FEELD OF INVENTION
This invention relates to a Solder Paste With Improved Shelf Life Properties. BACKGROUND
With the advent of surface mount technology, there is a substantial increase in the consumption of solder pastes. In both HMC'S and PCB'S solder paste is used to interconnect active and passive components/devices, connectors and other components onto the board. In the component assembly, solder pastes serve as joining materials for attaching leads, lead frames and the structural units to make end use components. The significant driving force for the use of SMT is the reduced package size and cost and improved board utilization as the package to board interconnection spacing changes from 2.54 mm to 1.27 mm or below.
Solder paste is a homogenous blend of solder alloy powder and flux. The flux has rosin as base dissolved in suitable solvent along with activator, thixotropy inducing agents and other special additives. Solder pastes are transferred to the pad by techniques such as: Stencil/Screen prining, pneumatic dispensing or pin transfer. The viscosity of the solder paste is adjusted depending upon the type of application required. After printing, the components are placed and reflowed to form a proper joint.
An ideal solder paste should provide good printability (i.e good release from the stencil
or pin etc), adequate adhesion of component leads (oivresistance to early drying). Post
_ reflow, a good solder paste should exhibit good solder filleting, no solder balls and easy
cleanability. Solder pastes are recommended to be stored at a temperature of 4-6°C. However in industrial use, the solder pastes are required to be frequently taken in and out
of the storage at room temperature. This temperature and humidity cycling cause evaporation of solvents leaving the paste dry and causing change in thixotropic properties
making it difficult to apply. Again if the solvents in the flux are highly volatile, the past, may dry up even before the components are placed. For the effective practical use, solder pastes are required to retain their consistency at least for a period of 6 months to one year. Hence, it is very important that solder paste is resistant to drying and it should retain its pasty nature for the minimum specified time.
OBJECTS OF THE INVENTION
An object of this invention is to propose a solder paste with improved shelf life properties.
Another object of this invention is to propose a solder paste wherein the solvent is prevented from evaporating faster from the printed/stored paste and also keeps the paste homogeneous when stored.
Still another object of this invention is to propose a solder paste having improved dispensing properties and can be dispensed by pin transfer or using dispensing machines.
Yet another object of this invention is to propose a solder paste which is smooth and retains its pasty nature for a substantial period and even if there be a time lapse between printing, placement of components and reflow, the paste remains tacky and on reflow
forms good joint between the components and the PCB.
Still another object of this invetion is to propose a solder paste and wherein there is no separation between solder powder and organics/flux after storing for long time.
A further object of this invention is to propose a solder paste and wherein the residue obtained after reflow is very less and can be easily cleaned by a common solvent containing a small fraction of reducing agent.
DESCRIPTION OF INVENTION
According to this invention there is provided a solder paste with improved shelf life properties comprising 85 to 94% by weight of solder powder and 6 to 15% by weight of a flux wherein solder powder is an oxide free solder powder of the kind as herein described and flux compring 17 to 22% of rosin, 0.5 to 8% of an activator, 2 to 5% of a polymer binder, 7 to 11% of parafinic compound such as petroleum jelly, 6 to 10% of castor oil and its derivatives, 3 to 6% of citric acid and other dibasic acids, and 45 to 60% of organic solvent.
SOLDER POWDER SPECIFICATIONS; -
There are four types of solder powders used for making solder pastes.
(Table Removed)

The solder powders used for preparing the paste mentioned in this patent were of two compositions:
a) 63Sn/37Pb
b) 62Sn/36Pb/2Ag
The particle size of these pastes was 25-45µm i.e. the powder was ofTYPE-III.

