WO1990008005A1

WO1990008005A1 - Preparing articles for soldering

Info

Publication number: WO1990008005A1
Application number: PCT/GB1990/000084
Authority: WO
Inventors: Norman Macleod
Original assignee: Cookson Group Plc
Priority date: 1989-01-19
Filing date: 1990-01-19
Publication date: 1990-07-26
Also published as: AU4947790A; GB8901114D0

Abstract

A method of preparing for soldering a surface of a composition comprising metallic particles distributed within an organic polymer binder characterised by including the step of irradiating at least part of the surface to be soldered with a controlled amount of ultraviolet electromagnetic radiation, preferably provided by a pulsed excimer laser, to ablate organic material from that surface and expose metallic particles.

Description

PREPARING ARTICLES FOR SOLDERING

The invention relates to a method of preparing articles for soldering. In particular, it relates to a method which is useful for improving the solderability of compositions comprising metal particles distributed within an organic polymer binder.

In recent years much research has been directed to the development of printed conductors employing thick film inks for use in electronic assemblies. Thick film inks are printable pastes comprising a conductive filler distributed within a curable or fusible binder. The conductive filler is generally in the form of powder or flakes of metal, for example silver or copper. When silver is used it is sometimes alloyed with palladium to inhibit electromigration while copper may be coated with silver or a silver palladium alloy to improve the solderability of the resulting ink. Nickel, gold aluminium, palladium and platinum/gold alloys have also been used.

The binders which are used fall into two classes; inorganic glasses and organic polymers. Silicate, phosphate or fluoride based glasses have been used. These bind by being fused at fairly high temperatures, for example in the range 600 to over 1000°C. Organic binders have the advantage that they bind at lower temperatures than most glasses, for example up to 300°C. They include compounds which would react together to form a polymer binder and thermoplastic polymers which would fuse in an analogous manner to glasses or dry in a similar fashion to paints. Reactive cross-linking polymers which, once cured, will withstand elevated temperatures are preferred. The most commonly used organic polymers are epoxies, polyesters, acrylics and silicones.

The use of thick film inks with organic binders in printed conductors for electronic assemblies has, however, been restricted by the difficulty of attaching components to the printed circuitry by soldering. The organic binder is non-wettable by solder unlike the metal filler and thus the solder will not adhere to the film. The difficulty in soldering despite relatively high metal filler contents is thought to arise from the fact that the organic polymers are effective wetting agents for metallic fillers and thus the filler.particles, especially those in the surface layers, are covered with a film of the organic binder and prevented from making direct contact with the solder.

Various techniques have been tried to improve the solderability of organic polymer thick films, for example by providing a copper layer on the surface of the polymer thick film by electroless copper plating, and success has also been claimed for electroless nickel coatings. Other workers have reported improvements using solvent based systems with complex organic mixtures, see EP 0169059A, and by using unconventionally high silver loading of between 88 and 95% by weight, see EP 0140585A.

However, there are disadvantages with using such specially formulated polymer thick films. For example, if there is a high metal content the cohesive strength is reduced and the films can fracture more easily leading to breaking away of devices attached to the films, as well as being more expensive.

The use of complex organic solvent mixtures on the other hand may lead to attack on the substrate and also can have environmental disposal problems. In our copending PCT patent application no. PCT/GB89/01083 filed on 14th September 1989 we describe a method of improving solderability of a polymer thick film by etching at least part of the surface to be soldered by bombardment with energetic atomic or molecular species. A drawback of this method is that it must be performed in a vacuum and, unless a mask is employed, the technique cannot be used selectively to etch one area but not another. It has now surprisingly been found that the solderability of polymer thick films can be improved by direct treatment of conventionally loaded films without the need for a vacuum and in a manner which provides a high degree of control. In accordance with the present invention there is provided a method of preparing for soldering a surface of a composition comprising metallic particles distributed within an organic polymer binder characterised by including the step of irradiating at least part of the surface to be soldered with a controlled amount of ultraviolet electromagnetic radiation to ablate organic material from that surface and expose metallic particles.

