JPH04300088A - Composition for depositing low melting point solder and method for producing low melting point solder precoated circuit board - Google Patents

Abstract

PURPOSE:To selectively deposit low melting solder on pad parts so that a precoat can be formed by applying the compsn. for depositing specific low melting solder on circuit patterns and heating the compsn. at a low temp. CONSTITUTION:The essential components of the compsn. for depositing the low melting solder are composed of metallic tin powder, metallic bismuth powder and lead salt of org. acid. The salt of org. acid comes into contact with tin having a high ionization tendency to generate an ion exchange reaction and the tin partly ionizes to form the salt of org. acid when this compsn. is heated at a low temp. The lead constituting the original org. acid salt deposits simultaneously to generate the alloy with the tin of the powder metal and to form the solder. The precoat is thus formed. Electronic parts are mounted on this precoated circuit board and the solder is subjected to reflow, by which the low melting solder is melted and the lead wires of the electronic parts are joined.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to a composition for depositing low melting point solder by heating, and a method for precoating low melting point solder onto pads of a circuit board using the composition. .

0002

2. Description of the Related Art In recent years, as various electronic devices have become smaller, electronic components have been surface mounted on circuit boards. In surface mounting, cream solder is applied to the pads of the circuit board, electronic components are placed on top of it, and the entire structure is passed through a reflow oven and heated to melt the cream solder and connect the lead wires of the electronic components. It is soldered directly to the pads of the circuit board.

However, with this method, as wiring patterns on circuit boards become finer, bridging phenomena in which conduction occurs between pad portions due to solder frequently occur, making it impossible to cope with higher circuit densities.

[0004] Therefore, the applicants invented a solder composition that can be soldered to fine circuit patterns, and filed a patent application in 1983.
- Filed as No. 156688, JP-A-1-15779
Published as number 6.

[0005] This invention comprises a powder of a metal that has the highest ionization tendency among the metals constituting the solder, and an organic salt of another metal. In other words, tin-lead solder is a composition consisting of metallic tin powder and organic lead salt, and when it is applied to a circuit board and heated, lead, which has a small tendency to ionize, is precipitated by an ion exchange reaction, and is combined with tin. An alloy is formed into solder that is selectively deposited on the pad to form a solder precoat.

[0006]

[Problems to be Solved by the Invention] The composition of the invention is
Although this is an extremely excellent means for forming a solder precoat made of tin and lead, it is not necessarily preferable for forming a three-component solder by adding other components.

When surface mounting electronic components on a circuit board,
Although it is necessary to heat the solder in a reflow oven to melt the solder, it is preferable to reflow at a lower temperature to prevent heat damage to electronic components and circuit boards and to save heating energy.

As a solder alloy having a low melting point, a ternary solder of tin-lead-bismuth is known.

[0009] If this solder composition were to be formed based on the above invention, since tin has the greatest tendency to ionize, a composition consisting of tin powder and organic acid salts of lead and bismuth would be used. Will be using it.

However, in this composition, bismuth with a small ionization tendency precipitates first to form a tin-bismuth alloy, and then lead precipitates to form a ternary alloy to form a low melting point solder. Because the melting point of the tin-bismuth alloy is high, it does not precipitate at low temperatures, and ultimately cannot be reflowed unless the temperature is raised to the same level as tin-lead solder.

Therefore, even if the melting point of the solder alloy finally obtained is sufficiently low, the solder alloy cannot be formed in a reflow oven unless the temperature is raised to a considerably higher temperature. The purpose of lowering the reflow temperature cannot be achieved.

The present invention was made in view of the above circumstances, and provides a composition capable of forming a three-component low melting point solder alloy at a low reflow temperature, and a low melting point solder pre-coated circuit board using the composition. The purpose of this invention is to provide a method for manufacturing.

[0013]

[Means for Solving the Problems] The first invention of the present application is characterized in that the main components are metallic tin powder, metallic bismuth powder, and organic acid lead salt. The invention is characterized in that the main ingredients are an alloy powder of tin and bismuth and an organic acid lead salt.

