Solder Fluxes Bearing Oxide Removers Generated by Light

The invention relates to solder flux formulations, and in one aspect, more particularly relates to solder flux formulations incorporating materials that react during soldering to produce useful volatile compounds.

Solder formulations, also known as solder creams or solder pastes, are homogeneous blends of a soft solder alloy typically in a powder form dispersed in a liquid medium conventionally containing a fluxing composition or flux, an organic solvent, and a thickening agent which will give the desired viscous or paste-like consistency to the solder formulation. Such solder formulations can be applied to the surfaces or locations requiring soldering in a number of various ways, such as by screen printing, or by means of a dispenser such as a syringe, or simply by dipping the site to be soldered into the solder paste formulation so that the viscous paste adheres to the site, such as an electronic component lead.

Recently, solder paste formulations have been used increasingly by the electronics industry, particularly in the automated manufacture of printed circuits in which leadless miniature electronic components are surface mounted on a printed circuit board (PCB) to which a solder pate formulation has previously been applied, such as by screen printing. The PCB is then subjected to a sufficiently high temperature, for example by means of a heated conveyor belt, to cause the flux and solder alloy in the formulation to liquefy and contact the electronic component leads so that on subsequent cooling of the PCB, the components will remain soldered in situ on the PCB.

For some uses in the electronics industry, it is desirable to use as the flux composition of the solder formulation a material which is non-corrosive and which will provide, after the heating and cooling steps, flux residues which are themselves non-corrosive and non-conducting. For this reason, rosin-based flux compositions are widely used in the commercially available solder pate formulations specifically made for use in the manufacture of surface mounted electronic components.

Alternatively, more reactive fluxing compositions may be used, which leave residues which are corrosive and/or conductive. Often a somewhat corrosive fluxing composition is desired so that the oxides which form on the metal surfaces may be removed to permit the subsequently formed solder bond to be stronger both physically and electrically. However, it is necessary to remove the residues formed by means of ether aqueous or organic solvent systems to ensure that the resulting soldered circuit is non-corrosive.

The use of the solder paste formulations containing such rosin-based or more reactive fluxes produces a number of disadvantages. First, because non-corrosive residues tend to be sticky, they prevent repetitive automatic testing of the circuit. Additionally, such residues are unsightly and therefor, as with the corrosive flux residues which are also unattractive, will need to be removed. The removal step involves extra production equipment, time and material.

Secondly, flux residues tend to be hygroscopic and may thereby cause spattering. Thirdly, some fluxes permit solder particles in the paste to move away from the solder site and give rise to the formation of numbers of discrete small balls of soft solder around the soldered joints, which can create electrical short circuits.

Because of these and other disadvantages, it is desirable and often essential to meet specifications, to remove the flux residues and any solder balls as much as possible. Often, however, their removal is difficult or impossible, particularly from areas of the PCB underneath the electronic components.

As noted, a common procedure is to use an aqueous or organic solvent in the removal of flux residues. Though water is preferred because it will not leave an objectionable residue itself, water typically is an ineffective agent, since many of the residues are only slightly soluble in water. Organic solvents are more effective, but less desirable because they are more expensive and particularly because they are more troublesome to dispose of. A particular class or organic solvents that had attained widespread use was the halocarbons, such as the chlorofluorocarbons (CFCs), because they would volatilize after cleaning. However, these materials are particularly inert and their eventual decomposition is suspected to be involved in the undesirable depletion of atmospheric ozone.

Thus, for these and other reasons the prior solder fluxing compositions are less preferred, and it would therefore be advantageous to discover a new fluxing composition that would avoid one or more of these disadvantages.

Accordingly, it is an object of the present invention to provide a solder fluxing composition that would easily and controllably generate an oxide removing agent during the soldering process.

It is another object of the present invention to provide a novel fluxing agent which generates an oxide removing agent that would either readily volatilize or be readily removed with water.

It is yet another object of the invention to provide a fluxing composition that would have little or no environmental concerns.

In carrying out these and other objects of the invention, there is provided, in one form, a fluxing composition for a solder formulation, which fluxing composition has a compound that will release an oxide removing agent upon exposure of the compound to light.

