Solid tin-palladium catalyst for electroless deposition incorporating stannous salts of organic acids

An improved method of producing a tin/palladium catalyst for use in electroless plating baths is disclosed. The method comprises the steps: PA0 (a) preparing a mixture of stannous halide, an alkali metal halide and water, PA0 (b) reacting said mixture with a palladium halide salt at an elevated temperature, PA0 (c) adding a stannous salt of an organic carboxylic acid to the reaction mixture obtained in step (b), and PA0 (d) continuing the heating of the reaction mixture to complete the reaction. The catalysts so produced exhibit improved activity and resist decomposition.

FIELD OF THE INVENTION 
Method for the production of improved tin-palladium catalysts useful in 
electroless deposition processes. 
DESCRIPTION OF THE PRIOR ART 
Electroless metal deposition refers to the chemical deposition of a metal 
on a conductive, non-conductive, or semi-conductive substrate in the 
absence of an external electric source. 
Electroless deposition is used for many purposes, for example, in the 
manufacture of printed circuit boards where, in one method, a metal, 
typically copper, is deposited on a dielectric substrate either as a 
uniform surface coating or in a predetermined pattern. This initial 
electroless deposit is usually thin and may be further built up by 
electroplating or may be deposited directly to full thickness. 
The substrate over which an electroless metal deposit is formed is most 
often a plastic panel which may have a metal foil such as copper laminated 
to one or both of its surfaces, for example, with adhesive, to form a 
metal clad substrate. Where both surfaces of the substrate are to be used, 
connections are typically provided therebetween by means of holes through 
the panel at appropriate locations, the walls of these holes being made 
conductive with the electroless coating. 
The electroless deposition of a metal on either a metallic or non-metallic 
substrate usually requires pretreatment or sensitization of the substrate 
to render it catalytic to the reception of such a deposit. Various methods 
have evolved over the years employing particular sensitizing compositions. 
One class of these sensitizing catalysts uses tin and palladium in 
combination. Several U.S. patents have been directed to improvements in 
the production, content and form of these tin/palladium sensitizing 
compositions. 
U.S. Pat. No. 3,011,920 (Shipley) describes a process in which a colloidal 
solution is prepared by mixing an aqueous acid solution of palladium 
chloride with an aqueous acid solution of stannous chloride and optionally 
including a tin salt such as sodium stannate. This is purported to produce 
a lyophilic colloid which, after acceleration with an acid or alkaline 
solution such as hydrochloric acid or sodium hydroxide provides a 
sensitizing layer for the subsequent electroless plating of a metal such 
as copper. 
U.S. Pat. No. 3,672,923 (Zeblisky) describes solid compositions dilutable 
to optically clear sensitizing solutions for electroless plating. These 
solutions are prepared by combining a dilute solution of a noble metal 
salt in hydrochloric acid with a hydrochloric acid solution of a stannous 
salt such as stannous chloride dihydrate. The mixture is heated and then 
subsequently cooled and evaporated to dryness under vacuum to constant 
weight. The solid composition, as described, may then be reconstituted in 
hydrochloric acid to provide an active sensitizing solution. 
U.S. Pat. No. 3,607,352 (Fadgen et al) describes the use of tartaric acid 
to improve the stability of a tin sensitizer. The theory is advanced that 
tartaric acid, which is one of the preferred hydroxy substituted acids of 
the present invention, inhibits tin oxychloride formation. 
U.S. Pat. No. 3,904,792 (Gulla et al.) discloses the advantages of using 
excess halide ions, in concentrations of at least 0.2 moles/liter in 
excess of the other chloride ion components, such as furnished by stannous 
and palladium chloride solutions. 
Nathan Feldstein, "Reliability in Printed Circuitry Metalization--A case 
for Improved Catalyzing Systems", Plating, June 1973. In the Feldstein 
article it is recognized that the inclusion of halide salts improves the 
stability of catalytic sensitizer solutions. 
U.S. Pat. No. 4,020,009 (Gulla) provides a method of producing improved dry 
tin/palladium catalyst compositions which represented an improvement over 
the colloidal suspensions of U.S. Pat. No. 3,011,920. 
