DD244768B1 - PROCESS FOR MAINTAINING LOADING OF NICKEL LAYERS - Google Patents

Abstract

The invention relates to a method for maintaining the solderability of nickel layers and is preferably used for the production of electronic components. The aim of the invention is to obtain the soldering capabilities of nickel-plated electronic components using an environmentally friendly gold plating process while conserving gold. According to the invention the object is achieved by the deposition of 10 to 50 nm thick, dense gold layers by the nickel-plated substrates for the period of 0.5 to 10 min in a known non-toxic, reductive gold bath with a pH in the range 7 to 13 and a temperature of 355 to 371 K at a bath load of 1 to 10 dm, / l are immersed.

Description

be adjusted by a sulfuric acid adjustable pH in the range 7 to 13 and a bath temperature of 355 to 371K at a bath load of 1 to 10dm ² / l.

2. The method according to claim 1, characterized in that the gilding bath 0.1 to 10 mg / l Thallium-I-nitrate are added.

Field of application of the invention

The invention relates to a method for maintaining the solderability of nickel layers, preferably those which are applied as contact layers in the manufacture of electronic components.

Characteristic of the known technical solutions

In electrical engineering / electronics components made of different materials such as _r . As silicon, ceramic, copper, tungsten, molybdenum, stainless steel and connected by soldering. A prerequisite for optimal soft solder joints is a good wetting of the nickel surface. It is known that can be formed on nickel during storage in air very quickly passive cover layers. In order to preserve the solderability of the nickel layers, it is common practice to protect them with a gold layer. Gold surfaces have a high corrosion resistance and promote wetting.

Known gold plating processes are vapor deposition, sputtering, galvanic and chemical (electroless) deposition. For economic reasons, the lowest possible precious metal consumption should be sought to achieve the required layer properties.

The above-mentioned physical methods are disadvantageous from this point of view, because an increased circulation of gold due to the unwanted gold deposition on parts of the coating apparatus (when sputtering still adds the problem of the residual target) is unavoidable.

Although the electrolytic deposition of gold is more economical, it requires an electrical connection to every surface that is to be gold-plated, which itself can again disturb the gold plating. In addition, a coarser grain is often deposited than with the corresponding electroless baths.

The electroless gold baths are divided into two groups, the cementation baths and the reductive baths. A cementation bath is described, for example, in DE-OS 2803147. However, like all baths of this type, it has the fundamental disadvantage of attacking the substrate. The layer thickness of the gold coatings that can be produced with this bath is limited, the adhesive strength is lower than in the case of known reductive baths.

The electroless gold plating baths also include the reductive bath, which is shown in WP 150762 and is characterized by its non-toxicity and relative stability. Commercially available reducing agents are used for this purpose (sodium hypophosphite, formaldehyde and hydrazine). To stabilize the solution, sodium sulfite, a di- or Triaminoalkyl- and / or di- or Triaminoarylverbindung, potassium bromide and a chelating agent are used. The temperature of the bath should be in the range of 353 to 371 K. A disadvantage of the bath with the specified tolerances of the bath composition is that a reliable preservation of the solderability takes place only at layer thicknesses of the bath. Layer of> 200nm occurs.

In DE-OS 2215364 a process for the gilding of tungsten or molybdenum electrodes is specified, in which 30 to 100nm thick gold layers are deposited electrolessly on nickel-plated and then silvered electrode bodies. A gold layer of this thickness is sufficient for a good soldering of the electrode body with other connecting parts or components. A disadvantage of this method is the use of a poisonous gold bath (Kaliumgoladcyanid) to see.

To reduce the gold layer thickness, coatings of the nickel layer with palladium and subsequent gold flash are also known. The disadvantage here is that an additional noble metal layer is needed.

Object of the invention

It is the object of the invention to obtain the solderability of nickel-plated electronic components while conserving gold using environmentally friendly gold plating methods.

Explanation of the essence of the invention

Starting from the object of the invention, the object is to find a method for maintaining the solderability of nickel layers, in which an adherent, thin and dense gold layer of an electroless non-toxic gold bath is applied to the nickel layer.

It was found, surprisingly, that with the below-listed narrow-tolerance composition of a gold bath known per se, coatings can be obtained which have such a tightness even at layer thicknesses of 10 to 50 nm that the solderability of the nickel layers when stored in air is permanent (several months ) is retained without restriction:

Gold as Na ₃ [Au (SO ₃ I ₂ ] 0.6g / l

Formaldehyde 0.9 to 1.1 g / l

Sodium sulfite 7.7 to 8.3 g / l

Ethylenediamine 0.7 to 0.8 g / l

Ethylenediaminetetraacetic acid 0.9 to 1.0 g / l

di-sodium salt

Potassium bromide 0.8 to 1.2 g / l

In addition, 0.1 to 10 mg of thallium I-nitrate may be added to the gilding bath to accelerate deposition.