It has been discovered that an excellent fluxing composition may be provided by incorporating a compound that will release or generate an oxide removing agent upon exposure to light or a combination of light and heat. It has also been discovered that there are a number of ways to implement this concept. The oxide removing agent may be an acid, but specifically a photoacid, that is, one which is generated or produced by a particular compound upon exposure to light or light and heat.

Onium salts are compounds which may be incorporated into solder flux compositions to allow the photoinitiation of acid production. As noted, the acid formed acts to remove oxides which would otherwise prevent efficient solder reflow operations. The onium salt is a cation/anion pair. Cations which are suitable for onium salts include, but are not limited to, aryldiazonium (ArN.sub.2 +), diaryliodinium (Ar.sub.2 l+), triarylsulfonium (Ar.sub.3 S+), diarylalkylsulfonium (Ar.sub.2 RS+), phenarylsulfonium, etc. and mixtures thereof, where Ar is an aryl group. The anion is selected from the group including, but not limited to tetrafluoroborate (BF.sub.4 -), hexafluorophosphate (PF.sub.6 -), hexafluoroarsenate (AsF.sub.6 -), hexafluoroantimonate (SbF.sub.6 -), etc. and mixtures thereof.

The salts produced by the cation/anion pairs of this list produce acids of the form HX, where X is any of the listed anions when exposed to light, particularly ultraviolet light. Heat may also be required for some compounds, but not necessary for others. Of course, the compounds will be heated during the soldering step. Non-metallic onium salts and non-ionic photoacid generators are also expected to useful. Such materials include, but are not limited to certain halogenated hydrocarbons and aromatics, including, but not limited to: o-nitrobenzyl alcohol sulfonates, oxime sulfonates, naphthoquinonediazide-4-sulfonates, trichloromethyl-substituted s-triazines, o,o'-dihalogenated phenols, dinitrobenzylesters, etc., and mixtures thereof. Organic polymeric materials may also be useful.

Compounds that undergo photodegradation are the furyl compounds which contain a carbonyl group. Such compounds may have the structure: ##STR1## where R is a substituent which contains a carbonyl group, for example the moieties --COOH or --CH.dbd.CH--COOH. Particular furyl compounds include, but are not limited to 2-furoic acid, furylacrylic acid, furoylacrolein, furylacetate, furylacetophenone, furylacetic acid, furylbenzoic acid, furylcarboxylic acid, furylglyoxylic acid, etc., and mixtures thereof. In this case the active compounds are the furyl compounds shown above and the compound is broken down by light into lower molecular weight components that readily volatilize after they clean oxides form the PCB. Typically, the decomposition products are furan and a low molecular weight acid, and may also include carbon monoxide or carbon dioxide. In other words, photovolatilization of solder flux components prevents the formation of fluxing composition residues when used with non-crosslinking chemistries that would otherwise react with the compound fragments. This volatilization again reduces or precludes the need for any board cleaning with ozone-depleting CFCs after solder reflow. Ultraviolet radiation-initiated Norrish cleavage reactions at elevated temperatures (such as those encountered during solder reflow) break down the residue forming solder flux components which clean the board and then volatilize, leaving a board with very little or no residue.

Although the system could be designed to respond to any specific wavelength range, a particularly useful region is the ultraviolet (UV). This specific wavelength range permits the photoinitiation to be controlled so that it cannot occur too early or late. Additionally, UV light sources are readily available.

With any of these compounds and methods of this invention, retooling would simply involve the installation of a UV transparent window and a UV exposure tool in the existing assembly line. While some benefit may be obtained form exposing the solder formulation to UV light before and/or after soldering, it is expected that the greatest benefit will be achieved by simply blanketing the assembly with UV light during the solder reflow operation. If some residue does remain with certain of these systems, it will further be appreciated that they will be relatively low molecular weight systems and in most, if not all cases, may be washed away with water. In the case of the metal onium salts, some metals may be entrained in the water, which metals may have to be removed. Depending on the exact organic fragments from the non-metallic compounds, water to rinse them may need to be treated as well. Nevertheless, these concerns are appreciably less than those presented by the CFC agents.

