CA1201253A - Electroless copper plating - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In an aqueous electroless copper plating solution comprising a source of cupric ions, hydroxyl radicals, sufficient complexing agent to render said cupric ions soluble in alkaline solution, and the add-ition product of formaldehyde and a formaldehyde add-ition agent, the improvement comprising a solution pH of at least 11 and a molar ratio of each of copper and free formaldehyde to hydroxide whereby the concentration of free formaldehyde in solution during plating detect-able by analysis is essentially 0.

Description

~3~53 `s This invention relates to a metal deposi-ting composition and more particularly, to a non-fuminy electroless copper pla-ting solu-tion.

Elec-troless copper deposition refers to the chemical plating of copper over a clean, catalyti.cally active surface, by chemical reduction in the absence of an external elec-tric current. Such processes, and compositions useful therefor, are known and are in substantial eommercial use. They are j 10 disclosed in a number of prior art patents, Eor example, U.S.
.~ Patents Nos. 3,663,242; 3,72~,137; 3,846,138; and 4,229,218.
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Known elec-troless copper deposition solutions ¦ yenerally comprise four ingredients dissolved in wate.r. They are (1) a source o:E cuprie ions, usually a copper salt such as 3 copper sulphate, (2) a reducincJ agent such as Eormalclehy(1e, or 3 preferably a Eorma].dehycle precursor such as pa:ra:Eormaldehycle, (3) hydroxide, generally an alkali metal hydroxide and usually sodium hydroxide, suEficient to provide -the required alkalinity necessary for said compositions to be effee-tive, and ~4) a 3 complexing agent Eor copper suEEieient to prevent its precipitation in alkaline solution. A large number oE
eomplexi.ng agents are known and described in the aforesaid cited patents and elsewhere `I ' I, it Known electroless copper plating solutions of the above type frequently provide a plate which, if mechflnically dense and strong, is britt]e such that it CRn withstand limited bending or thermal stress without fracture. This is not a substantiRl c7isadvantage where the electroless plate is of the order of millionths of 5 an inch in thickncss and is overplated with ductile electrolytic copper. Hoover where the entire desired thickness is provided by electroless plating, i.e., typically one to three mils in electrical fabrication such as in the manufacture of printed circuil bourds by additive tcchniques, limited ductility is a serious limitation.
One rncans of improving the bending or tensile characteristics of an 10 electroless copper p]nte is dcscribed in U. S. Pntent No. 3,213,430 which discloses the addition to ti1e coppcr plating so]ution of a wnter so111b1e compound of cyanide, vanadium, molybdenurn, niobium, tUrlgSten, rhenillm, arsenic, antimony, bismuth, rare earths of the actinium series and rare earths of the lanthanum series. Certain mernbers of the above groups, especially the vanadium compounds, provide 15 improved bending characteristics. The reason for this is not fully understood, but it is stated in the patent that the agents act on the catalytic sùrface so as- to prevent forrnation and release of hydrogen gas, thereby inhibiting the inclusion of hydrogen in the deposit as it forms. It has been found that where a complexing agent or a bath formulation is used permitting rapid deposition of copper smith rapid 20 eYO]ution of hydrogen gns at the surface, the improved duclility or bending characteristics are frequently sacrificed or lost.
An additional method for improving the bending or tensile characteristics of a copper plate is disc]osed in U. S. Patent No. 3,728,137 noted above. In this patent, -the copper solu-tion is characterized by the addition of a formaldehyde addition agent to the solu-tion which is believed to be responsible for forming an unstable addition product with formaldehyde. Because of the formation of the addition product with formaldehyde, it is theorized by the patentee that formaldehyde is slowly released into solution -thereby reducing -the evolution of hydrogen gas at the surface of the part being plated resulting in copper deposits having improved bending or tensile properties.
Though the improvement is obtained as noted in said patent, some free formaldehyde is still found in the bath at all times as a consequence of a breakdown of the addition produc-t. This Eree formaldehyde is released Erom solution as a vapor which is danyerous -to health and must be removed Erom the atmosphere, though the release oE this Eormalde~lyde to the atmosphere is decreased Erom that normally encountered absent the addition agen-t.

The invention herein is an improvement over that of the aforesaid U.S. Patent No 3,728,137 in that the desirable deposit properties are maintained and in many instances improved, but free formaldehyde is analytically absent from solution. The elimination of formaldehyde is accomplished by operating the solution a a pll of at least 11, while controlling the ratios of each of the free formaldehyde in solution and copper, to the hydroxide content in solution.