By "metallic particles" is meant both particles consisting wholly of metal and particles which have a metallic coating on a non-metallic substrate.

Irradiation of the surface of such a composition with a controlled amount of ultraviolet electromagnetic radiation removes a surface layer of the organic polymer by ablation and thus exposes metallic particles comprised within the composition and previously covered by polymer binder. In this way the solderability of the composition is improved. Most organic materials absorb strongly in the ultraviolet region and the energy absorbed at these wavelengths can break the bonds between the atoms. At higher wavelengths (lower energy) bond fission occurs less readily and may be further diminished by the fact that absorption rates are generally lower at higher wavelengths. The strong absorption with ultraviolet means that when such light is shone on organic material absorption tends to occur in only a thin layer near the surface. Thus it is in that surface layer that bonds are broken and the material tends to decompose ablatively, that is, volatile molecular fragments break off from the surface.

Depending on the material being ablated the ablation conditions, e.g. wavelength, energy density, can be adjusted so that there is little redeposition of molecular fragments and thus little contamination of the exposed surface or neighbouring surfaces.

Compositions which have been ablated in this way exhibit improved solderability compared with unablated compositions. This is thought to be because more metallic material is exposed in the new surface than in the original surface.

Ultraviolet absorption is known to be strong in epoxy groups and other organic groups and this absorption enables the removal of such organic material by the method of the present invention. The organic polymer binder in the composition may be any known binder such as epoxy, acrylic, polyester or silicone. A preferred binder is epoxy resin.

The metallic particles distributed within the organic polymer may be, for example, silver, a silver/palladium alloy, palladium, nickel, aluminium, copper or a platinum/gold alloy. Preferably they comprise silver.

Solderability of polymer thick films generally improves as the amount of metallic particles present in the composition increases. However, to maintain adequate mechanical strength in the composite, the amount of metallic particles will be as low as possible while still maintaining the required conductivity. Typically, the composition will comprise at least 20 vol% metallic particles (which is equivalent to at least about 70 wt% for particles consisting wholly of metal) but lower amounts may be used.

The ultraviolet radiation will preferably be produced by a laser. This is because lasers provide easily controllable amounts of radiation. The radiation from lasers is usually highly monochromatic coherent and of high power. Laser light can be focussed to one particular area of a composition to be ablated and soldered without causing significant ablation of other nearby areas of the composition. However, in some cases where it is desired to irradiate a more general area a broad-area beam may be used. A mask may then be employed to cover areas which are to be protected from ablation.

One particular class of lasers which emit ultraviolet light are excimer lasers which are generally used to provide pulsed radiation and such lasers are preferably used in the method of the present invention. These employ compounds known as excimers which have no stable ground state but that may have excited states when temporarily bound to other molecules. Examples are lasers which employ mixtures of noble gases with halogens. These include the following which provide outputs at the indicated wavelength.

argon fluoride (ArF) : 193 n krypton chloride (KrCl) : 222 nm krypton fluoride (KrF) : 248 nm xenon chloride (XeCl) : 308 nm xenon fluoride (XeF) : 351 nm

F₂ gas may also be employed and this produces an output at 157 nm.

An article entitled Excimer Lasers by Poulin et al, P.C. Fab. June 1988 describes the nature of excimer lasers and discusses some of the applications they are finding in the printed circuit industry. It discusses the process of ablation in respect of drilling dielectric films, removing dielectric films from metallic substrates or the metal films from dielectric substrates.

It is preferable to employ pulsed radiation of short duration such as that produced by excimer lasers in the process of the present invention to reduce the likelihood of heating the area beside and beneath the area being ablated. Heating is not desirable because it may cause deterioration of the heated article, e.g. printed circuit board, polymer circuit, or induce undesirable oxidation of the metallic filler. If the excimer beam is not collimated it may be focussed using quartz lenses or mirrors to produce a beam of the desired size, for example up to 2cm². In this way a broad area can be ablated by a pulse of the beam without the need to scan the area as is the case for conventional collimated laser beams.