U.S. Pat. No. 4,120,822 (Jameson et al) provides improved tin/palladium 
catalyst compositions prepared without the use of acid by reacting an 
aqueous halide solution of a palladium salt with a tin salt (typically 
chloride). The described method comprises the steps of: 
(1) melting a predetermined quantity of a hydrated stannous chloride 
composition; 
(2) adding an aqueous solution of palladium chloride and a water soluble 
halide salt, other than said stannous chloride composition and said 
palladium chloride, selected from the group consisting of bromide and 
chloride to the molten hydrated stannous chloride; 
(3) adding anhydrous stannous chloride to the mixture in a quantity 
sufficient to convert, at a minimum, all but 20% of the water in said 
aqueous solution to water of hydration association with said anhydrous 
stannous chloride; 
(4) reacting the mixture at a temperature between 35.degree. and 
140.degree. C.; and 
(5) cooling the product to yield a dry, friable material or a liquid or a 
semisolid concentrate. 
U.S. Pat. No. 4,182,784 (Krulik) describes tin-palladium catalysts which 
are stabilized through the use of hydroxy-substituted organic acids to 
prepare the catalysts from non-halide salts. The method involves placing a 
stannous salt in an aqueous solution containing a hydroxy-substituted 
organic acid and reacting the resultant mixture with a palladium salt. The 
method is designed to eliminate the use of halide salts which form noxious 
fumes when used in acidic plating baths. 
In the past, stabilized tin-palladium baths had been prepared by using 
either the chloride salt or bromide salt of palladium and/or tin; the 
concentrate then being dissolved in hydrochloric acid to produce the 
working bath. Other salts, such as palladium sulfate and tin sulfate have 
been used, but the presence of HCl has always furnished a large amount of 
Cl.sup.- anions regardless of the salt anion chosen. 
U.S. Pat. No. 4,120,822 (Jameson et al.) established that tin-palladium 
catalysts could be prepared without the use of acid by reacting an aqueous 
halide solution of a palladium salt with a compatible tin salt. 
U.S. Pat. No. 4,182,784 (Krulik) represented a departure from the prior art 
with the discovery that the Cl.sup.- anion could be substantially, or even 
completely, replaced by the conjugate anion of organic acids in the 
catalyst production method. This patent teaches the minimization, if not 
elimination, of halide ions from the catalyst production method to avoid 
the toxicity problem attendant to making acidic plating baths containing 
halide anions. The catalyst of the Krulik patent is a liquid. 
The contents of U.S. Pat. Nos. 4,120,822 and 4,182,784 are hereby 
incorporated by reference. 
SUMMARY OF THE INVENTION 
The present invention represents an improved method of incorporating 
organic acid into the catalyst structure yielding a catalyst whose 
activity is markedly improved over the prior art. This is accomplished by 
preparing a mixture of stannous halide, an alkali metal halide and water, 
reacting this mixture with a palladium halide salt at an elevated 
temperature, adding a stannous salt of an organic carboxylic acid to the 
reaction mixture and continuing the heating of the mixture until the 
reaction is complete. 
The resulting catalyst is a solid which facilitates easier and cleaner 
handling and avoids toxicity problems attendant to handling toxic liquids. 
Ease in handling is especially apparent if one considers the difficulty of 
replenishing an existing working bath. If the replenisher solution is 
added in relatively dilute liquid form, it is normal practice to remove an 
equivalent volume of the exhausted bath to make room for the addition. If 
the materials can be added in the highly concentrated solid form, it is 
only necessary to calculate the amount of composition needed to bring the 
bath up to working strength and then add the solid catalyst. The 
negligible volume of the solid catalyst, compared to a liquid concentrate, 
has little, if any effect on the volume of solution in the catalyst tank. 
Moreover, it is obvious that shipping and storage of a dry material would 
be more economical than for a liquid concentrate; and the fact that acid 
solutions are not involved reduces the safety hazards involved in handling 
the catalyst.

DETAILED DESCRIPTION 
The method of the invention comprises generally the steps of: 
(a) preparing a mixture comprising stannous halide, an alkali metal halide 
and water, 
(b) reacting said mixture with a palladium halide salt at an elevated 
temperature, 
(c) adding a stannous salt of an organic carboxylic acid to the reaction 
mixture obtained in step (b), and 
(d) continuing the heating of the reaction mixture to complete the 
reaction. 
