WO2008009475A2

WO2008009475A2 - Solder flux

Info

Publication number: WO2008009475A2
Application number: PCT/EP2007/006489
Authority: WO
Inventors: Frank Timothy Lawrence
Original assignee: Henkel Loctite Adhesives Ltd.
Priority date: 2006-07-20
Filing date: 2007-07-20
Publication date: 2008-01-24
Also published as: IE20060535A1; WO2008009475A3

Abstract

A soldering flux composition comprising: a carrier vehicle comprising a solvent; an activator component for activating a metal surface for soldering; a component selected from one or more compounds within the structural formula I, wherein each R is independently selected from C1-C15 alkyl, C6-C15 aryl, C2-C15 alkenyl; C2-C15 alkynyl; C1-C15 alkoxy, or derivatives thereof for example halogenated derivatives thereof. Quadrol is one suitable compound. The composition of the invention provides good through hole fill such as in wave flow soldering processes.

Description

Solder Flux

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to the field of fluxes in particular liquid flux material. Such liquid flux materials act as fluxes for solder materials. Of particular interest within the present invention are fluxes of the type suitable for use in processes where the article to be soldered is treated with flux and usually also conveyed over the solder material, such as in wave soldering processes. These are commonplace for use in electronic components to circuit boards such as printed circuit boards ("PCB' s") and the like.

Description of Related Technology

Soldering processes are well-known. These range from hand- soldering methods to machine soldering methods. It is well-known to use a flux material with the soldering material. The flux material fulfils a number of functions. Generally a main function of the flux material is to ensure that conductive material of a first component, for example a support substrate such as a PCB interconnects well with good electrical conduction across the solder interconnect to a second component such as an electronic component. Utilising solder alone will not form an interconnect. To form a successful interconnection it is necessary to apply a fluxing material. A number of different alternatives have been used. It is known for example to include a solid flux material within the soldering material. Generally such a soldering material would then be provided in a wire or other such solid form which would incorporate a core of flux material running through the solder. As the solder melts upon heating, the flux is activated, and the resulting interconnect formed in the soldering process is of an acceptable standard.

It is also known to use solder paste materials. Solder pastes are generally homogenous, stable suspensions of solder particles in a flux medium material.

Reflow soldering is used in the automated manufacture of PCB' s, wherein electronic components are surface mounted on PCB' s to which a solder paste material has previously been applied by a method such as screen or stencil printing or dispensing. The PCB is then subjected to a sufficiently high temperature, to cause the solder to liquefy and to join the components in place on the PCB. The heat can be supplied by, for example, infrared, heated conveyor belt or convective means.

In the past the majority of solder compositions used were of the tin-lead type which proved useful in most common applications. The flux materials of the present invention may be employed with such solder compositions.

The use of lead in solder composition has come under regulatory and political scrutiny and has become undesirable for environmental and health reasons. As a result, attempts have been made to eliminate the requirement for lead from solder compositions. The leading so-called lead-free solder compositions typically include tin, copper and silver (typically 95.5% by weight tin, 4% by weight copper and 0.5% by weight silver) . Bismuth may also be used together with tin, antimony and silver in a range of approximately 1.0% to 4.5% by weight. Patents directed to lead- free alloys include U.S. Patent Nos . 1,437,641, 3,607,253, 4,042,725, 4,170,472, 4,667,871, 4,670,217, 4,695,428, 4,758,407, 4,778,733, 4,806,309, 4,879,096, 4,929,423, 5,094,813, 5,102,748, 5,147,471, 5,242,658, 5,256,370, 5,316,205, 5,320,272, 5,328,660, 5,344,607, 5,352,407, 5,390,080, 5,393,489, 5,405,577, 5,411,703, 5,414,303, 5,429,689, 5,435,968, 5,439,639, 5,452,842, 5,455,004, 5,527,628, 5,538,686, 5,569,433, 5,580,520, 5,658,528, 5,698,160, 5,718,868, 5,730,932, 5,733,501, 5,755,896, 5,762,866, 5,817,194, 5,837,191, 5,843,371, 5,851,482, 5,863,493, 5,874,043, 5,918,795, and 6,231,691; European Patent Documents EP-A-O 251 611, EP-A-O 336 575, EP-A-O 629 463, EP-A-O 629 464, EP-A-O 629 465, EP-A-O 629 466, and EP-A-O 629 467; Great Britain Patent Document GB-A-2 , 158 , 459 ; Japanese Patent Documents JP-A- 5050286, and JP-A-8230598 ; and International Patent Publication WO-A- 94/2563 the disclosures of each of which are hereby expressly incorporated herein by reference.