The third invention is characterized in that the main components are metallic tin powder, metallic lead powder, and organic acid bismuth salt, and the fourth invention is characterized in that the main components are metallic tin powder, metallic lead powder, and organic acid bismuth salt. It is characterized by containing alloy powder and organic acid bismuth salt as main components.

Furthermore, a fifth invention is characterized in that the low melting point solder composition according to each of the above inventions is blended with a solvent, an adhesive, an activator, and a melt sag prevention agent to form a paste. .

A sixth invention is an invention of a method for manufacturing a pre-coated circuit board, in which the paste-like composition of the fifth invention is applied to a circuit board and heated to form a low-melting point coating on the pad of the circuit board. It is characterized by causing solder to precipitate.

That is, the low melting point solder precipitation composition of the present invention contains tin and either lead or bismuth among the components constituting the ternary low melting point solder of tin-lead-bismuth. It is formulated as a metal powder, and the other one of lead and bismuth is an organic acid lead salt.

In the conventional invention, in the three-component solder, only tin is mixed as a metal powder, and both lead and bismuth are mixed as organic acid salts, but in the present invention, is not only tin, but also
Either lead or bismuth is also blended as metal powder.

Either lead or bismuth may be blended as a metal powder or as an organic acid salt, but it is more preferable to use bismuth as a metal powder and lead as an organic acid salt.

[0020] Tin, lead, or bismuth may be used as a mixture of individual metal powders, or an alloy powder of the two may be used.

[0021] As the organic acid constituting the organic acid salt of lead or bismuth in the present invention, carboxylic acid is suitable;
In particular, it is preferable to use rosin or its derivatives. Rosin originally has an excellent function as a soldering flux, and the lead or bismuth salt of such rosin has both the functions of a part of solder and a flux.

As such rosin, in addition to gum rosin, tall oil rosin, wood rosin, etc., pure substances such as abietic acid and pimaric acid, which are the main components of these rosins, can also be used.

As derivatives of these rosins, asymmetric rosins, hydrogenated rosins, maleated rosins, or fumarated rosins can also be used. In particular, maleated or fumarized rosin is a tribasic carboxylic acid by introducing carboxyl groups by maleating or fumarizing the rosin, so by converting these three carboxyl groups into metal salts, , the content of lead or bismuth can be increased.

Other organic acids include formic acid, acetic acid,
Lower fatty acids such as propionic acid, fatty acids obtained from animal and vegetable oils such as caproic acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, and linoleic acid, 2,2-dimethylpentanoic acid, 2-
Synthetic carboxylic acids such as ethylhexanoic acid, isononanoic acid, 2,2-dimethyloctanoic acid, n-undecanoic acid,
Various organic acids can also be used, such as naphthenic acid.

The composition of the present invention is mainly composed of the metal powder and an organic acid metal salt, and is used after being dissolved in an organic solvent, if necessary. Furthermore, an adhesive, an activator, and a melt sag prevention agent can be added to form a paste.

To form a precoat of low melting point solder on a circuit board using these compositions, a paste of the composition is applied to the circuit board and heated to form a low melting point solder onto the pads of the circuit board. is precipitated.

When applying the paste to the circuit board, the paste can be supplied to the soldering area by screen printing, a dispenser, or the like, similar to conventional cream solder. At this time, the paste can be applied to the entire surface of the circuit board, and there is no need to print according to a fine circuit pattern.

By heating the circuit board coated with solder paste in a heating furnace, the metal constituting the organic acid salt precipitates, mixes with the metal blended as a powder, and forms tin-
A lead-bismuth ternary solder alloy is formed and selectively deposited on the pad portion to form a precoat.

Further, an electronic component is placed on the pad portion of the circuit board, and this is heated in a reflow oven to reflow the pre-coated solder, thereby joining the pad portion and the lead wire of the electronic component.

[0030] It is appropriate that the maximum temperature of the heating furnace and reflow furnace is 180 to 190°C. Tin-lead-bismuth ternary solder has a melting point of approximately 135 to 160°C.
Therefore, by heating it to 180 to 190°C, it can be sufficiently melted and soldered.

