JPH09234588A - Solder paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a solder paste which has good printability, little development of fine solder ball and good soldering property without developing residual crack, by adding a specific quantity of specific component in a flux of the solder paste. SOLUTION: In the solder paste admixing powdery solder and pasty flux, ester trimellitate is added by 1-30wt.% in the flux of the solder paste. By this constitution, the residual crack can be prevented and also, excellent printability and high adhesiveness are obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a solder paste suitable for soldering an electronic component and a printed circuit board.

[0002]

2. Description of the Related Art Recent electronic devices tend to be light, thin, short and small.
The electronic components used in this electronic device are also becoming very small and multifunctional. Small and multifunctional electronic components are
This is a so-called fine pitch component in which a large number of leads are installed in the main body and the distance between the leads is very narrow.

When the fine pitch component is soldered to a printed circuit board by a dipping method, a solder bridge is formed between adjacent leads to form a bridge. That is, in the immersion method, flux is applied with a fluxer installed in an automatic soldering device, preheating is performed with a preheater, solder is attached in a solder bath, and cooling is performed with a cooler for soldering. Since the distance between the leads is narrow, when the molten solder in the solder bath is brought into contact with and then pulled up, the molten solder adhered between the leads is not broken by the surface tension but becomes a bridge.

However, when fine pitch components are soldered by a reflow method using a solder paste, the generation of bridges can be eliminated. This is because a small amount of solder paste is applied only to the soldering portion on which the leads are mounted, and therefore even if the solder paste melts, the solder does not spread to the adjacent leads. Therefore, the reflow method has been widely used for soldering fine pitch components and printed circuit boards.

The solder paste used in the reflow method is
It is a viscous mixture of powdered solder and paste-like flux, which can be applied by printing on a required part of the printed circuit board using a metal mask or silk screen, or by dispensing with a dispenser. To do

The flux of the solder paste is a paste prepared by dissolving solid components such as pine resin, activator, thixotropic agent, etc. in a solvent.

The pine resin used for the flux of the solder paste has adhesiveness, activity, and a property of covering the soldering surface after soldering (hereinafter referred to as "coverability"), and is a very suitable property as a material for the flux of the solder paste. have.

The adhesiveness of pine resin means that even if the printed circuit board is coated with solder paste and then electronic components are mounted on the printed circuit board and the printed circuit board is conveyed, it may be vibrated or tilted. It helps prevent the parts from sticking and falling off the printed circuit board.

The activity of pine resin improves the solderability by reducing and removing the oxide film of the land of the printed board and the lead of the electronic component during soldering. However, as the flux of the solder paste, the activity of only pine resin is not sufficient, so an activator that improves solderability is added separately.

The covering property of pine resin is that it becomes a flux residue after soldering and covers the entire soldered portion to protect it from the outside air. In other words, the electronic device
When the power is turned on, the inside temperature rises, and when the power is turned off, the temperature drops. Therefore, when the temperature is lowered, moisture in the atmosphere is condensed and water drops adhere to the printed circuit board. However, if the flux residue covers the soldered portion of the printed circuit board, the condensed water drops are prevented from directly contacting the land of the printed circuit board and the lead of the electronic component by the flux residue, and the land and the lead are not rusted.

[0011]

By the way, in a printed circuit board after soldering, a checker pin is used as a method for inspecting a soldering failure such as a bridge or unsoldered solder. The checker pin inspection is a method in which a large number of checker pins each having a sharp tip are brought into contact with all the soldering portions at once to electrically inspect a defective soldering portion.

In the inspection using the checker pin, the tip of the checker pin is penetrated through the film of the flux residue, and the solder and the checker pin are electrically energized. Therefore,
Flux residue, when passing the checker pin,
Even if there is no small hole, it must not peel or crack. This is because if the flux residue is peeled off or cracked in the checker pin inspection (hereinafter referred to as residue cracking), the waterproof effect is lost. Residue cracking during inspection with a checker pin is due to the thermoplasticity of pine resin. In other words, the flux containing pine resin has the property of becoming brittle even if it becomes a flux residue after soldering, and in particular, it becomes more brittle in winter and cracks easily occur in the residue.