There has been a specific requirement to have good through-hole fill in certain commercial applications. Many substrates for electronic components such as PCBs have through holes, that is non-blind vias, which run through the substrate (perpendicular to the longitudinal axis of the substrate) . Generally such through holes are dimensioned and disposed to receive a conductive pin or leg of a component to be affixed to the substrate. The diameter of the through hole may vary depending on the application in question. For example typical through-hole diameters can range from 0.4 mm to 2.0 mm. Generally the side walls of the PCB about the through-hole will be metallised for example with copper forming a metal jacket about the hole. It is important therefore to have good electrical conductivity from the electronic component to the metal jacket of the through-hole.

The conductive pin of the component will generally extend into and through the through-hole. Generally, the conductive pin will occupy a good part of the through-hole. It is important in such cases to ensure that a sufficient amount of solder is present in the through-hole to secure the pin of the component in place, and that an electrical interconnect is formed by way of a copper-tin intermetallic compound. Generally the conductive pin will be a wire of relatively small diameter.

Some components may be initially held in place by some form of adhesive material. In this way, the substrate is first populated with one or more electronic components which are affixed to the substrate. Soldering occurs afterwards. In cases where the components have pins, those pins will extend into the through-hole and possibly extend out the other side.

Components which generally sit on the substrate surface are often referred to as surface-mount components.

It is important therefore where at least a portion of through holes of a substrate are populated with electronic components, to ensure sufficient solder makes its way into the through-hole to firstly secure the (pin of the) component in place and secondly ensure good metal to metal electrical conduction across the solder for example from a pin of the component to the metal of the through-hole.

It is important therefore that sufficient solder makes its way into the through-hole. Once a component is in-situ it is usually necessary to have the solder applied from what is generally considered to be underneath or the underside of the substrate - that is the side of the article opposite to the side on which the electronic component is attached. For example where an assembly, comprising the assembled component and the substrate to which it is to be conductively attached, is conveyed over the solder material, such as in a solder wave application, it is often times difficult to ensure sufficient solder makes its way into the through-hole. That is, it is difficult to achieve a desirable through-hole fill. Generally speaking, the larger the diameter of the through-hole the more difficult it becomes to achieve a complete fill. This is because wicking/capillary travel of the solder lessens with increasing hole diameter.

Furthermore, the oxidisation state of the metal jacket of the through-hole very dramatically influences the wicking/capillary travel of the solder. Where the metal is unoxidised, good through-hole fill is achieved. Where however the metal is partially or completely oxidised, poor through-hole fill is generally achieved.

In the past, achieving a 50 to 75 percentage hole fill was generally deemed acceptable. As technology advances however, electronic component manufacturers generally are requiring a higher fill rate. In particular, the ultimately desirable result is a 100 percent fill of the through-hole. There is thus a need for technology which can achieve good and reliable hole fill.

It is particularly desirable to provide a technology which can be utilised in contact applications of the wave flow type. The present invention provides such a solution.

It has been generally recognised in the art that providing a flux material which achieves a number of functions is desirable. In particular it is desirable that the flux material provides good surface activation. In this respect, it has been known to include an activator component within the flux material which will act to remove oxidised material from a metal surface, thereby allowing better solder to metal interconnection and ultimately metal (PCB) to metal (electronic component) interconnection.

