2-6 g S P P i v

Water Requirements for Clean PCBs

By Lyle Kirman

ater is the most used and often the least appreciated material required in the pro- duction of PCBs. This is particularly true in the Great Lakes area where we are

right next door to over 20% of the world’s supply of fresh water. Long taken for granted or treated as an afterthought, water quality is now an important pro- cess variable that can be quantified, monitored and controlled. While the wet processes in PCB manufac- turing are always changing, it appears that the need for higher quality water is already here and increas- ing, particularly as manufacturers and assemblers turn away from the use of CFCs.

One of the most important problems facing assemblers today is how to clean boards after soldering. Table 1 shows six alternatives to CFC cleaning and some advan- tages and limitations of each. I t is impossible to predict which of these technologies will dominate and which one will be in use two to five years from now. Many of us will need to be familiar with several different approaches.

Today, most people appear to be using rosin-based fluxes or water-soluble fluxes cleaned in either aque- ous or semiaqueous chemistries. Each of these alterna-

tives requires a final deionized water rinse to meet cleanliness and ionic contamination specifications. As spacing on PCBs decreases and component densities increase, cleanliness will be just as important before solder paste is applied as it is after soldering. This is good news to manufacturers of water purification equipment, but it may pose a bewildering array of choices to the assembler.

Types of Cleaners Aqueous cleaning is a mature, well-developed technol- ogy. Often saponified, these cleaners contain a source of alkalinity, wetting agents, alkyl glycol ethers and other comDonents. such as butvl carbitol and chelators.

Semiaqueous cleaners, which are based on terpenes, are some of the most environmentally friendly clean- ing chemicals. They are natural prod- ucts that are already being released to the air and water in large quantities. Cleaners based on terpenes are being used for a wide variety of other clean- ing tasks, including household prod- ucts and general metal cleaning. No-clean soldering would make life a lot simpler if it is developed to the point where it can achieve the same low reject rates as the more estab- lished fluxes. Water-soluble fluxes offer interest- ing alternatives in terms of their environmental consequences. Aque- ous and semiaqueous cleaners may contain too much lead to be sent to the sewer without treatment. On the

TABLE 1: A matrix of the advantages and disadvantages of some CFC alternatives.

other hand, it is likely that the rinses following the cleaners will meet most

72 CIRCUITS ASSEMBLY JUNE 1994

placement and alarms notify the oper- ator when the media reach this point.

The wastewater treatment system was removed from its cabinets due to tight spacing on the manufacturing floor and was mounted in its own ex- ternal location where it takes up less space than the version in a cabinet but is still accessible for service. It is linked to both cleaners. During oper- ation, wastewater from the semi- aqueous cleaner is piped to the recy- cling unit where any residual terpene, typically trace, is separated using the separator module. This waste stream then joins with the waste- water from the aqueous cleaner, and both enter the recycler and are puri- fied. The essentially pure, hot D1 water returns from the recycler to both cleaners, balanced by flow con- trols to ensure that each cleaner gets the right amount of return water.

The system has been in place and working well for over a year without problems. The combined wastewater discharge from both cleaners averages 6.5 GPM, well within the purification capabilities of the system. No waste- water is discharged to the drain, and we have achieved our goal of imple- menting a closed-loop cleaning pro- cess, with the exception of the periodic disposal of contaminants as solid waste. The purification media in the recycler must be changed every two weeks, but this is not considered ex- cessive in view of the manufacturing volume and required level of water pu- rity. It had been feared that incom- patibility of the wastewater chemis- tries might result in more frequent changeover of the media, making the recycling process costly or cost prohib- itive. This has not been the case.

Conclusion Dictaphone’s implementation of closed- loop wastewater recycling with mul- tiple chemistries has been a success. This is due, in large part, to detailed preliminary studies and by working with knowledgeable equipment sup- pliers who were flexible enough to match a custom system to our needs and to anticipate future changes in processing requirements.