Because Eree formaldehyde is analytically absent from solution during plating, formaldehyde fuming Erom the solution is eliminated lessening the health hazard caused by formaldehyde in the air. This decreases the need to rely upon expensive exhaust equipment.

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It has been Eound that the absence of :Eree formaldehyde in solution may eEEec-t initiation ox plating from a fresh solution and a minor amount of free formaldehyde may have to be added to start the plating reaction. This free formaldehyde is quickly consumed and once plating is initiated, the addition product provides adequate formaldehyde Eor continued plating. Consequently, the addition of the free formaldehyde initially appears to act as a catalytic initiator for plating t but as formaldehyde is consumed during the plating reaction, the formaldehyde required for continued plating is released and immediately consumed, with the concentration of the formaldehyde in solution during plating essentially 0~

any wa-ter soluble copper salt hereto;Eore used for preparing electroless coppe.r deposition solutions may be used Eor the solutions o:E this inven-tion. For ox ple, the halides, nitrate, acetate, sulphates and other organic and inorganic acid salts of copper are generally suitable as is known in the art. Copper sulphate is preferred.

Suitable complexing agents for the copper ions are also well known in the are and include Rochelle salts, the sodium salts oE ethylenediamine tetraacetic acid, nitrilotriacetic acid and its alkali metal salts, -triethanolamine, modified ethylenediamine te-traacetic acids such as N-hydroxyethylenediamine -triacetate, hydroxya:Llcyl substituted dialkaline triamines such as pentahydroxypropyl-die-thylenetriamine, and the like. A preFerred class oE
complexing agents are those described in Ursa Patent No.
3,329,512 and include hydroxylalkyl substituted tertiary amines such as tetrahydroxypropylethylene diamine, pentahydroxypropyldie-thylene triamine, trihydroxyproply amine (-tripropanolamine), trihydroxyproply hydroxyethyl %~3 _5 ethylene diamine, etc.

The rate of copper deposition is, to some exten-t, dependent upon the selec-tion of the complexing agent.
.~ 5 Complexing agents such as pentahydroxypropyldiethylene -triamine provide a reasonably East ra-te of topper deposition, usually in excess of 1 mil per hour. Though the copper solutions of this invention provide copper deposits prom solutions containing any of the known complexing agents Eor copper ions, they are particularly well adap-tea for copper solutions having complexing agents that provide a more rapid rate oE copper `~ deposition and which themselves proviae more ductile deposits.;,~
,¦ The formaldehyde addition agent, Eor puxposes of this inven-tion, is one that feats with formalde]ly~e to form a relatively stable .Eormaldehyde adduct and which may 'it be ~c1ded in su:E:ficient concentration without pOiSOllillCJ the solution. reactions oE this nature and :Eo:rmaldehycle adclition agents are well known in the art and are described in various publications such as "Formaldehyde" by J. Frederick Walker, Reinhold Publishing Company, 3rd Edition, 1964, Pages 219 to 221. Preferred formaldehyde addition agents are sulfites, bisulfites, nitrites and phosphites of a metal having a cation non-interfering with the copper solut.ion and preferrably an alkali metal cation. The most pre:Eerred formaldehyde addition agen-ts are sodium sulfite, sodium or potassium bisulEite and sodium phosphi-te.

The formaldehyde addition agent and formaldehyde, or preferrably paraformaldehyde, are reacted with each other to form the adduct prior to - addition to the rernflining components of the copper solution as in the aforesaid V. S. Patent No. 3,728,l37. The arnount of formaldehyde addition agent used may 5 be less than or in molar excess of the amount of formaldehyde required for the solution, ranging from about 0.5 to 3.0 moles per mole of forms1dehyde, though the addition agent is preferably used in amounts of from 0.8 to 1.2 moles per mole of formaldehyde, and more preferably, in amounts equimolar with the formaldehyde, it being understood that amounts of addition agent in excess of the formaldehyde 10 are prcferred to arnollnts less than tlle amount of formaldehyde.
The total îorrnaldehyde content in soll~tion, inclusive of free formaldellyde fldded lo initiate the plating reaction and forrnaldellyde contained in thc addition product is substantially as in the prior art. In this regard, the total formaldehyde content in solution is at least equimolar with the metal content in 15 solution and preferrably, in substantial excess of the metal ion content in solution.
Preferrably, the total formaldehyde content is at least 2 moles per mole of dissolved metal, preferrably varies between about 3 and 15 moles of formaldehyde per rnole of dissolved rnetal and most preferrflbly, ranges between about 5 and lO
moles of formaldchyde per mole of disso]ved metal.