Activators are added to decompose and remove any oxide film existing in the portion where soldering is going to be carried out, and are usually organic compounds containing halides, typically amine hydrohalides , such as diethylamine hydrochloride or weak halide-free organic acids.

In particular, it is desirable to use a carboxylic acid which melts at a temperature in the range of from 75° to 250⁰C as described in European Patent Publication No. 0 620 077 A and in particular as defined in the flux compositions claimed therein.

SUMMARY OF THE INVENTION

The present invention provides a liquid soldering flux composition comprising: a carrier vehicle comprising a solvent; an activator component for activating a metal surface for soldering; and qυadrol, or a quadrol derivative or combinations thereof .

The quadrol or quadrol derivative of the soldering flux composition maybe selected from one or more compounds within the structural formula:

where each R is independently selected from Ci-Ci₅ alkyl, C₆-C₁₅ aryl, C₂-Ci₅ alkenyl; C₂-Ci₅ alkynyl; Ci-Ci₅ alkoxy, or derivatives thereof for example halogenated derivatives thereof .

In one embodiment each R is selected from C_x-C₅ alkyl and suitably at least one R is CH₃. It is desirable that each R is CH₃.

The flux of the invention provides good through-hole fill. The flux of the invention acts well across the various properties attributable to a good flux, for example removal of oxide layers from the metal surfaces on the PCB or other substrate, and protecting the clean joint surfaces from oxidation until soldering has taken place. This allows for an improved inter-metallic contact. The carrier vehicle of the flux composition may comprise at least one of a high boiling point solvent and a low boiling point solvent. The low boiling point solvent will generally act as initial carrier for application of the flux and will generally evaporate quickly leaving the remainder of the composition in-situ in the desired location.

A high boiling point solvent will not evaporate at ordinary room temperatures and will generally remain in place with the other components of the flux on the substrate to which it is applied thus the flux retains a fluid phase when substantially all the low boiling point solvent is no longer present .

When the flux is exposed to the higher temperatures typical of the heating station (s) of a wave soldering machine any remaining low boiling point solvent will generally be evaporated early in the pre-heating stages of the wave soldering machine . For example in a typical machine a substrate such as a PCB treated with flux is heated stepwise to a temperature of approximately 230⁰C. The high boiling point solvent thus provides a medium that will carry the active material in the absence of the low boiling point solvent carrier.

The low boiling point solvent is preferably at least one of alcohol or water. Isopropanol is one suitable alcohol .

The high boiling point solvent of the flux composition may comprise at least one Ci₀-Ci₅ alkane . Preferably the high boiling point solvent comprises at least one of glycol ethers, methyl ethyl ketone, and white spirit. Desirably the high boiling point solvent is a glycol ester for example of the formula: CH₃CH₂OCH₂CH₂OCH₂CH₃.

The present inventor has found that white spirit is a suitable high boiling point solvent. Other suitable high boiling point solvent include esters (for example di-basic esters) of a carboxylic acid which may be a least one methyl ester of a C₄- C₆ dicarboxylic acid. The carboxylic acid can be selected from at least one of succinic, glutaric or adipic acids.

The low boiling point solvent may have a boiling point below 150⁰C, for example the low boiling point solvent may have a boiling point of up to 120⁰C. The high boiling point solvent may have a boiling point above 180⁰C, for example the high boiling point solvent may have a boiling point of 200⁰C or greater.

The activator component of the flux composition is at least one of a carboxylic acid and a rosin.

Activators are usually added to decompose and remove any oxide film existing in the portion of the substrate where soldering is going to be carried out . For copper substrates, such as those conventionally used in PCBs, removal of oxidised copper is desirable.

The rosin may be used for its properties as a weak acidic activator but it is also added to protect the metals from oxidation for example as a protective coating. The overall effect of the flux of the present invention is that of a more definite, improved and robust interconnection between the electronic component and the metal on the substrate, for example copper on the board.