Keith Dunne is a senior manufactur- ing engineer with Dictaphone Corp., Melbourne, FL. John Russo is president of Separation Technolo- gists Inc., Metheun, MA.

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JUNE 1994 CIRCUITS ASSEMBLY 71

t

trations of these impurities vary great- ly from region to region.)

Common water purification tech- niques, such as softening, reverse os- mosis (RO), electro-deionization, sep- arate- and mixed-bed deionization and filtration, require some form of pre-filtration. Fine filtration at 0.2 microns or less is required in many applications to remove particles and bacteria in order to produce “particle- free” water.

Each of these technologies has ap- plications and each has limitations- either to the quality of water it can produce or to the environmental re- quirements for effluent treatment (Table 2).

Softening is a simple, inexpensive technique that requires no effluent treatment. But softening removes only calcium, magnesium and iron and then replaces them in solution with so- dium. (This takes place when resin beads absorb these materials from the solution while at the same time leav- ing their sodium content in the solu- tion.) The water leaving a softener has

~~~ ~

TABLE 2 A listing of some typical water purification systems and their advan- the Same conductivity and TDS (total dissolved solids) as .~

tages and limitations.

effluent restrictions and be acceptable for discharge. Since there are no concentrated cleaner dumps associ- ated with the use of water-soluble fluxes, it is possible that environmental problems associated with cleaning after rosin-based fluxes can be avoided by the use of a water-based flux.

Water Quality Tap water contains an abundance of ions and varying amounts of organic materials. Surprisingly, for constitu- ents present at 1 mg/l or greater, water from almost any source in the country has the same immrities. (Concen-

FIGURE 1: A typical small plant design for >I megohm- cm water-deionization by exchange tanks.

the water entering it. RO literally pushes the water through a membrane at

high pressure, leaving behind fine particles, ions and large organic molecules. RO is a well-established tech- nology that usually requires no effluent treatment. Its chief limitations are that it requires pre-treatment of the water, yields only 65 to 75% of the feed water as purified water and cannot produce very high-purity water. Typi- cally, an RO system will reject 90 to 98% of incoming ions. If you are starting with a feed water that contains 400 mg/l of TDS, an RO system will produce water with 10 to 30-mg/l of TDS.

Electro-deionization is a relativelv new commercial

FIGURE 2 A typical small plant design for >I megohm- cm water-electro-deionization or RO/mixed-bed ex- change tanks.

JUNE 1994 CIRCUITS ASSEMBLY 73

FIGURE 3 A purification system for recycling water- throw-away/exchange resin or regenerated off-site. development, combining both membrane and ion ex- change resin technologies. I t produces water with a low- er TDS than an RO system. Like RO, electro-deioniza- tion requires pre-treatment, generally requires no effluent treatment and has a significant flow of untreated water to the drain. Typically, 80 to 90% of the feed water will be delivered as purified water.

Deionization can produce high-quality water and is available in a variety of types. For the small user, service exchange tanks are the most convenient way to get high- quality water with 5 1 megohm-cm resistivity. For the large user, a regenerative system is more cost-effective, but it requires a neutralization system.

Separate-bed deionization can be used to make wa- ter < I megohm-cm, but in general, the pH of the wa- ter produced is not neutral. Mixed-bed deionization, either by exchange tanks or by a regenerative system, produces the highest quality of all technologies-up to 18.2 megohm-cm.

Because of the relative advantages, costs and techni- cal limitations, hybrid systems that incorporate two or more of these approaches are common to the small- to medium-sized user. (Most PCB manufacturers and as- semblers fall into this category.) For example, it is common to see an RO, electro-deionization or a sepa- rate-bed deionization system feeding mixed-bed ex- change tanks (Figures 1 and 2). This combination ne- gates the need for an effluent treatment system, produces very high-quality water and minimizes oper- ating costs.