FLUX COMPOSITION
The flux contains about 17-22% of rosin as base material. The rosin may be either tall oil, wood rosin, gum rosin or modified rosin. The activator forms 0.5-8% of the flux, which may be halogenated amines, primary aliphatic amines such as adipic acid or malic acid. Viscosity of the flux was adjusted by polymer binders such as ethyl cellulose and its derivatives, starch etc. in the range of 2-5%, parafinic compounds in the range of 7-11%, 6-10% of castor oil and its derivatives and glycol ester of acid such as adipic acid, Citric acid and other dibasic acids constituting about 3-6% of the flux. Organic solvent, which is a mixture of glycols, cellosolves and carbitols, etc formed about 45 to 60% of the flux composition.
SOLDER PASTE MAKING PROCEDURE
Solder paste is prepared by mixing the separately prepared flux and the solder powder in the desired proportion depending upon the required viscosity and application needs. The paste mentioned in this patent is prepared in the following way:
FLUX MAKING PROCEDURE
The required amount of solids including thixotropic agents are accurately weighed and added to the organic solvents in a silica crucible and mixed. The activator is added and mixed subsequently. The mixture is then further mixed manually for half an hour. The flux is then stored in air tight storage bottles and kept binder refrigeration untill required for paste preparation.
PASTE MAKING PROCEDURE
Depending upon the dispensing method of the solder paste, the flux and solder powder are mixed in the appropriate proportion to give the required viscosity. The solder powder constitutes about 85-94% of the solder pastes and flux percentage may be in the range of 6-15%. The oxide free solder powder and the flux are blended together in the desired ratio to make a homogeneous paste. The paste by the above mentioned process leads to excellent dispensing and reflow properties (tested using an IR reflow station). Most importantly the paste shows improved shelf life property and even after 6-7 months shows good dispensing and tackiness. This can be attributed to the special paraffmic compounds used in the flux. The residue obtained after reflow is minimal and is easily cleanable with a warm mixture of common cleaning solvent containing a small fraction of reducing agent and a saponifier.
EXAMPLES Example I:-
Solder flux was made by dissolving 4gms of ethyl cellulose, 6gms of castor oil, 2 gms of hydrogenated castor oil, 7gms of petroleum jelly and 4 gms of ethylene glycol ester of citric acid and 19 gms of WW rosin in 50 gms of a mixture of organic solvents containing terpineol, butyl carbitorl acetate, butanediol, dibutyl phthalate. 1 gm of hydroxyl amine hydrogen choloride was added and mixed to form a homogeneous blend. Solder paste was made by mixing 85 gms of type -III solder powder with alloy composition 62Sn/36Pb/2Ag, and 15 gms of the above mentioned flux. The paste showed excellent dispensing and wetting properties and could be cleaned with common organic solvents.
Example II
Solder flux was made according to the procedure mentioned above. Solids constituted of 3 gms of ethyl cellulose, 5 gms of castor oil, 3 gms of hydrogenated castor oil, 6 gms of petroleum jelly, 5 gms of ethylene glycol easter of citric acid and 22 gms of WW rosin. All these materials were dissolved in 55 gms of solvent mixture containing terpineol, butyl carbitol acetate, butanediol and dibutyl phthalate. 2 gm adipic acid was used as activator. Solder paste was then made with 87 gms of type III solder powder - with alloy composition 63Sn/37Pb.

WE CLAIM:
1. A solder paste with improved shelf life properties comprising 85 to 94% by weight of solder powder and 6 to 15% by weight of a flux wherein solder powder is an oxide free solder powder of the kind as herein described and flux comprising 17 to 22% of rosin, 0.5 to 8% of an activator, 2 to 5% of a polymer binder, 7 to 11% of parafinic compound such as petroleum jelly, 6 to 10% of castor oil and its derivatives, 3 to 6% of citric acid and other dibasic acids, and 45 to 60% of organic solvent.
2. A process paste as claimed in claim 1, wherein said rosin is selected from tall oil, wood rosin, gum rosin or modified rosin.
3. A process paste as claimed in claim 1, wherein said activator comprises halogenated amines, primary aliphatic amines such as adipic acid or malic acid of the kind as herein described.
4. A process paste as claimed in claim 1, wherein said polymer binders comprises ethyl cellulose and its derivatives, starch of the kind as herein described.
5. A process paste as claimed in claim 1, wherein the said castor oil and its derivatives of the kind as herein described.
6. A process paste as claimed in claim 1, wherein said organic solvent is a mixture of glycols, cellosolves and carbitols of the kind as herein described.
7. A solder paste substantially as herein described and illustrated in examples.