The rosin component may be at least one material selected from diproportionated rosin; hydrogenated rosin, dehydrogenated rosin, or unmodified rosin.

The water insoluble properties of rosin mean that, in general, when rosin is present in the composition the low boiling point solvent used is predominantly alcohol. The high boiling point of rosin ensures that it will remain in place until soldering occurs.

Rosin is a natural material obtained from natural sources such as the sap of pine trees and contains isomers of abietic acid. Synthetic resins may also be used. These materials may be referred to generically as resins and as used herein the term "resin" includes natural resins such as rosin, chemically modified rosin, and synthetic resins. To form compositions of the invention resins may be dispersed in solvents, together with additional activators.

The flux composition will generally comprise activator component present in the amount 0.1% to 20% by weight of the total composition. More desirably the flux composition will comprise activator component present in the amount 1% to 10% by weight of the total composition, for example the flux composition will comprise activator component present in the amount 0.5% to 5% by weight of the total composition such as 1.1% to 1.5% by weight of the total composition. The flux composition will preferably comprise at least one quadrol or quadrol derivative above present in the amount of 0.1% to 20% by weight of the total composition. More preferably the at least one compound will be present in the amount 1% to 10% by weight of the total composition, such as 0.5% to 5% by weight of the total composition for example 1.1% to 1.5% by weight of the total composition. Such compositions are suitable for many applications including those termed "no-wash" where there is no washing away of residue from the soldering process. For applications where there is a washing process, for example a water wash, it may be desirable to use an amount of quadrol or quadrol derivative in the range from 5% to 20% by weight of the composition.

The flux composition may comprise a high boiling point solvent present in the amount 2% to 50% by weight of the total composition. More desirably the flux composition will comprise a high boiling point solvent present in the amount 4% to 25% by weight of the total composition, such as 5% to 20% by weight of the total composition, for example 7% to 12% by weight of the total composition.

The remainder of the composition will generally be made up by the low boiling point solvent such as isopropyl alcohol .

The flux material is generally in liquid form and can be applied easily by spraying. The solute content of the flux can be 0.1%-10% by weight of the total composition such as 05%-12% by weight of the total composition, for example the solute content may be l%-8%, in particular the solute content can be 1.5%-6% by weight of the total composition. Desirably the solute content will be 3%-5% by weight of the total composition and most desirably the solute content will be 3%-4%. In general the remaining percentage will substantially be made up with solvent.

Solder can be used to attach an electronic component, for example a connection pin thereof, to a substrate with a metallic through hole defined therein by substantially filling the hole with solder composition, after the substrate has been treated with the flux composition of the present invention. The electronic component may be a surface component or it may be a through hole component comprising a pin which extends at least part of the way through the through-hole.

The present invention also provides a method of soldering a component to a metallic substrate comprising the steps of:

(i) treating the metallic substrate with a flux composition of the invention as described above; and

(ii) simultaneously or subsequently soldering the component to the metalised substrate.

The treatment step can be easily achieved by spraying the liquid flux material onto the substrate. The soldering step can be achieved by a wave flow process.

The present invention extends to the use of quadrol or quadrol derivative for example a compound within the structural formula:

where each R is independently selected from Ci-Ci₅ alkyl; C₆-Ci₅ aryl ; C₂-C₁₅ alkenyl ; C₂- Ci₅ alkynyl; C₁-Ci₅ alkoxy, or derivatives thereof for example halogenated derivatives thereof ; as a component in the manufacture of a liquid flux for use in a soldering process such as a wave flow process. In particular use of the flux composition achieves good and reliable through-hole fill.

The invention will now be described in more detail as set out below.

Brief Description of the Drawings

Fig. 1 shows a side view with partial cross section of a pin of an electronic component soldered to a PCB through- hole using existing solder flux liquids; and Fig. 2 shows a similar view of the same arrangement soldered using a solder flux composition of the present invention.