It has been known that if the flux residue after soldering is eliminated from brittleness, residue cracking will not occur during inspection with a checker pin. It has been practiced to add plasticizers, resins, etc.

The plasticizers added to the flux of the conventional solder paste are phthalic acid esters, aliphatic dibasic acid esters, orthophosphoric acid esters, and the resin is a polymer rubber-like resin. It was

[0015]

By the way, the conventional solder paste containing a plasticizer or resin (hereinafter referred to as "residual crack prevention solder paste") does not have sufficient residual crack prevention or poor printability. There are problems such as insufficient tackiness, poor solderability, unsoldered solder, poor wetting, and frequent occurrence of fine solder.

[0016]

Means for Solving the Problems The present inventor has conducted diligent research to prevent residue cracking and to improve the problems of the conventional residue crack prevention solder paste. The present invention has been completed by finding out that it has the effect.

According to the present invention, in a solder paste in which powdered solder and a paste-like flux are mixed, 1 to 3 trimellitic acid esters are contained in the flux of the solder paste.
The solder paste is characterized in that 0% by weight is added.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The trimellitic acid ester is represented by the following general formula.

[0019]

Embedded image

The trimellitic acid ester used in the present invention includes tri-2-ethylhexyl trimellitate, trinormal octyl trimellitate, triisononyl trimellitate, triisodecyl trimellitate and the like. In the present invention, one or more of these trimellitic acid esters are added.

[0021]

EXAMPLES Examples and comparative examples of the present invention are shown in Table 1 below.

[0022]

[Table 1]

The solder pastes of Examples and Comparative Examples are
The composition of the powdered solder is 63Sn-Pb, the spherical powder having a particle diameter of 50 μm or less, and the flux content is 10% by weight.

The characteristic tests shown in Table 1 are as follows.

Test sample QFP: Lead pitch 0.5 mm, number of leads 100 Printed circuit board: Printed circuit board formed with lands capable of mounting 10 of the above QFP

(1) Printability A solder paste is printed on a land of a printed board with a metal mask and a squeegee, and the printed state is observed. If the solder paste is uniformly applied to all lands, ○,
Up to 20% of non-uniformly coated lands are marked with Δ, and when non-uniformly coated lands exceed 20%, they are marked with x.

(2) Adhesiveness After mounting the QFP on the solder paste by printing and printing, the printed board is tilted by 90 degrees and the number of QFPs dropped off is counted. Dropout is 0, o is up to 10%, and if dropout is over 10%, x.

(3) Solderability After mounting the QFP on the solder paste application portion, the solder paste is heated in a reflow furnace to melt the solder paste, and the state after soldering is observed. A defect of 0 such as unsoldered or insufficient wetting is evaluated as O, a defect of 10% is evaluated as Δ, and a defect of more than 10% is evaluated as X.

(4) Fine solder balls The state of fine solder balls generated after soldering is observed. When the number of minute solder balls is 0 to 3, it is ◯, when 4 to 10 is Δ, and when 11 or more, it is x.

(5) Peeling State The printed circuit board after soldering is inspected with a checker pin, and the state of residue cracking of the flux residue after inspection is observed. Residual cracking is 0 when it is ○, residual cracking is up to 20% when it is Δ, and residual cracking when it exceeds 20% is marked as x.

[0031]

As described above, the solder paste of the present invention has no residue cracking, good printability, few fine solder balls, and high adhesiveness.
Further, it has an excellent feature that the solder paste has a good solderability, which is not present in the conventional residue crack preventing solder paste.

Claims (1)

[Claims] 1. A solder paste in which powdered solder and a paste-like flux are mixed, wherein the trimellitic acid ester is 1 to 30% by weight in the flux of the solder paste.
Solder paste characterized by being added.