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Home , Soldering

Do’s & Don’ts

Donald W. Baudrand, Consultant 621 NE Harrison St. Poulsbo, Washington 98370-8453 E-mail: [email protected]

Do’s and Don’ts of Soldering, DI and Wire Bonding to Electroless Plated Deposits

It is generally thought that it is not pos- At 600°C, the phosphorus begins to migrate silicide, and/or -phosphorus-silicide. sible to to electroless nickel (EN) to the surface. In moist hydrogen, the phos- The result is a brittle joint that could cause plated deposits unless a strong acid phorus is removed at 600 to 800°C. The delamination from the silicon device. is used. The fluxes can be harmful to other moist hydrogen atmosphere is reducing for Electroless nickel-boron deposits do not components in an assembly, particularly the nickel, and phosphorus is oxidized by need the thermal excursion to form good if it is an electronic device. So, soldering the oxygen supplied by the dissociation of quality bonding to silicon chips. is often avoided where electroless nickel the water and is vaporized from the surface, could provide many advantages. Likewise leaving a pure nickel surface. Wire bonding onto electroless DI attachment and or wire bonding are not nickel-boron plated surfaces considered due to the fear of the use of Solder fluxes Electroless nickel plated deposits wire bond electroless nickel plated surfaces. Nickel Solder fluxes are many. They are classified well using aluminum wire and ultrasonic and electroless nickel are good conductors by their degree of activity, inorganic being bonding systems. Higher ultrasonic energy compared with thick films used to metal- the strongest and resin the weakest. Fluxes is required for bonding to electroless nickel lize devices. are used to remove surface oxides and sul- than for gold plated deposits. Bonding gold Electroless nickel deposits also provide fides, reduce the surface tension of molten wire to electroless nickel-boron requires a good corrosion resistance, good adhesion solder and prevent oxidation during the thermosonic technique and higher energy. and have excellent uniformity of thick- heating cycle. R fluxes are , RMA are Long term reliability of wire-bonded joints ness. rosin-mildly activated and RA fluxes are using aluminum wire 0.000125 in. in the most active. Soluble organic acid and diameter wire bonded to electroless nickel- Soldering to electroless nickel inorganic acid fluxes complete the list. boron deposits were run by destructive When soldering, intermetallic compounds A suitable inorganic flux is sulfamic pull tests at time zero and after 2000 hr at form to provide high adhesion values. acid. In tests, a dilute solution of sulfamic 85°C and 81% relative humidity. The Mil

Compounds such as Ni3Sn, Ni3Sn2 and acid did indeed act as a suitable flux alone. standard requires a pull strength of 3.0 g.

Ni3Sn4 are formed that provide excellent R, RMA, RA and organic acid fluxes can The samples tested ranged from 13.5 to 14 adhesion. Also nickel-gold, nickel- be used in addition to the sulfamic acid g at time zero. At the end of 2000 hr test, and nickel- compounds can form, film that was left on the item for long times all samples recorded pull strength values although slowly. (several months.) Any water soluble flux within range. Typical surface films that interfere with can activate the sulfamic acid, including adhesion are soils, phosphorus and oxide. DI water. If sulfamic acid is applied the Do’s and Don’ts of soldering to Electroless nickel deposits contain phos- unit to be later soldered should be stored in electroless nickel phorus or boron and oxides. To form good a dry atmosphere. Sulfamic acid is a solid bonding to electroless nickel-phosphorus, when dry and becomes an acid when water Do soils, oxides and surface phosphorus must is added. Know that phosphorus in the deposit be removed. For electroless nickel-boron causes poor soldering, and adhe- deposits, the boron aids soldering (Adding Di bonding sion and therefore must be removed from boron to the electroless nickel-phosphorus Di bonding to electroless nickel-phos- the surface before soldering. Boron in solution, the NiB-P deposit aids solder- phorus plated deposits requires a heat nickel-boron deposits does not have to be ing.). Nickel-boron electroless nickel excursion above 700°C, usually 800°C removed for soldering. easily, but is much more expensive. for 8 to 10 minutes in a nitrogen/hydrogen To solder, oxides on the surface must Low boron content EN-B works best. atmosphere. This treatment leaves pure be removed from both nickel-phosphorus To make electroless nickel-phosphorous nickel on the surface that is needed for and nickel-boron deposits. Select active plated deposits easy to solder requires a good bonding. An electroless gold or gold , or mild acid such as sulfamic acid strong acid flux or a heat-treat process. will minimize oxidation of to remove oxides, or heat at 600 to 800°C There are several heat-treat processes that the nickel so that Di bonding can take place (1110 to 1471°F) in a moist hydrogen can remove phosphorus from the surface by a nickel silicide intermetallic. along with oxide to make EN-P solderable. Phosphorus inhibits the formation of nickel Continued on page 27.

