us to Asia. If they want Asia, they go there, as there are still some advantages if your supply chain is steady and large. However, if our customers are looking for some sourcing in North America that comes close to Asia pricing and better than the United States, Mexico is a great fit. Not only is it very competitive, it also is just a couple of days away via ground shipment, and none of our customers have more than a three-hour time difference. The staffing and technical expertise is an added bonus.”

Pros and cons. Ultimately, leading EMS providers weigh multiple factors before expanding operations in either China or Mexico. Sometimes, comparing the pros and cons of each location requires exceptional considering.

For example, Creation Technologies sees Mexico as a viable option, but has some concerns. “We would certainly look at Mexico as a manufacturing location,” Besse says. “Since we are focused on a niche market, we are seeking the right location. Because we are a private company, we are more cautious about entering Mexico these days, but the interest is definitely there.”

Creation’s supply base is 65% North American, 30% Asian, and 5% from elsewhere. Despite the fact that most of its suppliers are in North America, the company’s desire for an eventual Mexican location is customer-driven.

“Depending upon the customer and how they structure their manufacturing, it can be cheaper to build in North America. It’s not as simple as saying, ‘We should just buy it in Asia.’”

Creation Technologies provides services for high-mix, high-complexity, medium-volume electronic assemblies or total box builds (all commercial). The company’s customers are primarily North American OEMs looking for manufacturing services for North American or overseas markets. Besse says a growing segment of customer OEMs want to get their products into Asia, mostly China, because Asia by far is the largest market opportunity –but the bulk of customer products are still built and purchased in North America. A Mexico location would provide those customers with access to a Western hemisphere low-cost manufacturing area; lower shipping costs, and a more favorable time zone – but it still comes down to the importance of those factors in the OEM’s overall strategy.

“Are those issues of time and distance so overpowering that I can’t overcome them, or do I have the process and people to deal with it? Many OEMs will say, ‘I have an office in Asia already, and I am prepared to deal with the complexity because Asia.’ Looking at it from a cost point of view, Asia is still less expensive than Mexico. The labor-cost gap is steadily shrinking, and most likely will continue to do so over time, but as of this moment, when you do a straight comparison of manufacturing from a cost standpoint, Asia is the leader.”

Mexico: growth opportunity. Despite the attractiveness of a growing market in Asia, Mexico represents a growth opportunity for Creation because of its commercial niche, and when safety is no longer a concern, the company likely will establish a location in Mexico.

“In the past, companies have moved manufacturing to China because of cost savings, and on the consumer side, I don’t think we’ll ever see it come back,” Besse says. “However, on the commercial/industrial side, the cost of Asia versus Mexico and even North America, that gap is closing. We are also very interested in looking at the shelter business model that is available to companies that manufacture in Mexico. Using such as service provider is not a possibility in China.”

China is still a huge growth market, he says, but some customers are bringing products that they moved to Asia years ago back to North America because it is now no longer economically feasible to produce them there profitably.

The CBA research concludes that neither China nor Mexico provides a full-service solution across the manufacturing life-volume continuum, and each is best suited for specific manufacturing processes, but with one distinct difference: “Volumes appear to matter more in China than Mexico.” As way of qualification, Barnhart quotes one respondent’s description of the optimal build volumes in China: ‘The volumes need to be minimum revenue of $3 million to get into a decent Tier 2 EMS, and above $10 million for consideration at a Tier 1 EMS.’”

CBA has documented that the regional approach is “gaining traction, as this approach is now more cost-effective than a cross-hemispheric solution. The only known remaining exceptions are the extremely large consumer, communication and computing sector relationships, which are now atypical to the balance of the EMS/ODM industry.

“The return to regionalization, which is how the outsourcing industry actually began, should continue to benefit Mexico, especially in serving North American-based OEMs and also global OEMs that are selling their products into the North American market. Clearly this is influenced by the type of products, location of end-markets, and volume levels as mentioned above.”

EE Tech’s Newman says the company’s Empalme location in The Offshore Group’s Bella Vista Industrial Park is definitely part of the company’s future growth strategy.

“We still will primarily do all of the upfront prototyping, engineering, purchasing, and planning out of our Reno facility, but we will use our Empalme site to produce the volume at an economical North American price. Most of our new customers are coming to us because they ultimately want to use our Mexico price for volume builds.

“The hidden benefit to this is that we do most of all of the procurement of parts, planning, finance, customer service, and logistics out of the Reno facility, which ultimately creates business and jobs in Reno. I feel that if our Mexico facility did not exist, we would probably have about 75 Reno employees instead of the 150 that we have today. Operating in Mexico under a shelter plan has allowed us to successfully bid and take on volume work that we might have otherwise had no shot at winning.”

Tonya Vinas is a freelance writer.