The stannous halide used in step (a) above may be any of the stannous 
halide salts in their various hydrated and non-hydrated forms or mixtures 
thereof. Examples of stannous halides include: SnBr.sub.2, SnCl.sub.2 and 
SnCl.sub.2.2H.sub.2 O. The more water-soluble salts are preferred. Of the 
anions, chloride is preferred and, of these compounds, SnCl.sub.2 is 
preferred. 
The alkali metal halide used in step (a) above may be any alkali metal 
halide or mixtures thereof. Examples include: NaCl, KCl, KBr, NaBr, LiBr 
and RbCl. 
The preferred of the alkali metal halides are the potassium salts, 
particularly potassium chloride. 
The water used in step (a) is preferably deionized water which helps to 
prevent contamination and spurious reactions in the mixture. However, all 
or part of this water may be omitted if hydrated salts are employed. 
In step (b), at least one palladium halide salt is added to the mixture. 
The palladium halide salt(s) are formed by reacting at least one palladium 
halide with at least one alkali metal halide. Examples of palladium 
halides which can be used to form the palladium halide salt are: 
PdBr.sub.2, PdF.sub.2, PdCl.sub.2 and PdCl.sub.2.2H.sub.2 O. Of these 
PdCl.sub.2 is preferred. Examples of alkali metal halides which may be so 
used include NaCl, KCl, NaBr, KBr, and LiBr and RbCl. The preferred alkali 
metal halides are the potassium metal salts and, of these, potassium 
chloride is preferred. 
The amounts of the palladium halide and alkali metal halide added in step 
(b) should preferably be chosen to yield a molar ratio of palladium to 
alkali metal of at least about 1:2. 
The mixture resulting from step (b) is maintained at an elevated 
temperature, preferably in the range of from about 70.degree. to about 
110.degree. C. A more preferred temperature range is from about 85.degree. 
to about 95.degree. C. Heating is continued for an effective time to allow 
the palladium and tin salts to react. This is generally in the range of 
from about one-half hour to about three hours or more. The reaction is 
normally complete after one hour when maintained in the 
85.degree.-95.degree. C. temperature range. 
After step (b) is completed, a stannous salt of an organic carboxylic acid 
is added to the reaction mixture. Mixtures of stannous salts can be added. 
A wide variety of Sn(II) carboxylates may be used including salts of mono- 
and polycarboxylic acids. These carboxylic acids need not be of any 
particular size, however lower carboxylic acids, i.e. from 2 to 12 
carbons, are preferred. Examples of carboxylic acids from which suitable 
stannous carboxylate salts are derived include: 
tartaric acid (2,3-dihydroxy butanedioic acid) 
citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) 
oxalic acid (ethanedioic acid) 
succinic acid (butanedioic acid) 
glycolic acid 
acetic acid 
lactic acid (2-hydroxy-propanoic acid) 
maleic acid (cis-butanedioic acid) 
butanoic acid 
2-hydroxy-butanoic acid 
4-hydroxy-butanoic acid 
3-hydroxy-propanoic acid 
2-hydroxy-2-methyl-propanoic acid 
pentanoic acid 
2-hydroxy-pentanoic acid 
4-hydroxy-pentanoic acid 
2-hydroxy-4-methyl-pentanoic acid. 
The preferred acids for use as stannous salts in the present invention are 
oxalic acid and the mono- and polyhydroxy-substituted acids, preferably 
tartaric acid. 
The stannous carboxylates used in the method of the invention may be 
obtained from commercial sources or may be produced by known methods of 
reacting at least one stannous cation source with at least one carboxylic 
acid or salt of said acid. Examples of suitable stannous cation sources 
include SnO, SnCl.sub.2.2H.sub.2 O, SnSO.sub.4, Sn(NO.sub.3).sub.2 and 
SnBr.sub.2. The preferred stannous cation source is stannous sulfate. 
The stannous carboxylate forming reaction is carried out by reacting at 
least one stannous cation source with at least one carboxylic acid or salt 
of said acid. 
The temperature of the stannous carboxylate forming reaction is preferably 
maintained in the range of from about 15.degree. C. to about 110.degree. 
C. and more preferably in the range of from about 20.degree. C. to 
30.degree. C. The reaction time is generally in the range of from about 
one-half hour to about three hours, although most of the reaction is 
normally complete after one hour at a temperature within the 
20.degree.-30.degree. C. range. 