[0031]

[Function] In the low melting point solder deposition composition of the present invention, the organic acid salt comes into contact with tin, which has a strong tendency to ionize, to cause an ion exchange reaction, and some of the tin is ionized to become organic acid tin, and the organic acid salt is The metal that made up the acid salt precipitates and forms an alloy with the powdered metal to form solder.

When the metal constituting the organic acid salt is lead and bismuth is blended as metal powder, heating causes an ion exchange reaction between tin and lead, and the metal bismuth does not contribute to the reaction.

[0033] Bismuth also constitutes an organic acid salt,
If lead is mixed as metal powder, tin-
In addition to the ion exchange reaction between bismuth, it is believed that an ion exchange reaction between lead and bismuth also occurs, since bismuth has a smaller tendency to ionize than lead. However, the lead that has been ionized by ion exchange with bismuth causes the tin-lead ion exchange reaction again, and metallic lead is precipitated.Finally, both bismuth and lead are precipitated as metals, and tin is ionized.

Therefore, in either case, the lead or bismuth salt of the organic acid is precipitated as a metal, resulting in a tin-lead-bismuth ternary solder alloy.

However, when bismuth constitutes an organic acid salt, the reaction follows a complicated route as described above, and depending on the conditions, there is a possibility that a portion of lead or bismuth may remain as the organic acid salt.

Therefore, the product using tin and bismuth metal powder and lead organic acid salt is more effective because bismuth does not contribute to the reaction and the reaction occurs only by a simple ion exchange reaction between tin and lead. preferable.

The process of solder formation during heating in the composition of the present invention is not necessarily clear, but it is presumed to be as follows.

In the present invention, lead or bismuth precipitated by ion exchange with tin appears to be extremely reactive as a nascent metal immediately after precipitation.

In addition, the surface of tin, bismuth, or lead mixed as metal powder is activated by the action of an organic acid, and the bismuth or lead powder partially penetrates the surface of the tin powder, and is partially activated. forming an alloy.

Since the above-mentioned nascent lead or bismuth acts on this, the nascent lead or bismuth quickly penetrates into the tin powder particles and comes into contact with the bismuth or lead powder on the surface of the tin powder particles. It constitutes a low melting point alloy of the component system and melts by heat.

[0041] By forming a solder alloy in a part of the tin powder and melting it, the penetration of bismuth or lead into the tin powder is further promoted, and the nascent lead or bismuth acts thereon, so that the three components are It is thought that alloying progresses and a low melting point solder alloy is rapidly formed.

[0042] When an alloy powder of tin and bismuth or lead is used as the metal powder, the nascent lead or bismuth penetrates the surface of the alloy and immediately forms a ternary solder alloy. .

Therefore, if the reflow temperature is higher than the melting point of the tin-lead-bismuth ternary solder alloy, solder will be generated even at a temperature sufficiently lower than the melting points of the individual metals, and the solder will melt and soldering will not be possible. It is considered possible.

When a tin-lead-bismuth ternary solder is formed by the conventional method described above, bismuth, which has a small ionization tendency, precedes the precipitation of lead and bismuth.
A bismuth layer is formed on the surface of the tin powder particles, and even if lead subsequently precipitates, it rarely comes into contact with tin, making it difficult to form a three-component solder.

[0045]

Therefore, according to the present invention, by applying a composition for depositing low melting point solder onto a circuit pattern and heating it at a low temperature, low melting point solder is selectively deposited on the pad portion, forming a precoat. .

By placing electronic components on this pre-coated substrate and performing reflow, the low melting point solder melts and the lead wires of the electronic components can be joined.

Further, according to the present invention, even if the composition for depositing low melting point solder is applied all over the circuit board, the low melting point solder is selectively deposited on the pad portions of the circuit pattern, so that it is possible to print according to the circuit pattern. There's no need.

Furthermore, in the present invention, the surface of the produced low melting point solder is extremely clean and maintains an active surface state, and the absolute amount of metal powder particles in the composition is lower than that of conventional cream solder. Since the number of particles is small, the mobility of the metal particles is large and the followability to the circuit pattern is good. Therefore, it is possible to follow even fine circuit patterns that cannot be handled with conventional cream solder without causing a bridging phenomenon.