Particularly, favourable results have been obtained using an argon fluoride laser providing ultraviolet radiation of wavelength 193nm and operating at an average energy density of 40 to 50₀ mJ/cm² and a krypton fluoride laser providing ultraviolet radiation of wavelength 248 nm and operating at an average energy density of 100 to 5₀₀ mJ/cm². The action of the ultraviolet laser radiation causes the break up and spontaneous removal of material from the surface of an organic polymeric solid by a process which has been termed "ablative photodecomposition". Over an area that is defined by the light beam, the surface of the solid may be etched away to a depth of 0.1 μra. to 4 μ_m at every pulse and the products expelled at supersonic velocities of 1000 to 2000 m/s. The angular distribution of the ablated material for e.g. 193 nm radiation directed at a polyimide film is generally within 25 - 30° of the surface normal for photochemical ablation.

For most polymers, significant etching occurs when the laser fluence (average energy density impinging on a unit area of the surface to be ablated expressed in mJ/cm²) is above a certain threshold.

Close but below the threshold fluence, etch rates are often insignificant to measure, and polymer materials may suffer thermal degradation as energy is converted into heat. The threshold fluence varies with laser wavelength and the organic material. It ranges from e.g. 10 mJ/cm² for polymethyl methacrylate at 193 nm to 130 mJ/cm² for polyimide at 351 nm. Above the threshold fluence, each pulse ablates the same amount of material and the etch depth per pulse (If at wavelength λ) roughly obeys the relationship

If = In (F/F_t)

Where a is the absorption coefficient of the polymer (cm^-1) at the wavelength of the laser beam, F the laser fluence and F-t the threshold fluence. The threshold fluence for three typical cured epoxy resins is given below for radiation of 193 nm and 248 nm.

Wavelength/nm

Threshold fluence /mJ/cm²

The compositions of the epoxies is given below.

Epoxy 1 : Dow Chemical resin DER 351 cured with an acid anhydride (hexahydrophthalic anhydride) in an amount of 90 parts per hundred of resin.

Epoxy 2 : Dow Chemical resin DER 351 cured with a modified polya ine (DP 500 from Anchor

Chemicals) in an amount of 70 parts per hundred of resin.

Epoxy 3 : Dow Chemical resin DER 351 cured with an imidazole (2E4MZ-CN from Anchor Chemicals) in an amount of 10 parts per hundred of resin.

It can be seen that for a particular epoxy resin, in this case DER 351, and different curing agents the threshold fluence varies very little at 193 nm although the variation is a little more marked at the higher wavelength of 248 nm.

Bearing in mind these threshold fluences it is preferred in the present invention, particularly for epoxy resin, to provide ultraviolet radiation with an average energy density of 40 to 500 mJ/cm² at a wavelength of 193 nm and an average energy density of 100 to 500 mJ/cm² at a wavelength of 248 nm.

Typically the radiation provided by an excimer laser will be in pulses having a fixed duration in the range 10 to 25 ns and a variable frequency. Particularly favourable results have been obtained at a frequency of about 1Hz. Higher frequencies up to about 200 Hz may be employed but as the frequency increases the likelihood of producing unwanted heating increases and so lower frequencies of the order of 0.5 to 10 Hz are preferred.

It does not take very many pulses to achieve the desired effect and, indeed, if too many pulses are employed metallic material itself may be removed from the ablated area leaving a depleted metal content and having detrimental effect on the solderability. This metallic material may also redeposit around the ablated area leading to undesirable non-uniform effects. Thus preferably the surface to be ablated will be irradiated by 1 to 20 pulses, more preferably 5 to 10 pulses.

Ease of ablation may be improved by irradiating the surface with broad spectrum ultraviolet before employing the laser. Thus in a preferred embodiment of the invention the surface is irradiated with an ultraviolet lamp prior to irradiation by a laser. Compositions comprising metallic particles distributed within an organic polymer binder which have been irradiated in accordance with the present invention may be used in printed circuits which are to be soldered. Thus the present invention also includes a printed circuit prepared by the method of the invention and to which a component has subsequently been soldered. The present invention also includes a method of soldering a surface of a composition comprising metallic particles distributed within an organic polymer binder comprising the steps of

(i) irradiating at least part of the surface to be soldered with a controlled amount of ultraviolet electromagnetic radiation to ablate organic material from that surface and expose metallic particles, and thereafter

(ii) soldering to the ablated surface.