The amounts of the ingredients of the mixture achieved upon performing step 
(c) should be measured to yield a stannous halide:palladium halide molar 
ratio of at least about 2:1. 
The molar ratio of the stannous carboxylate salt ingredient to that of the 
palladium halide ingredient should be in the range of from about 10:1 to 
about 50:1. 
After the addition of the stannous carboxylate to the reaction mixture in 
step (c), the temperature of the reaction mixture should be in the range 
of from about 70.degree. to about 110.degree. C. These elevated 
temperatures are maintained until the reaction is complete. A more 
preferred range is from about 85.degree. to about 95.degree. C. The 
reaction generally takes about one hour at a temperature in the 
85.degree.-95.degree. C. range. 
The reaction mixture is finally cooled, and the catalytically active 
product is recovered. The product is recovered by pouring the mixture into 
trays and allowing it to cool and dry by air exposure. The dry catalyst 
may be stored without regard to ambient temperatures or air exposure. 
Typically, other dry compositions, that are mainly SnCl.sub.2.2H.sub.2 O, 
absorb oxygen forming oxychlorides of stannic tin. 
The catalyst composition prepared by the method of the invention should 
contain a molar ratio of stannous ion to palladium ion of at least about 
6:1. 
In order to better understand the present invention, the following example 
is set forth as an illustration only. 
EXAMPLE OF A PREATION OF A CATALYST 
This exemplary procedure is carried out as a two-step process using two 
containers, one of approximately 10-gallon (hereinafter "container A") and 
the other of approximately 30-gallon (hereinafter "container B") 
capacities. Both containers should have means for stirring and heating 
their contents (i.e. a stirring apparatus capable of 300 rpm and steam 
heating apparatus), and container A should have means to transfer its 
contents quickly to container B (i.e. a hose). Container A is charged with 
2.01 kilograms of deionized distilled water and heated to a temperature of 
about 65.degree. C. To this are added 2.3 kilograms of potassium chloride 
and 2.735 kilograms of palladium chloride. The container is then covered, 
brought to a temperature of about 90.degree. C., and stirred until the 
reaction is complete. 
Container B is charged with 7.8 kilograms of deionized distilled water and 
42.86 kilograms of stannous chloride. Container B is then sealed and 
stannous chloride dihydrate is formed by heating the contents to a 
temperature of about 70.degree. C. until the contents melt. If the 
contents do not melt, the temperature should be slowly increased and the 
contents checked at about every 5.degree. C. increase until melting 
occurs. The melt is then stirred for 15 minutes. 
An aliquot of 6.9 kilograms of potassium chloride is then added to 
container B while the temperature is maintained and the stirring is 
continued for another 15 minutes. The temperature is then increased to 
about 85.degree.-95.degree. and maintained for about one hour. 
The contents of container A are then transferred quickly to container B by 
means of a hose. The transfer preferably is made within a period of 40 to 
50 seconds. If both A and B are at 90.degree. C. at the time of addition, 
the reaction forming the catalyst generally will be indicated by an 
exotherm of about 5.degree.-10.degree. C. 
Container B is then sealed and stirring and heating are maintained for 1 
hour. The temperature should be kept at about 95.degree. C. 
Afterwards, 100.4 kilograms of stannous tartarate are slowly added to 
container B while the mixing and stirring are maintained. Container B is 
then resealed and a temperature of about 95.degree. C. is maintained, with 
stirring for 1 hour. The temperature preferably is maintained above 
85.degree. C. 
The final product is then drawn from container B and allowed to air dry for 
about 48 hours. Drying is facilitated by occasionally breaking the solid 
cake that forms to allow increased surface exposure. The dry catalyst may 
then be packaged and stored as desired. 
The product prepared in this manner was found to be about 1.75% by weight 
in PdCl.sub.2 and about 40% by weight in Sn(II). 
The stannous tartrate used in this exmaple is prepared by reacting stannous 
sulfate with tartaric acid. Stannous sulfate (413.5 kilograms) and 363.66 
kilograms of tartaric acid are dissolved in about 115 gallons of deionized 
water and mixed for one hour. Thereafter, the product is allowed to settle 
and the supernatant liquid is drawn off. The product is then washed on a 
filter with three successive 25 gallon aliquots of deionized water. The 
resulting filter cake is dried at 100.degree. C. 
Generally, the catalyst composition of the invention can be used for 
electroless plating of non-conductive materials with any metal, primarily 
nickel or copper, according to practice known in the art. 