The pH of the solution is adjusted in the range 7 to 13. The bath temperature should be kept between 355 and 371K.

The bath load can be between 1 and 1OcJmVl. The deposition time is in the range of 0.5 to 10 minutes. The deposition rate is relatively constant until near the exhaustion of the bath.

The adhesion of the gold layers produced on the nickel is very good and exceeds z. B. the breaking strength of silicon wafers.

The best results in terms of tightness of the gold layers, d. H. a particularly fine-grained and planar structure, resulting in the gold plating of electrolessly deposited nickel layers. Even with galvanically deposited or sputtered nickel layers results satisfactory results.

As a rule, before gilding, it is necessary to decap the nickel surfaces in a known manner. After that, a thorough rinsing of the substrates is necessary in each case to avoid the carryover of acid into the gilding solution.

It is beneficial to preheat the nickel plated substrates immediately prior to immersion in the gold bath in hot deionized water.

Below, the invention will be explained in more detail in some embodiments:

1st embodiment

It should be gilded glass passivated silicon wafers, which were chemically nickel-plated on both sides by a known method. For this purpose, the silicon wafers are placed upright in a plastic magazine and etched in the first step 10s in buffered hydrofluoric acid, then rinsed for 60 seconds in flowing deionized water, then dipped for 30 seconds in deionized water with a bath temperature of 363K and after Expiration of this time immediately converted into a gilding bath with the following composition:

GoHaIsNa ₃ [Au (SO ₃ I ₂ ] 0.6g / l

Formaldehyde 1.0g / l

Sodium sulfite 8.0 g / l

Ethylenediamine 0.75g / l

Ethylenediaminetetraacetic acid - 1.0 g / l

di-sodium salt

Potassium bromide 1.0g / l

The temperature of the gold bath is 355 K, the pH is 12.0, the bath load is 9.5 dmVl.

After a gilding time of 2 minutes, the discs are removed from the gold bath, rinsed intensively in deionized water and dried. The gold layer thickness is 15 to 25 nm, the gold-plated chips are storable for several months in air, the solderability of the metal surfaces is retained without restriction.

2. Execution example

On both sides galvanically nickel plated molybdenum sheets with the dimensions (300 χ 50) mm ^{2 are to be} gold plated. Since the gilding takes place immediately after nickel plating, activation of the nickel surfaces can be dispensed with. After the rinsing process following galvanic plating, the racks of molybdenum sheets are immediately converted to a gold bath having the composition as in the first embodiment. The temperature of the gold bath is 358 K, the bath load 2dmVl.

During the period of 5 minutes 25 to 40 nm gold are deposited. After gilding, the substrates are rinsed in a known manner and dried. The solderability of the metallized molybdenum sheets is guaranteed for several months when stored in air.

3rd embodiment

Silicon wafers are to be gold-plated on one side, which were nickel-plated on the reverse in a known manner by sputtering. First, the masking of the wafer front pages with a commercially available protective coating. The stored substrates are dipped in 10% sulfuric acid for 60 seconds and then rinsed in flowing deionized water for 30 seconds. Thereafter, the slices 30s are preheated in deionized water, the temperature of which is 363K, and reacted in a gold bath containing 2.0 mg of thallium I-nitrate in addition to the composition shown in the first embodiment. The pH of the gilding solution was adjusted to 7.4 by the addition of sulfuric acid. At a bath temperature of 363 K, a gold layer of 35 to 50 nm thickness is deposited in 5 min at a bath load of 5dm ² / l, which allows the gilded substrates to be stored in air for several months, without the solderability is changed negatively.

Claims (1)

A method of preserving the solderability of nickel layers with a known pretreatment to activate the nickel layers, characterized in that 10 to 50 nm thick gold layers are deposited on the nickel layers by the nickel-plated substrates for the period of 0.5 to 10 min in a a known non-toxic, reductive gold bath with the composition Gold as Na ₃ [Au (SOa) ₂ ] 0.6g / l Formaldehyde 0.9 to 1.1 g / l Sodium sulfite 7.7 to 8.3 g / l Ethylenediamine 0.7 to 0.8 g / l Ethylenediaminetetraacetic acid 0.9 to 1.0 g / l di-sodium salt Potassium bromide 0.8 to 1.2 g / l,