Detailed Description of the Drawings Figure 1 shows a cross sectional view of a typical PCB board 3 having a through hole 5 with a metal jacket 2. In the embodiment the jacket is made of copper. A conductive pin 4 of an electronic component (not shown) can be seen to extend into and through the hole extending to a position beyond the underside 6 of the PCB . The pins of the component are soldered to a circumferential pads 7 on the underside. A solder interconnect 1 has been put in place using a wave flow machine. A commercially available flux material has also been employed. The Examples below detail the various materials which may be used. The solder interconnect 1 surrounds the pin 4 so that the entire diameter of the pin has an electrically conductive connection with the copper of the PCB. The solder interconnect shown in Figure 1 has achieved a less than 50% penetration/fill of the hole, illustrating the effect of inadequate solder flow up the gap between the pin and the copper via capillary action. The poor hole fill means that the intermetalic interconnect that exists between the pin and the copper on the PCB may be unsatisfactory and less robust. Thus a poor hole fill will increase the possibility for loss of electrical conduction.

Figure 2 shows an analogous view to Figure 1 showing instead what is believed to be good hole fill. Figure 2 shows a typical result of good hole fill which can be obtained with the materials of the present invention. The intermetallic connection between the pin and copper is achieved along the entire length of the hole. This provides for a robust connection that is less susceptible to loss of conductivity .

The words "comprises/comprising" and the words "having/including" when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

EXAMPLES

Tests were conducted using a wave-soldering machine charged with 96SC solder at 260⁰C. The respective preheat settings were 250, 300 and 450⁰C and the conveyor speed was 3.5 feet/minute. The main solder wave was on, but the chip wave (which is intended for surface mount components) was off.

Flux formulations are given in Table 1. A standard low solids liquid flux was used as a benchmark.

A test PCB with empty plated through holes (PTHs) was used to gauge PTH fill. The through holes had various diameters including 0.55mm, 0.65 mm, 0.75mm, 0.85mm 0.95mm, 1.05, 1.15mm, 1.25 mm. These were pre-conditioned by passing them through the wave- soldering machine with the wave turned off. This provided a less solderable, and more discriminating, substrate. In addition, tests were conducted with un-oxidized PCBs. Soldered PCBs were inspected for the percentage of PTHs that exhibited perfect fill. Results are given in Table 2.

There was excellent soldering performance when the PCB was un-oxidized. However, the oxidation process exerted a significant impact, with the benchmark liquid flux conferring poor PTH fill with oxidized PCBs.

Significantly, improved PTH fill was restored by QUADROL, with Flux 6 (1% QUADROL) giving similar performance to that exhibited by the benchmark liquid flux on un-oxidized PCB.

It is to be noted that good through hole fill was achieved in all holes of varying diameters despite the fact that wicking/capillary travel of the solder is likely to fall off with increasing diameter. Good through hole fill was observed up to and including 0.85mm, 0.95, mm, 1.05mm, 1.15mm and 1.25mm. It is believed that all compositions of the invention will exhibit this property. In particular the present invention believes compositions of the present invention will work well also for through holes with diameters in excess of 1.25 mm.

Table 1 Flux formulations

QUADROL is N, N, N' ,N' -Tetrakis (2-hydroxyproplyl) ethylenediamine .

Table 2; Wave Soldering

Claims

1. A soldering flux composition comprising: a carrier vehicle comprising a solvent; an activator component for activating a metal surface for soldering; and quadrol, or a quadrol derivative or combinations thereof .

2. A soldering flux composition as claimed in Claim 1 wherein the quadrol or quadrol derivative is selected from one or more compounds within the structural formula I :

wherein each R is independently selected from Ci-Ci₅ alkyl, C₆-Ci₅ aryl , C₂-Ci₅ alkenyl ;

C₂-Ci₅ alkynyl; Ci-Ci₅ alkoxy, or derivatives thereof for example halogenated derivatives thereof .