In one aspect, the proportion of oxide remover generating compound should be at least 1 wt. % of the overall fluxing composition. It is preferred that the compound proportion be at least 3 wt. % It will be appreciated that the proportion of compound as a proportion of the overall solder formulation will vary depending on the particular formulation. The balance of the fluxing composition may be any of the customary materials, for example, propylene carbonate may be used as a carrier material. It will be appreciated that although the other common materials, such as the rosins, may be used in conjunction with the compounds of the invention, that some of these typical materials contribute to flux residues and should not be employed to take full advantage of the little or no residues provided by the flux compositions of the present invention.

While the fluxing compositions described herein may be integrally mixed with the solder powder, such as a lead/tin powder, solvent and thickener in a conventional manner, it has also been discovered that the fluxing compositions may also be effective simply by being coated over the solder surfaces prior to reflow.

The fluxing compositions of the present invention may also include an ultraviolet sensitizer. Specifically preferred are ultraviolet triplet sensitizers. These triplet photosensitizers are used to shift the optimum wavelength for UV radiation into the region supplied by most common Hg--Xe exposure sources. These are compounds which readily absorb UV energy to excite their electron pairs from the singlet electronic state to the triplet electronic state. As the pairs go back to the singlet state energy is discharged to the compounds causing them to cleave and release the oxide cleaning agents. The triplet photosensitizers are specific molecules that help the fluxing compositions absorb UV energy and transmit it to the compounds for use. The sensitizers are selected based on optimum triplet excited state lifetimes and intersystem crossing efficiencies. Some seemingly related compounds release energy too fast or in an usuable form and are thus unsuitable.

Acceptable triplet photosensitizers include, but are not limited to acetophenone, benzophenone, Michler's ketone (tetramethyldiaminobenzophenone), triphenylene, naphthalene, derivatives of acetophenone, 2-phenylacetophenone and its derivatives, ketocoumarins, thioxanthones, and the like, and mixtures thereof. Solder flux compositions utilizing the acetophenone triplet sensitizer were found to be particularly effective as triplet energy transfer agents in solder paste formulations. The proportion of sensitizer in the fluxing compositions should be about 30 wt. % of less, for example, from 0.1 to about 30 wt. %, and in one aspect from about 3 to about 10 wt. %.

The invention will be described in more detail with respect to the following illustrative examples.

Fluxing compositions having the indicated compositions in Table 1 were prepared and reflowed at 230.degree. C. Other components were used as solvents, and they are noted. The onion salt system for all Examples, except Examples 5, 9 and 10, was (C.sub.6 H.sub.5).sub.3 S+/AsF.sub.6 -. Comparative Examples 5, 9 and 10 used no onium salt. The amount of residue for each example is indicated. In each of these Examples, UV light was present during and preceding the reflow. When no UV light was used for these same materials, considerably more residue was noted, if reflow occurred at all. The relative notations for residue are based on the maximum residue observed when acid content was insufficient to promote reflow. It may be seen that the formulations of Examples 4 and 7 using the onium salt gave appreciably less residue.

These Examples were conducted similarly to Examples 1-10 with the indicated results. Surprisingly, in the case of the furyl compounds, little or nor residue was noticed when UV light was used, as compared with Example 11 when no UV assistance was employed.

A copper tape was dipped in malic acid and then rinsed in deionized (DI) water. A fluxing composition of 619.4 mg. ethylene carbonate (265.2+344.2), 267.6 mg. acetophenone and 35 mg. furylacrylic acid. The proportions in this composition were 67 wt. % ethylene carbonate, 29 wt. % acetophenone and 4 wt. % furylacrylic acid. Three solder balls were placed on the tape and coated with the fluxing composition. A 50 sec. reflow of this material at 245.degree. C. on the tape which included a 40 sec. UV treatment produced bonded solder balls with extremely little residue, even before washing. Thus, this experiment indicates that the triplet photosensitizers can materially help transfer energy to make the photoinitiated acid production more effective, and also that the fluxing compositions may be added topically to the solder as an alternative to being integrally mixed therewith.

It will be appreciated that modifications may be made in the exact implementation of the invention illustrated in the above examples which would still fall within the spirit and scope of the invention as claimed herein. For example, it is anticipated that the processing conditions, modes or sequences of addition of the fluxing compositions, and exact combinations of flux components may be altered to optimize the invention by one skilled in the art. It is also expected that the method of this invention could be used to assemble PCBs, including ones bearing surface mount devices, more economically by potentially eliminating a cleaning step, but without the use of CFCs.