The catalyst solution is prepared by dissolving the catalyst composition in 
a solution of at least one mineral acid, preferably HCl. The acid 
concentration is in the range of from about 1/2N to about 4N, preferably 
from about 1N to about 3N. 
The concentration of the catalyst composition in the bath is at least about 
0.15 millimoles/L, measured as PdX.sub.2, where X is the halide associated 
with palladium in the halide salt. 
The following specific examples illustrate the use of the catalysts of the 
invention and demonstrate some of the improved results achieved as 
compared to catalysts of the prior art. 
The catalyst of the current invention, hereinafter called DriCat-3X, and 
the catalytic compositions as taught by U.S. Pat. No. 3,011,920 (Example 
2), U.S. Pat. No. 3,532,518 (Example 1), U.S. Pat. No. 3,672,938 (Example 
1) and U.S. Pat. No. 4,120,822 (Example II), all yield excellent 
electroless nickel coverage when employed in working baths at catalyst 
equivalent concentrations (CEC) of 200 mg/L PdCl.sub.2. In order to 
demonstrate the superior catalytic activity of DriCat-3X over the 
catalytic compositions of the prior art, working baths of the above 
catalysts were prepared at low concentrations and employed in an 
electroless nickel preplate cycle. 
Standard plateable grade ABS test plaques were sequenced through a preplate 
cycle consisting of the following steps: 
1. etching the surface of a standard test plaque of ABS resin in a chromic 
acid-sulfuric acid etch bath; 
2. neutralizing any Cr(VI) remaining on the surface; 
3. immersion in a low concentration bath of either DriCat-3X or DriCat-3 to 
activate/sensitize the surface; 
4. accelerating the activated/sensitized surface with an acid dip; 
5. immersion in an electroless nickel bath containing nickel ions, 
hypophosphite, stabilizers and buffering agents. 
Intervening deionized water rinses were employed between steps 1 and 2, 2 
and 3, and 4 and 5. The etching step was performed at an elevated 
temperature, about 75.degree.-80.degree. C., while all other steps were 
completed at room temperature. 
The following steps were taken to process the test plaques: 
______________________________________ 
AGITA- 
STEP TYPE TEMP. TIME TION 
______________________________________ 
1. Etch chromic-sulfuric 
75-80.degree. C. 
12 min. 
Moderate 
acid bath 
2. Rinse Deionized H.sub.2 O 
Room -- Flowing 
3. Neutralize 
D-400 Room 1 min. 
Rapid 
4. Rinse DI H.sub.2 O Room -- Flowing 
5. Activate 
DriCat-3X Room 3 min. 
Manual 
or other 
6. Accelerate 
1:1 HCl Room 2 min. 
Still 
7. Rinse Deionized H.sub.2 O 
Room -- Flowing 
8. Plate Electroless Nickel 
Room 3 min. 
Slow, gentle 
Bath 
______________________________________ 
Manual agitation refers to a transverse movement of the panels through the 
catalyst bath. Agitation is believed necessary for the more dilute 
catalyst solutions that can be employed in the method of the invention to 
aid transport of the active species to the surface of the material to be 
treated. 
The following examples demonstrate the method of the invention and results 
are compared to prior art methods. For purposes of qualitative comparison, 
it should be noted that coverage values less than 100% do not yield 
commercially valuable products. 
EXAMPLE I 
Working baths containing a CEC of 50 mg/l PdCl.sub.2 in 3M hydrochloric 
acid were prepared for DriCat-3X and the prior art compositions identified 
above. Standard plateable grade ABS panels were processed through the 
preplate cycle employing the individual baths at the "Activate" step. The 
results obtained are given below. 
______________________________________ 
CATALYST COVERAGE 
______________________________________ 
DriCat-3X 100% 
U.S. Pat. No. 3,011,920 
98% 
U.S. Pat. No. 3,532,518 
100% 
U.S. Pat. No. 3,672,938 
100% 
U.S. Pat. No. 4,120,822 
100% 
______________________________________ 
EXAMPLE II 
Working baths containing a CEC of 25 mg/L PdCl.sub.2 in 3M HCl were 
prepared from DriCat-3X and the catalytic composition of U.S. Pat. No. 