The criteria for selecting the best water purification system will depend upon the tap water analysis, the quantity of water needed, whether or not there are exist- ing neutralization facilities and the quality of the water needed. There is no single best system, and several alter- natives usually need to be evaluated for capital and oper- ating costs before a good decision can be made.

Specificat ions Water quality specifications are the most important factors to consider when deciding on a system. The most common specification of control is conductivity or resistivity; it is also referred to as the ppm of TDS.

FIGURE 4: A purification system for recycling water- regenrative/throw-away/exchange resin. The various units used to measure the electrical prop- erties of water are often confusing. I t is common to use conductivity for water with > 1 mg/l of ionic constitu- ents and resistivity for water with > 1 mg/l. The unit for conductivity is the microsiemen (pS) , and the units for resistivity are in ohms, typically megohm-cm. A helpful way to relate them is to remember that water with a conductivity of 1 pS also has a resistivity of 1 megohm-cm.

Resistivity is the easiest unit to explain. If two conduc- tive parallel plates, each 1 cmz in area, are spaced 1 cm apart and are immersed in water and the resistivity is measured with an ohm meter, then the measured resis- tance is the resistivity.

While conductivity and resistivity are equivalent ways to express the same thing, the same is not true in general for TDS. There are many tables and charts that relate ppm to conductivity, and for typical tap water they are approximately correct. For this type of water, 1 ppm is about 1.6 pS. When it comes to con- ductivity in high-purity water, however, all ions are not equal. At one extreme, 1 ppm of hydrochloric acid or sodium hydroxide is approximately 7 to 10 pS. At the other extreme, 1 ppm of silica is only a fraction of a microsiemen.

Other specifications that are common include pH, hardness, specific ions, TOC, bacteria and particles. The ASTM (American Society for Testing and Mate- rials) has established specifications for four types of electronics grade water. Surprisingly, these are rarely used in specifying equipment. Nearly every company prefers to have its own set of specifications. This makes life interesting for the water purification indus- try. Sometimes the specifications are contradictory or difficult to measure.

In this industry, there seems to be no consensus on wa- ter quality specifications. In the last year, we have re- ceived bid specifications a t 1, 2 and 3 megohm-cm and one that called for 5 megohm-cm particle-free water.

How Clean is Clean? How clean does water need to be for final rinsing and cleaning? It really depends on how it is used. The quality

74 CIRCUITS ASSEMBLY JUNE 1994

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of water a system produces is much less important than the quality of the water on the board when it dries.

Adequate agitation and contact time are necessary to remove ionic contaminants. Water quality in the final rinse, not water quality leaving the purification system, is most important. It makes little sense to feed 18 me- gohm-cm water into a final rinse, which contains 20 ppm of ionic contamination. For this reason, spray rinsing with water > 1 megohm-cm is usually the final step.

Since tests for cleanliness on assembled PCBs use resistivity as criteria, it appears that the only specifi- cations needed for a water purification system are flow rate and resistivity. It is important to remember, how- ever, that even 10-megohm-cm water may contain sig- nificant quantities of TOC, silica, bacteria and parti- cles. At the present time, it is unknown how these affect a PCB, if a t all.

Once Through vs. Recycling Rinses No discussion of final cleaning with high-purity water would be complete without considering the differences between once through and recycled rinses. The argu- ments for recycling water are compelling: it saves water and heat; and it usually costs less than deionizing tap water because the water leaving a final rinse usually has a lower TDS than tap water. Now, ask yourself the fol- lowing questions: Which of the CFC alternatives is com- patible with water recycling? Which technologies work best? What are their characteristics and limitations?

