Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/31—Coating with metals
  • C23C18/32—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron
  • C23C18/34—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents
  • C23C18/36—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents using hypophosphites

Definitions

• the invention relates to an aqueous, acidic, reductive depositing bath for electroless deposition of nickel.
• nickel / gold adhesion requires a nickel layer that is as pure as possible, whereby a phosphorus content of less than 10% is desirable.
• the well-known nickel baths contain the nickel to be deposited in the form of nickel sulfate or nickel chloride.
• the nickel present as a cation must be reduced for the purpose of its deposition become. This task is performed by sodium hypophoshite.
• As a complexing agent for the nickel are lemon, glycol, milk, apple, vinegar, amino vinegar, Propion, Succinic acid, ammonium, sodium, sodium hydroxyacetate, sodium citrate, Sodium glyconate, hydroxylamine sulfate and mixtures of some of these acids known.
• TU Thiourea
• Stabilizers needed. Lead, tin, arsenic and molybdenum compounds are known. However, the stabilizer reduces the deposition rate.
• a disadvantage of the known baths is that in order to achieve acceptable deposition rates, the bath must be operated at relatively high temperatures of over 90 ° C. At these temperatures, the thiourea is no longer sufficiently stable and its effects are no longer fully effective. To maintain the deposition rate, thiourea must therefore be added continuously. There is a gradual accumulation of the decomposition products of the thiourea in the bath and thus an increased deposition of sulfur in the nickel layer. This makes the deposited nickel layer less noble. This leads to an increased occurrence of corrosion after the exchange gold coating, in particular to pitting corrosion in holes in the printed circuit board. The corrosion values deteriorate with increasing bath age, since more and more thiourea has to be added in order to maintain the deposition rate.
• MTO is derived from M etal T urn - O ver. that is, all of the nickel contained in the bath can only be reacted five times until the bath must be discarded.
• the present invention is based on the object of finding a composition for a bath of the type mentioned at the outset, with which a high-quality nickel layer formation with as few foreign deposits as possible is possible over a longer lifespan of the bath with a constant deposition rate.
• the limits for the complexing agent are chosen so that at the lower limit all of the nickel can still be complexed during the concentration exceeding the upper limit would be uneconomical.
• the accelerators used according to the invention are thermally more stable and more efficient than the accelerators previously used. This enables both the concentration of the accelerator and to keep the concentration of the stabilizer lower than before. This has the advantage that fewer decomposition products are produced in the bathroom than before, especially the concentration of the accelerator and the one following it Concentration of the stabilizer can be lower from the outset than in State of the art. This not only leads to a longer lifespan, but also also for better controllability of the bathroom. First experiences show that lifetimes of up to 8 MTO are achieved with baths according to the invention become.
• the accelerator should be in 1 to 3 times the molar concentration of the stabilizer be included in the bathroom. Will the relationship between stabilizer and accelerator shifted towards the stabilizer, inhibition of Conductor edges and drill holes. If there is an excess of accelerator, it can increase Pitting corrosion occurs.
• the acidic character of the nickel bath is due to the carbon and hydroxy carboxylic acids certainly; the pH value is expedient by adding ammonia set to a value between 5 and 6.
• a certain buffer effect occurs through the complexing agent used. Over time it will Bath is becoming increasingly acidic due to the chemical process. If the pH drops too much from; so it is raised again by adding ammonia.
• Hypophosphite unfolds its reducing effect in the weakly acidic range. At a pH above 6 the reducing effect of the hypophosphite decreases. The same thing applies to a pH below 5. The hypophosphite reaches its optimum reducing effect for pH values between 4.6 and 5.5.
• the present bathroom is very well suited to printed circuit boards with a noble nickel adhesive layer with a Coating phosphorus content less than 10%.
• a subsequent one Coating the noble nickel layer with an exchange gold layer causes one Reduction of corrosion compared to a less noble one - coated with gold - Nickel layer.
• the exchange gold bath can be in a wide pH range be driven between 5.5 and 6.5.
• the bath temperature is expediently between 80 ° and 90 ° C. is preferred a temperature of 85 ° to 90 ° C. is maintained in these temperature ranges one particularly economical deposition rates.
• the accelerator is above 90 ° C increasingly decomposes and foreign atoms accumulate in the bathroom then can be deposited together with the nickel and pitting could cause.
• the nickel content is preferably between 6 and 8 g per liter of bath liquid and the concentration of the reducing agent between 25 and 35 g per liter of bath liquid.
• the stoichiometric ratio is in these concentration ranges particularly advantageous between the dissolved nickel and its reducing agent, so that there is a constant deposition rate over several MTOs.
• a mixture of milk, apple and Acetic acid used, the molar ratio is preferably about 11: 4: 1. This relationship has proven particularly beneficial in terms of complexation of ionic nickel.
• a particularly advantageous development of the invention provides that in addition a complexing agent specifically complexing the stabilizer is added. It is preferably methylglycine diacetic acid, which is preferably in a concentration ratio of 0.5 to 5 ml per liter of bath liquid is clogged. This prevents the stabilizer cation from failing and preferentially precipitates on edges or boreholes. This will make the counteracted so-called edge weakness, this means that on the edges there is no inhibition by the stabilizer that causes the nickel deposition would inhibit locally. Another advantage is that the stabilizer does not need to be consumed and replenished as quickly. By clogging the further complexing agent remarkably becomes the stability of the bath not affected. A genalt in the range mentioned is advantageous, however is not a deterioration in bathing properties due to a higher concentration to recognize.
• the preferred accelerator is N-ethyl-thiourea or benzyl-isothiourea used. These two substances are thermally stable, so that hardly any Decomposition takes place, which would necessitate subsequent dosing. By the low formation of sulfur gives you a very pure nickel layer few built-in foreign atoms, which leads to good corrosion resistance leads.
• the pH of the bath should be 5.0 and is adjusted with ammonia solution.
• the bath is operated and enabled at a temperature of 88-90 ° C a deposition rate of 15 to 21 ⁇ m per hour.
• the new accelerator especially N-ethyl thiourea
• the risk of pitting corrosion is reduced because the storage of Sulfur in the deposited nickel layer is reduced.
• Particularly advantageous affects the good adhesion of gold on the nickel layer - due to a Phosphorus content less than 10% - on printed circuit boards. You also get with these small amounts of phosphorus in the nickel layer easily solderable and bondable Nickel / gold layers on the circuit board over the entire bath age of the Nickel baths.
• a second embodiment differs from the first only in that 3 ml of methylglycine-diacetic acid is added to the nickel bath. This ensures that the stabilizer is partially complexed, which surprisingly the accumulation of lead, e.g. on a conductor edge, is reduced.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemically Coating (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Catalysts (AREA)

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• 1997
  • 1997-08-06 DE DE19733991A patent/DE19733991A1/de not_active Withdrawn
• 1998
  • 1998-07-31 AT AT98114391T patent/ATE210207T1/de not_active IP Right Cessation
  • 1998-07-31 DE DE59802304T patent/DE59802304D1/de not_active Expired - Fee Related
  • 1998-07-31 EP EP98114391A patent/EP0897998B1/fr not_active Expired - Lifetime

Patent Citations (4)

Non-Patent Citations (1)

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