20 & • April 2010 April 2010 • Plating & Surface Finishing 21 factant blends also soften and lift buffing • Analysis can be accurately and quickly and polishing compounds. determined by measuring the cleaner’s NASF Welcomes Recycling and purification of cleaners conductivity, a direct function of the New Members* have become wise investments to extend concentration. Conductivity or tor- bath service life. Acknowledged cost sav- roidal probes activate a pump, which ings are infrequent cleaner dumps, less dispenses a sufficient quantity of the downtime, reduced demands in waste treat- concentrate. Desired cleaner concentra- ment, less sludge and, most importantly, tion is maintained at the set point. satisfactory cleaning on a per-shift basis. • A 75 - 85 percent reduction in sludge. Sophistication of mechanical filtering aids • Tank dumps and new makeups are range from simple belt and disk skimmers quicker. to coalescers to micro- and ultrafiltration. • In many waste treatment applications, Based on the specific user’s operating cost metals flocculate and settle faster as a data and cleaning application, the capital result of reduced dissolved salts in the expense and maintenance of these devices cleaner solution. can be readily justified. During recent • OSHA-compliant because of reduced years, oil displacement has become a worker direct contact. more popular method of alkaline cleaning, thereby increasing the useful application Unlike standard cleaner blends avail- of mechanical filtering devices. Many of able in the past, today’s powder and liquid the oils can be recycled by certified firms concentrates offer different additive pack- (costing as low as just a freight charge to ages to meet specific requirements. These haul off the oils) used as an additive with include surfactants only, or surfactancy/ fuel oil in plant boilers. additive systems. The finisher can maintain Liquid products have really contributed a cleaning system with additional deter- to improving the overall technology of gency, as needed, without exceeding alka- aqueous cleaning. Concentrated blends, as lis or other constituents. To optimize the SUPPLIER MEMBERS the name implies, are not bulk water-based, cost of cleaning, proprietary concentrates but rather water present in just sufficient can be added in ratio with the finisher’s T & J Industries, Inc., Paso Robles, CA quantity for blending. Additives, such as source of caustic. Polyfab Plastics & Supply, Springfield, MO alkalis, conditioners and surfactants, are Some cleaners are formulated to be used the heart of these concentrates - just as they in both soak and electrocleaning. This sim- INDIVIDUAL MEMBERS are in powdered blends. Many liquid con- plifies application, inventory and analysis centrates are equal in dry weight of certain control. Other concentrates improve rins- Jon Barrows, Springfield, MO additives to their powdered counterparts. ing characteristics, silicates, chelates, com- Alain J. Bergeron, Cedar City, UT This even includes caustics. Liquid clean- plexors or other targeted components. Vladislav Dimitrov, Boylston, Bulgaria ers are just as effective as powders. Field Many of the changes in soak cleaning Curis Goad, Ellisville, MO benefits that are attributed to liquid clean- technology, therefore, are the result of Xiao Hai Hu, City of Industry, CA ers include: meeting a new range of requirements: oper- Eric Todd, Harrison Township, MI • Safer, easier to handle; can be pumped ations, environmental and health-related. through pipes to the process tank from These items continue to foster change and Liz Hergatt, San Carlos, CA another plant location. improvement in the science of soak clean- Barbara Knecht, Mentor, OH • No dust, caking, splash-back or local- ing. In the shadow of the 21st century, the Kyeyul Lee, Ansan, Korea ized boiling when making additions. first step in many metal finishing processes Elijah Castillo Losbanos, San Diego, CA definitely has “the right stuff.” P&SF Todd Miller, Springfield, MO Joe Montano, Marlborough, MA Richard Morgan, Monroe, WA John Mulder, Grand Rapids, MI Do’s & Don’ts Gary Perillo, Tolleson, AZ Continued from page 20. Peter F. Pulcini, Southampton, MA Richard Reeves, Cleveland, TN atmosphere for 12 to 15 min. Solder using Use electroless nickel-boron deposits Joseph Renaud, Springfield, MO RMA flux. The same is true for Di bonding for soldering, wire and Di bonding (more Stefanie Lee Wallburg, Huntsville, AL except flux is not used. Wire bonding to EN expensive than Ni-P). requires higher energy, not more pressure. Add boron to the EN solution for solder- Don’t ing of nickel phosphorus plating systems. Delay attaching after the heat treat pro- This adds cost but aids activated rosin cess. fluxes in oxide removal without the high Delay after fluxing for soldering. * As of March 29, 2010. temperature excursion. Boron is added by Use fluxes for wire bonding or for Di including a small amount of dimethyamine bonding. P&SF boron in addition to the usual sodium hypo- phosphite. This adds cost.

26 Plating & Surface Finishing • April 2010 April 2010 • Plating & Surface Finishing 27