Ion exchange is being used to recycle rinses after both semiaqueous and aqueous cleaning; and rinses after water- soluble flux soldering. It is also used for rinses after solder- wave processing. Among the technologies listed, only ion exchange can produce water with a high enough quality to meet the demands for (3-megohm-cm resistivity.

In principle, there is no reason why an RO system could not be used to recirculate the water. But there are disadvantages. A large percentage of the water would be sent to the sewer, and RO may not produce the quality of water required.

There are at least three ways ion exchange can be used to recycle final rinses. A common way is to use throw-away resin (Figure 3). The same general arrange- ment can be used with regenerative service exchange tanks. But be careful, the resins may contain lead, and only vendors permitted to treat and dispose of hazardous wastes can legally perform this exchange.

There are several limitations with using ion ex- change in this application. The first concern is tem- perature. Anion exchange resins have a significantly shorter life span as the temperature of the water being processed rises above 100°F. Heat and thermal cy- cling destroys the cation and anion exchange resin crosslinking and functional groups, and their capacity.

Lastly, some of the materials present from the flux and cleaner are not easily regenerated and act as cumu- lative poisons to the resins. In other words, they are re- moved by the resin but do not come off during normal

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76 CIRCUITS ASSEMBLY JUNE 1994

regeneration cycles. Lead is difficult to regenerate from cation resins, and several aromatic and high-molecular weight aliphatic organic compounds are irreversibly ab- sorbed on anion resins. Progressive loss of capacity is normal in this application, unlike tap water deionization where resins can last five to 15 years.

Despite these problems, a regenerative ion exchange system, polished by mixed-bed resins, can be the most cost-effective way to recirculate the final rinses (Figure 4). Carbon is necessary to remove organics that would otherwise foul the resins.

Conclusion The choice to recirculate or not to recirculate is a question, first, of economics. Then it must be evaluat- ed in the context of the quality of raw water available, the type of CFC alternative you choose and the envi- ronmental regulations at your facility. If you choose to recycle, you must further decide between non-regen- erative and regenerative systems. If you choose the re- generative system, you must choose between service exchange and a system that regenerates on site. Whether or not you already have a wastewater treat- ment system is an important consideration for all of these choices. There are many equipment vendors will- ing to help you make these choices, but only you should make the final decision. That selection should be an informed one, based upon a good working knowl- edge of the choices available.

Bib1 iography “An Analysis of Three Aqueous Cleaning Methods,”

“From Solvent to Aqueous Cleaning,” Circuits Manu-

“How SMT Boards are Assembled,’’ Printed Circuit

“The Latest Information on Cleaning Solutions,” Finishers Management, Vol. 38, No. 2, February 1993.

“New HCFC Material Passes Phase 2 Test,” Circuits Manufacturing, Vol. 30, No. 4, p. 19.

“The Reality of No-Clean Fluxes,” Circuits Manu fac- turing, Vol. 30, No. 4, p. 58.

Rohm and Haas Engineering Manual. “Semiaqueous: A Progress Report,” Circuits Manufac-

“Semiaqueous Steps Forward,’’ Circuits Manufacturing,

“Soldering Without Cleaning,’’ Circuits Manufacturing,

“User Qualifies Terpene Cleaning,” Circuits Manufac-

“Water to the Rescue,” Assembly, Vol. 35, No. 8, Octo-

Circuits Manufacturing, Vol. 30, No. 4, p. 44.

facturing, Vol. 30, No. 4, p. 41.

Fabrication, Vol. 16, No. 2, February 1993. p. 42. ~~

turing, Vol. 30, No. 4, p. 22.

Vol. 30, No. 4, p. 36.

Vol. 30, No. 4, p. 66.

turing, Vol. 30, No. 4, p. 28.

ber 1992.

Lyle Kirman is vice president-technology with Kinetic0 Engineered Systems Inc., Newbury, OH.

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i ’ . .

Bull Announces New Procurement Strategy Bull H N Information Systems Inc. (Billerica, MA) announced a new procurement strategy for the com- pany’s U.S. manufacturing opera- tions and the formation of strategic business relationships with Hamil- ton/Avnet and Wyle Laboratories, which will now supply most of the firm’s standard components.