The present invention is illustrated by the following Examples and by the drawings in which:

Figures 1 to 4 are photographs showing the results of copper deposition tests on samples ablated under different conditions.

Figure 5 is a plot of current density against number of pulses for conductivity tests carried out on samples ablated under different conditions. For the examples, an excimer laser (model

Questek 2460) which delivers a 2cm² pulsed beam with pulses of about 20 ns duration was used.

Example 1 A four inch square of FR4 epoxy fiber glass board was printed with silver loaded epoxy resin thick film ink of the composition given below using a screen printer to produce a test pattern of the thick film ink on the FR4 board. The epoxy resin was then cured for 1 hour at 180°C.

Certain parts of the test pattern were then subjected to ultraviolet irradiation from an argon fluoride excimer laser with an output at 193 nanometres to ablate the surface. The laser beam was kept in a fixed position while the board was moved in front of it to enable irradiation of different parts of the board. The irradiated parts of the board were each subjected to 20 pulses (1 Hertz) each pulse having an average energy density of 240mJ/cm².

The test pattern on the board was then tested for solderability using Alpha solder wire of 62 wt% tin, 36 wt% lead and 2 wt% silver with 2% by weight of the wire of a core of activated rosin flux as supplied by Alpha Metals Inc. The areas of the test pattern which had been irradiated were easily solderable with the solder forming a strong bond with the polymer thick film. Parts of the test pattern which had not been irradiated were also tested for solderability as a control. These areas were not solderable; the solder balled up, did not wet the substrate, and would not adhere.

The composition of the thick film ink was:

% by weight Epoxy resin 20.42

Dicyandiamide 1.02 Silver flake 77.00

Gamma glycidyloxypropyltrimethoxysilane 1.56

Example 2

Because it is difficult to measure solderability numerically a copper electroplating technique has been used to give an indication of the quantity of metal exposed by ablation of the surface of the polymer thick film so that the effect of different radiation conditions could be assessed. Using a laser beam with a wavelength of 248 nm polymer thick films of an epoxy resin containing 78% by weight of silver were irradiated with different numbers of pulses at a rate of 1 Hz and at different energy densities. The copper deposition tests were then carried out on the ablated areas at a current density of 10 mA/cm² during 90 s. Two different polymer thick films were employed (PTF1 and PTF2) . The compositions of these are given below.

PTF1 wt%

Epoxy resin (DER 351 from Dow Chemicals) 19.47 Curing agent (DMPF, Casamid from Thomas 0.97 Swan & Co. )

Silver flake (SF 282 from Handy & Harman) 78.00 Silane (A 187 from Union Carbide) 1.56

PTF2

Epoxy resin (DER 351)

Curing agent (cishexahydrophthalic anhydride)

Silver flake (SF 282)

Silane (A 187)

The Experimental conditions for the tests are as follows:

Experiment 1 epoxy : PTF1 wavelength : 2 8 nm

Test strip Energy Density No. of Pulses /mJ/cm²

1 300 1, 5, 10, 20, 50 2 500 1, 5, 10, 20, 50 3 1000 1, 5, 10, 20

4 2000 1, 2, 5, 10, 20, 50 - 13 -

Experiment 2 epoxy : PTF2 wavelength : 248 nm

Experiment 3 epoxy : PTF1 wavelength : 193 nm

Experiment 4 epoxy : PTF2 wavelength : 193 nm

Photographs showing the results of the copper deposition tests are shown in Figures 1 to 4. All spot diameters are 3.5 mm (equivalent to the diameter of the fσcussed laser beam) except for the fluence 2000 mJ/cm² where the diameter is 2.0 mm. The number of pulses increase from left to right in accordance with the conditions quoted above.