3,011,920. Two standard plateable grade ABS test plaques were 
simultaneously processed through the preplate cycle. The panels were 
separated at the "Activate" step for immersion in the individual baths. 
The panel processed through the DriCat-3X bath yielded 100% electroless 
nickel coverage, while the concurrent panel of U.S. Pat. No. 3,011,920 
promoted only 94% coverage. 
EXAMPLE III 
Working baths having a CEC of 25 mg/L PdCl.sub.2 in 3M HCl were prepared 
from DriCat-3X and the catalytic composition of U.S. Pat. No. 3,532,518. 
Two standard plateable grade ABS test plaques were simultaneously 
processed through the preplate cycle employing these baths as described in 
Example II. While DriCat-3X promoted 100% coverage, the catalyst of U.S. 
Pat. No. 3,532,518 was only 81% effective. 
EXAMPLE IV 
Working baths having a CEC of 25 mg/L PdCl.sub.2 in 3M HCl were prepared 
from DriCat-3X and the catalytic composition of U.S. Pat. No. 3,672,938. 
Two standard plateable grade ABS test plaques were simultaneously 
processed through the preplate cycle employing these baths as described in 
Example II. While DriCat-3X promoted 100% coverage, the catalyst of U.S. 
Pat. No. 3,672,938 was only 95% effective. 
EXAMPLE V 
Working baths having a CEC of 25 mg/L PdCl.sub.2 in 3M HCl were prepared 
from DriCat-3X and the catalytic composition of U.S. Pat. No. 4,120,822. 
Two standard plateable grade ABS test plaques were simultaneously 
processed through the preplate cycle employing these baths as described in 
Example II. While DriCat-3X promoted 100% coverage, the catalyst of U.S. 
Pat. No. 4,120,822 was only 95% effective. 
EXAMPLE VI 
Examples II through V were repeated using a residence time of 30 seconds in 
the electroless nickel bath at the "Plate" step. Again, each catalytic 
composition of the prior art was processed simultaneously with DriCat-3X 
as described in Example II. Working baths of 3M HCl containing a CEC of 25 
mg/L PdCl.sub.2 were employed. 
Although no catalytic composition promoted full electroless nickel 
coverage, the number and distribution of the sites where electroless 
nickel deposition had begun was far greater for the DriCat-3X bath than 
any bath prepared from the compositions of the prior art. 
It should be noted that Examples II through V were performed in triplicate 
for each identified catalytic composition. DriCat-3X promoted 100% nickel 
coverage for every single run, while the stated values for the prior art 
compositions are the best obtained. 
RESULTS 
Although we do not wish to be limited by theory, we believe the superior 
activity of DriCat-3X is the result of a modification of the reaction 
media by the stannous carboxylate. This modification controls the form 
and/or proportion of the active specie(s) to yield the best possible 
product. Moreover, the dry catalyst is resistant to atmospheric oxidation 
and thermal degradation. It should be stressed that these aforementioned 
properties are only obtained from the stannous carboxylate salt and 
preferably from stannous tartrate. 
The validity of the final statement above was tested. A catalytic 
composition was prepared whereby tartaric acid was substituted for 
stannous tartrate. This composition was not active when employed in a 3M 
HCl working bath at a CEC of 50 mg/l PdCl.sub.2. Similarly, the addition 
of tartaric acid to catalytic compositions of the prior art either before 
or after their use in working baths at a CEC of 25 mg/L PdCl.sub.2 did not 
improve their catalytic activity. 
The improvement in physical properties obtained by employing stannous 
carboxylate salts, and preferably stannous tartrate may be due to the 
complexity of these salts and their hydrophobic behavior. Infrared data on 
stannous tartrate lack frequency bands that can be assigned to OH 
stretching. Similarly, the IR spectra of DriCat-3X also lack these bands. 
Claiming no theory limitations, we suggest that this composition may be a 
clathrate. Analogous structural data on other tartrate salts suggest the 
prevalence of cage type structures. We assume that stannous tartrate may 
also have a cage type structure although we do not limit its effect to 
this structure. If this is true, then we suggest that this structure is 
the perfect size to enclose the active species. This would explain its 
physical properties. 
Although the invention has been described and illustrated by reference to a 
particular embodiment thereof, it will be understood that in its broadest 
aspects the invention is not limited to such embodiment, and that 
variations and substitution of such equivalents may be resorted to within 
the scope of the appended claims.