Bull’s new alternate supplier strat- egy will enable the company to con- solidate material from over 40 ven- dors into two key distributors who will create efficiencies in the pro- curement process. Hamilton/Avnet and Wyle will hold materials deliv- ered from over 40 manufacturers of standard parts. These distributors will provide a delivery service direct- ly to Bull’s production floors, allow- ing the company to eliminate in- house storage of parts and related paperwork. E3

Growth Motivates Avex’s Fremont Relocation Continued growth and the need for improved efficiency have led Avex Electronics Inc. (Fremont, CA), a supplier of contract manufacturing and engineering services, to relocate into a single 41,500-sq.-ft. building in Fremont.

Certech Awarded Service Contract from Spectradyne The Cerplex Group Inc. (Anaheim, CA) has established a new affiliate, Certech Technology Inc. (CTI, Rich- ardson, TX). The new affiliate will provide contract manufacturing ser- vices to Texas and the Southwest. CTI has established a 5O,OOO-sq.-ft. manufacturing and repair facility in Richardson. In addition, CTI has en- tered into a service agreement with Spectradyne (Richardson, TX) to provide contract manufacturing and repair services for Spectravision in- room electronics and head-end equip- ment. Under the agreement, CTI be- comes the manufacturing and repair provider for the entire line of Spec- traVision products. El

Comptronix Restructuring Comptronix Corp. (Guntersville, AL) has secured eight new customers

since a major corporate restructur- and Acuna, Mexico. It also main- ing that took place in late 1992. In tains engineering and materials sourc- addition, the company recently pur- ing offices in Elk Grove Village and chased a second Hewlett-Packard Taipei, Taiwan; engineering support 3070 combinational test system. services in Acuna, Mexico; and war-

The HP system purchase was pos- ehousing services in Del Rio, TX, sible in part because Comptronix se- and Huntsville, AL. El cured a $40 million credit line in November 1993 and increased its Group Technologies and KME Join

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Forces to Install COB System Group Technologies (Tampa, FL) has purchased a fully automated, high volume, in-line chip-on-board (COB) system. The company has

duced COB design services t OEMs in converting designs

vanced packaging technol- OEM client ba new bare-chip mounting duces over thre system augments Group Technol-

ogies’ high-speed surface-mount ca- pabilities and will enable the com- pany to offer an array of packaging technologies on a single assembly, in- cluding flip-chip, MCM and COB, as well as placement of BGAs and ul- trafine-pitch SMDs.

chnologies will purchase y automated COB line from Matsushita Electric Co. Ltd.

, Japan). The company also been granted I S 0 9001 cer- ation by ABS Quality Evalua-

and San Jose, CA. E l

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memory, modem and fax capabili- ties, p e r the next three months. Qtron to Open

The company is also creating an New San Diego Facility infrastructure to support its custom- Qtron Inc. (San Diego, CA) has ers’ needs for responsiveness and ad- opened a new 35,OOO-sq.-ft. facility. vanced technology access. El Qtron made a $2 million capital

equipment investment in the auto- SigmaTron Completes mated facility, which opened in Feb- Initial Public Offering ruary with 75 employees. The com- SigmaTron International Inc. (Elk pany offers real-time statistical Grove Village, IL) announced its ini- process control techniques and pro- tial public offering on February 9 of vides design, engineering, turnkey 1.1 million shares, which consists of manufacturing, testing and complete 750,000 sold by the company and product assembly services. 0 350,000 sold by selling to sharehold- ers a t $7 per share. The company’s SEMCO Appoints President shares of common stock are trading Guntis Melbardis has been appoint- on NASDAQ/NMS under the tick- ed president and CEO of Southern er symbol “SGMA.” Electronic Manufacturing Co. Inc.

SigmaTron is an independent con- (Lexington, SC). The company pro- tract manufacturer of electronic com- vides electronic contract manufac- ponents, PCB assemblies and turn- turing and engineering services.

78 CIRCUITS ASSEMBLY JUNE 1994