Examination of the photographs clearly shows that the main parameter affecting the effectiveness of the ablation and subsequent copper deposition is fluence and to a lesser extent the number of pulses, the nature of the polymer, and the laser wavelength.

The change in behaviour with increasing energy density (fluence) for a set number of pulses is well illustrated by the pulse samples of Experiment 4 (Figure 4) subjected to radiation of 193 nm. In test 1 (150 mJ/cm²) slightly non-uniform copper deposition is obtained in the irradiated spot. In test 2 (250 mJ/cm²) much better coverage is achieved. In test 3 (500 mJ/cm²) the coverage appears even more dense and uniform. However, in test 4 (1250 mJ/cm²) too much energy has apparently been supplied causing the metallic particles in the polymer thick film to be themselves removed from the ablated area and redeposited outside it. This has led to the formation of a copper halo around the ablated spot during deposition. In practice this effect on a printed circuit could bridge adjacent conductors and cause the circuit to fail.

In Experiment 1 at 248 nm (Figure 1) the effect of increasing fluence for a set number of pulses is slightly different but still detrimental. Here, as the fluence increases the centre of the ablated area may itself become non-conductive and so no copper plating is observed. This effect increases with increased numbers of pulses. This effect may be because most of the available metallic particles are removed from the centre of the ablated area.

The effectiveness of the ablation at exposing metallic particles has also been measured by the conductivity method described below.

Conductivity tests were performed on two samples of PTF2 ablated at 150 mJ/cm² and 250 mJ/cm² respectively. Radiation of 193 nm was employed for differing numbers of pulses. The tests were carried out in a standard electrochemical cell with a 0.5 M CΛ1SO4 solution containing 50 ml/1 H2SO₄ and 1 ml/1 HC1. A platinum foil served as a counter-electrode. The potential versus S.C.E. was set at a fixed value of 0.040 V and the current was measured as a function of time. The results of normalised current density against number of pulses are plotted in Figure 5.

These show a progressive ablation and thus increased current density with increase in number of pulses from 1 to 10 but little improvement is seen for greater numbers of pulsesfor these particular conditions.

Claims

CLAIMS :

1. A method of preparing for soldering a surface of a composition comprising metallic particles distributed within an organic polymer binder characterised by including the step of irradiating at least part of the surface to be soldered with a controlled amount of ultraviolet electromagnetic radiation to ablate organic material from that surface and expose metallic particles.

2. A method as claimed in claim 2 characterised in that the organic polymer comprises an epoxy resin.

3. A method as claimed in claim 1 or claim 2 characterised in that the metallic particles comprise silver.

4. A method as claimed in any one of claims 1 to 3 characterised in that the surface is irradiated with ultraviolet radiation produced by a laser.

5. A method as claimed in claim 4 characterised in that the laser is an excimer laser providing pulsed ultraviolet radiation.

6. A method as claimed in claim 5 characterised in that the laser is an argon fluoride laser providing ultraviolet radiation of wavelength 193 nm and operating at an average energy density of 40 to 500 mJ/cm² or a krypton fluoride laser providing ultraviolet radiation of wavelength 248 nm and operating at an average energy density of 100 to 500 mJ/cm².

7. A method as claimed in claim 5 or claim 6 characterised in that the radiation is provided in pulses of 10 to 25 ns duration at a frequency of about 1 Hz.

8. A method as claimed in any one of claims 5 to 7 characterised in that the radiation is provided in pulses with a frequency in the range 0.5 to 10 Hz.

9. A method as claimed in any one of claims 5 to 8 characterised in that the surface is irradiated by 1 to 20 pulses, preferably 5 to 10 pulses.

10. A method as claimed in any one of claims 5 to 9 characterised in that the surface is irradiated with an ultraviolet lamp prior to irradiation by a laser.

11. A printed circuit prepared for soldering by a method as claimed in any one claims 1 to 10 to which a component has subsequently been soldered.

12. A method of soldering a surface of a composition comprising metallic particles distributed within an organic polymer binder comprising the steps of

(ii) soldering to the ablated surface.