Furthermore, by precoating the pads of the circuit board, the surface area of the solder becomes smaller than when using cream solder, and the precoated surface is less likely to be oxidized, making it easier to use cream solder with the same solder composition. Electronic components can be bonded at lower temperatures than in the case of bonding.

[0050]

EXAMPLES Pastes of low melting point solder deposition compositions of Examples and Comparative Examples were prepared according to the formulations shown in Table 1.

[0051]

[Table 1]

Experiment 1 The composition pastes of Examples 1 to 4 and Comparative Examples 1 to 2 were applied onto a copper plate and heated on a hot plate at 150°C and 210°C. The deposited metal was analyzed by DSC and its melting point was measured.

The results of the measurements are shown in Table 2.

[0054]

[Table 2]

As can be seen from the results in Table 2, solder precipitated at 150° C. in all of the compositions of the present invention. On the other hand, in Comparative Example 1, a solder similar to that of the example of the present invention was obtained when precipitated at 210°C, and although the metal composition constituted a low melting point solder, Therefore, it cannot be precipitated, and its precipitability at low temperatures is poor. Moreover, Comparative Example 2 is a tin-lead solder, and 150
It cannot be precipitated at ℃.

[0056] Among the Examples, in Examples 1 and 2, the solidus line and the liquidus line coincide with each other in the case where the precipitation was carried out at 150°C and the case where the precipitation was carried out at 210°C.
It shows that the reaction is completely progressing at ℃. On the other hand, in Examples 3 and 4, the liquidus line of the precipitate at 150°C is slightly higher than that of the precipitate at 210°C, and it cannot be said that the precipitation of bismuth in the organic acid salt is complete.

[0057] Therefore, it can be understood that a composition containing bismuth as a powder and lead as an organic acid salt is more superior.

Experiment 2 A 0.5 mm pitch, 48 pin QFP pattern board was coated with the composition paste of Example 2 to a thickness of 500 μm, and the paste was left in a heating oven at 180° C. for 2 minutes to precipitate solder. Ta.

Thereafter, the substrate was washed with toluene to obtain a mounting board in which a pad width of 0.25 mm was precoated with a low melting point solder having a height of about 60 μm in a pedestal shape.

When this substrate was examined for the presence or absence of bridges, no bridges were observed at all.

Experiment 3 A temporary fixing agent/flux (Finebond BF-211 manufactured by Harima Kasei Co., Ltd.) was applied to the mounting substrate obtained in Experiment 2.
After printing on the entire surface to a thickness of 100μ, components were mounted on the pad portion and reflowed at a maximum temperature of 190°C.

[0062] The state of bonding between the lead wire and the pad portion on the board after reflow was observed using a 50x microscope.

[0063] The bonding condition was good, with no solder balls that did not contribute to bonding observed, and no bridging.

For comparison, a low melting point solder cream (Micro Solder PS110R-450A-F17 manufactured by Harima Kasei Co., Ltd.) was printed on the patterned substrate, parts were placed thereon, and reflow was performed under the same conditions.

[0065] The bonding condition was also good in this case, and no bridging occurred, but there were about 3 solder balls per 16 pins, and it seems that bridging may occur if the conditions are bad. Ta.

Claims (5)

[Claims] [Claim 1] A composition for low melting point solder precipitation, characterized in that the main components are metallic tin powder, metallic bismuth powder, and organic acid lead salt. [Claim 2] A composition for low melting point solder precipitation, characterized in that the main components are a powder of an alloy of tin and bismuth and an organic acid lead salt. [Claim 3] A composition for low melting point solder precipitation, characterized in that the main components are metallic tin powder, metallic lead powder, and organic acid bismuth salt. 4. A composition for low melting point solder precipitation, characterized in that the main components are a powder of an alloy of tin and lead and a bismuth salt of an organic acid. 5. A low-melting point solder precipitation composition, which is made into a paste by blending the composition of claims 1 to 4 with a solvent, an adhesive, an activator, and a melt sag prevention agent.
6. A method for producing a low melting point solder pre-coated circuit board, comprising applying the composition of claim 5 to a circuit board and heating the composition to precipitate the low melting point solder onto the pads of the circuit board.