The hydroxide content and pH for the solutions of the invention are higher than those genera]ly used in the prior art. For example, where pH typically varies between 9 and 10.5 in prior art plating solutions, tlle solutions of this invention employ a pl-I of at least 11, preferably a pH ranging bet~ieen 11 and 14, and more preferably, betwcen 11.5 and 12.5. Iiigh hydroxide content forces the reaction of the formaldehyde and the addition agent to the r;ght thereby maintaining the solution analytically free of formaldehyde.
Important to the elimination of free formaldehydc is the molar ratio of copper content and free formaldehyde content prior to initiating plating, each to hydroxide content. In this respect, the ratio of each to the hydroxide may vary between about 1 to 10 and 1 to 40 and preferably varies between about 1 to 12 and 1 to 25~
Take-off, i.e., the tirne for initial deposition of copper from a fresh plnting so]l~tion formulated in accordance with this invention is slov~ hecause the formaldehyde is tightly bound to thc nddition agent at the high pll. Therc~ore, it is desirable that a sma]l arnollrlt of free forma]dehyde be added to a fresh plhting solution to initiate deposition. This free formaldehyde is used in an amount of from O.ûl to 0.05 moles per liter of solution. It is consumed as deposition proceeds lS and it cannot be analytically detected during use of the plflting solution.
The plating baths of this invention may be used at widely varying temperatures, e.g., at least room temperature and preferably up to about 140F.
As temperature is increased, it is customary to find an increase in the rnte of plating. Ten)perature is not critical and within usual operQting ranges, electrolcss copper deposits having excellent tensile properties are obtflined.
In using the electroless copper solution to plate rmetal, the surface to be plated should be catalytically active and free of grease and ContaminQting materials as is known in the art. ~Yhere a non-metallic surface is to be plated, the ~-~0~ 3 surface area Lo receive the deposit musl first be sensitized to render it catalytically active as by the well known treatment with the product of admixture of stanllous chloride and a precious metal cMoride, particularly palladium chloride, the stannous chloride being present in molar excess of the precious metals chloride.
5uch catalysts are disclosed in U. S. Patent No. 3,011,920f.

The inventioll will be beLter understood by reference to the following examples ~here all parts were plated using the following procedure:
(n) Cut a pllenolic substrate to a size of 2" by 2";
(by Scrub part cleall USillg an abrasive cleallcr;
(c) Uinsc till colcl water:
(d) ln~lnerse in n solution of a wetting ngent identifie(l is Shiplfoy (: onditioner 117~ at 150 to 160F for three to five minutes;
(c) Rinse in co]d water;
(f) lmmerse in a stannic acid-palladium catalyst (identified as Cuposit Catalyst 44) maintained at 110 - to 120F for three to five minutes;
(g) Rinse in cold water;
(h) Immeïse in Cuposit accelerator 19 maintained at room temperature for five to seven minlltes;
(i) Rinse in cold water;
(j) Immerse in an electroless copper solution of this invention maintained at between 110 and 130F for a period sufficient to provide a deposit of 100 millionths of an inch;

~,Q~ 3 (k) Dry parts end examine deposits for ap?eùrance and ductility. Ductility is deterrnined by peeling a copper deposit from thc substrate and bending it through 180 in one direction, creasing Rt the fold, then returning it to its original position with pressing along the crease to flatten it. This cycle corlstittJtes OMe bend. The 5 procedure is repeated until the sample breaks at the crease. A sarnple unnble to withstand at least 1/2 bend is considered brittle.
example 1 Cupric sulfate pentahydrate (gm~ 10 Addition product (ml) ~0 1 0 Pentahydroxylpropyl-diethylelletri~rnirle (gm) 30 Sodium hydroxide (50~'6 solution -ml) 40 Water to 1 liter In the above example, thc addition product was preforIned by mixing 18 grsms 15 of sodium meta bisulfite and 7 grams of paruformaldehyde in 40 ml of water and adding the solltion of the adduct to the plating path.
Plating from the abo~/c formulation was initiated by ndding 0.9 gms of formaldehyde to a made up bath. The formulation yielded a copper deposit of 100 millionths of an inch in tllickness within a period of time of 30 minutes at 120F.
20 Tlle deposit was able to ~lithstand 6 bends.
The formulation was analyzed initially following the addition of the paraformaldehyde and after 4 hours of continuous use, and though the 0.9 grams of paraformaldehyde were detected following its addition, no free forrnaldehyde was detected after the four hour plating time.