3. A flux composition according to Claim 2, wherein each R is selected from C_x-C₅ alkyl.

4. A flux composition according to Claim 2 or Claim 3 wherein at least one R is CH₃.

5. A flux composition according to any one of Claims 2 to 4 wherein each R is CH₃.

6. A flux composition according to any preceding claim wherein the carrier vehicle comprises at least one of a high boiling point solvent and/or a low boiling point solvent .

7. A flux composition according to Claim 6 wherein the low boiling point solvent is at least one of alcohol or water.

8. A flux composition according to Claim 7 wherein the alcohol is isopropanol.

9. A flux composition according to Claim 6 wherein the high boiling point solvent comprises at least one Ci₀-Ci₅ alkane .

10. A flux composition according to Claim 6 wherein the high boiling point solvent comprises at least one of glycol, glycol ethers, methyl ethyl ketone or white spirit

11. A flux composition according to Claim 6 wherein the high boiling point solvent is a glycol.

12. A flux composition according to Claim 10 wherein the Glycol ether has the formula CH₃CH₂OCH₂CH₂OCH₂CH₃.

13. A flux composition according to Claim 6 wherein the high boiling point solvent is white spirit.

14. A flux composition according to any one of the preceding claims wherein the activator component comprises a carboxylic acid.

15. A flux composition according to any one of the preceding claims wherein the activator component comprises rosin.

16. A flux composition according to Claim 6 wherein the high boiling point solvent is an ester or di-basic ester of a carboxylic acid for example at least one methyl ester of a C₄- C₆ dicarboxylic acid.

17. A flux composition according to Claim 14 or Claim 16 wherein the carboxylic acid is selected from the group consisting of succinic, glutaric, adipic acids, and combinations thereof.

18. A flux composition according to any one of the preceding claims wherein the activator component comprises a rosin component.

19. A flux composition according to Claim 15 wherein the rosin component is selected from the group consisting of diproportionated rosin, hydrogenated rosin, dehydrogenated rosin, or unmodified rosin, and combinations thereof.

20. A flux composition according to any preceding claim comprising activator component present in the amount 0.1% to 20% by weight of the total composition.

21. A flux composition according to any preceding claim comprising activator component present in the amount 1.1% to 1.5% by weight of the total composition.

22. A flux composition according to any one of the preceding claims wherein the quadrol or quadrol derivative is present in the amount of 0.1% to 20% by weight of the total composition.

23. A flux composition according to any one of the preceding claims wherein the quadrol or quadrol derivative is present in the amount of 5% to 20% by weight of the total composition.

24. A flux composition according to any one of the preceding claims wherein the quadrol or quadrol derivative is present in the amount of 1.1% to 1.5% by weight of the total composition.

25. A flux composition according to Claim 6 comprising a high boiling point solvent present in the amount of 2% to 50% by weight of the total composition.

26. A flux composition according to Claim 6 comprising a high boiling point solvent present in the amount 7% to 12% by weight of the total composition.

27. The composition according to any one of the preceding claims wherein the remaining percentage weight of the composition is taken up by a low boiling point solvent such as an isopropyl alcohol .

28. An assembly comprising: (i) a substrate with a metallic through-hole defined therein (ii) an electronic component attached to the substrate substantially filled by a solder composition after treatment of the metallic through hole by a flux composition according to any one of Claims 1 to 27.

29. An assembly according to Claim 28 wherein the electronic component comprises a pin which extends at least part of the way through the through-hole.

30. A method of soldering a component to a metallic substrate comprising the steps of:

(i) treating the metallic substrate with a flux composition according to any one of Claims 1 to 27; and (ii) simultaneously or subsequently soldering the component to the metallic substrate.

31. A method according to Claim 30 wherein soldering is carried out by wave flow process.

32. Use of a quadrol or quadrol derivative as a component in the manufacture of a liquid flux for use in a soldering process .