-~ZQ~ ;3 Example 2 Cupric sulfate pentahydrate (grn) 10 Additjon product (ml) 40 Pentahyc~o~propyldiethylene triamine (gm) 30 S Tri}3ydroxypropyl arrline (gm) 20 Sodium hydroxide (50% solution -ml) 40 water to I liter Example 3 Cupric Sulfate Pentahydrate (grn) 10 Addition product (ml) 40 ml Ethylenediaminetetrancetic ncid (gm) 35 Sodiurn hydroxidc (5U% solution--ml) 40 Water to 1 liter Solul:ions of Examples 2 and 3 performed in a manner similar to that of 15 Example 1. Thereafter, Example 1 was repeated ith the hydroxide convent lowered to 9 ml. Formaldehyde fumes were readily detectable throughout a 4 hour plating sequence.
Copper solutions of this invention find utility for all purposes for which electroless copper solutions have heretofore been used inc]uding decorative and 20 electrical applications. They are especially useful for the formation of printed circuit boards where the dcposit acts as ductile conductors and as ductile connectors plated onto the walls of through-holes. The formation of a printed circuit board with conductive througl~holes is illustrated in the following example.

:~'2,~ i3 Exûmple 4 (a) Scrub clean copper on a copper clûd epoxy circuit board base materisl and etch to activate the copper.
(b) Drill through-holes at desired locations.
(c) Immerse in a one-step pa11adium sensitizing solution m&intained at room temperature for A period of five minutes.
(d) Silk screen a reverse image of a printed circuit board pattern onto the roughened surface of the phenolic substrate using an epoxy plating resist and dry.
(e) Deposit electroless copper from the solution of Exumple 1 with copper deposition taking place on thc walls of the thro~lg~holes nnd on the e~poscd copper of the clndding. No copper dcpositioll should tuke place on tic cpoxy resist.
(f) Plate electrolytic solder onto the exposed copper conductors.
(g) Remove lie plating resist.
(h) Irnmerse in a stripping solution of 10 grarns of copper chloride, 100 grams of 37% hydrochloric acid Qnd ton to I liter muintained at room temperature for six minutes to remove exposed copper.
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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electroless copper plating solution compris-ing dissolved copper, hydroxide, sufficient complexing agent for the dissolved copper to render the same soluble in alkaline solution and the addition product of formalde-hyde and a formaldehyde agent, said solution having a pH of at least 11 and a molar ratio of copper and free formalde-hyde to hydroxide varying between 1 to 10 and 1 to 40.

2. The solution of claim 1, where the pH of the solution varies between 11 and 14.

3. The solution of claim 1, where the pH of the solution varies between 11.5 and 12.5.

4. The solution of claim 1, where the ratios of copper and free formaldehyde to hydroxide vary between 1 to 12 and 1 to 25.

5. The solution of claim 1, where the addition product of formaldehyde and the addition agent are reacted together before addition to the copper plating solution.

6. The solution of claim 1, where the addition agent is selected from the group of salts of sulphites, bisulphites, nitrites and phosphites, said salt having a cation non-interfering with the plating solution.

7. The solution of claim 6, where the formalde-hyde addition agent is bisulfite.

8. A process for initiating plating from a freshly prepared electroless copper plating solution comprising a source of dissolved copper, hydroxide, complexing agent for the dissolved copper in an amount sufficient to render the same soluble in alkaline solution and the addition product of formaldehyde and a formaldehyde addition agent, said solution having a pH of at least 11 and a molar ratio of copper and free formaldehyde to hydroxide varying between 1 to 10 and 1 to 40, said process comprising the addition of free formaldehyde to the solution in an amount sufficient to initiate deposition.

9. The process of claim 8, where the concentration of free formaldehyde added to the solution varies between about 0.01 to 0.05 moles per liter of solution.