JP3314856B2 - Solder paste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 In recent years, electronic devices have tended to be light, thin and small.
Electronic components used in such electronic devices have also become very small and multifunctional. Small, multifunctional electronic components are
This is a so-called fine pitch component in which a large number of leads are provided on the main body, and the distance between the leads is very narrow.

If a fine pitch component is soldered to a printed circuit board by an immersion method, a bridge is formed between adjacent leads where the solder straddles and adheres. In other words, in the immersion method, soldering is performed by applying flux with a fluxer installed in an automatic soldering device, preheating with a preheater, applying solder in a solder tank, cooling with a cooler, and fine pitch parts. Since the space between the leads is narrow, when the molten solder is brought into contact with the molten solder in the solder bath and then pulled up, the molten solder adhering between the leads is not broken by the surface tension and becomes a bridge.

However, when fine-pitch parts are soldered by a reflow method using a solder paste, the occurrence of bridges can be eliminated. This is because a small amount of solder paste is applied only to the soldering portion on which the lead is to be mounted, so that even if the solder paste melts, the solder does not spread to adjacent leads. Therefore, the reflow method is often used for soldering a fine pitch component and a printed circuit board.

[0005] The solder paste used in the reflow method is
This is a mixture of powder solder and paste-like flux to make it viscous. This can be printed and applied only to the required parts of the printed circuit board using a metal mask or silk screen, or it can be applied by dispensing with a dispenser. Or

The flux of the solder paste is obtained by dissolving solid components such as rosin, an activator, and a thixotropic agent with a solvent to form a paste.

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

[0008] The adhesiveness of rosin can be measured by applying a solder paste to a printed circuit board, mounting electronic components thereon, and then transferring the printed circuit board. It helps to prevent the components from sticking and falling off the printed circuit board.

The activity of rosin reduces the oxide film on the lands of the printed circuit board and the leads of the electronic components during the soldering to improve the solderability. However, the flux of the solder paste is not sufficient only with rosin, so an activator for improving the solderability is separately added.

[0010] The coating properties of rosin are flux residues after soldering and cover the entire soldered portion to protect it from the outside air. In other words, electronic devices
When the power is turned on, the temperature inside rises, and when the power is turned off, the temperature drops. Therefore, when the temperature drops, moisture in the atmosphere condenses and water droplets adhere to the printed circuit board. However, if the solder residue on the printed circuit board is covered with the flux residue, the condensed water droplets are disturbed by the flux residue and do not directly adhere to the land of the printed circuit board or the lead of the electronic component, and the land or the lead does not rust.

[0011]

On the printed circuit board after soldering, inspection using checker pins is performed as a method for inspecting soldering defects such as bridges and unsoldering. Inspection with checker pins is a method in which a large number of sharpened checker pins are brought into contact with all the soldered portions at once, and a defective soldering portion is electrically inspected.

In the inspection using the checker pin, the tip of the checker pin is made to penetrate through the coating of the flux residue, and the solder and the checker pin are electrically connected. Therefore,
When the flux residue penetrates the checker pin,
Even if there is no way to make a small hole, it must be one that does not largely peel or crack. This is because if the flux residue is peeled or cracked (hereinafter referred to as residue crack) in the checker pin inspection, the waterproof effect is lost. Residual cracking during inspection with checker pins is due to the thermoplastic nature of rosin. In other words, the flux containing rosin has the property of being brittle even if it becomes a flux residue after soldering. In particular, in the winter season, the flux becomes more brittle and the residue cracks easily occur.

It has been known that if the flux residue after soldering is made brittle, residue cracking will not occur when inspecting with a checker pin. , A plasticizer, a resin, and the like have been added to the solution.

The plasticizers added to the conventional solder paste flux include phthalates, aliphatic dibasic esters, orthophosphates and the like, and the resin is a polymer rubber-like resin. Was.

[0015]

By the way, conventional solder pastes to which a plasticizer or a resin is added (hereinafter referred to as "residual crack preventing solder paste") do not sufficiently prevent residue cracking or have poor printability. There were problems such as insufficient adhesiveness, poor solderability, unsoldering, poor wetting, and frequent occurrence of minute solder.

[0016]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on preventing residual cracks and improving the problems of the conventional solder paste for preventing residual cracks. The present inventors have found that the present invention is effective and completed the present invention.

[0017] The present invention provides a solder paste mixed with solder powder and a paste flux Sorudapesu
The resin flux and the activator are contained in the flux of trimethacrylate and tri-2-ethylhexyl trimetate
Rimellitate or tri-normal octyl trimellitate
Bets are solder paste, characterized in that the addition of 1 to 30 wt%.

[0018]

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

[0019]

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The trimellitate 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 trimellitate 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 as follows:
The composition of the powder solder is 63Sn-Pb, the particle size of the powder solder is 50 μm or less, a spherical powder, 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 on which lands on which 10 QFPs can be mounted are formed

(1) Printability Solder paste is printed on a land of a printed circuit board with a metal mask and a squeegee, and the printed state is observed. ○, if solder paste is uniformly applied to all lands
If the land of non-uniform application is up to 20%, then mark Δ; if the land of non-uniform application exceeds 20%, mark x.

(2) Adhesiveness After printing and applying the solder paste and mounting the QFP, the printed circuit board is tilted by 90 degrees and the number of QFPs dropped is counted. If the dropout is 0, it is rated as ○, the dropout is up to 10%, and the dropout is more than 10%, ×.

(3) Solderability After mounting the QFP on the solder paste application section, the solder paste is heated in a reflow furnace to melt the solder paste, and the state after soldering is observed.不良 indicates a defect such as unsoldering or insufficient wetting, ○ indicates a defect up to 10%, and × indicates a defect exceeding 10%.

(4) Fine solder balls Observe the state of the fine solder balls generated after soldering. The number of micro solder balls is 0 when the number is 3 or 4;

(5) Peeling State The printed circuit board after soldering is inspected with a checker pin, and the state of cracking of the flux residue after the inspection is observed. 0 indicates residual cracks, Δ indicates residual cracks up to 20%, and X indicates residual cracks exceeding 20%.

[0031]

As described above, the solder paste of the present invention is free from residue cracks, has good printability, has little occurrence of fine solder balls, and has high adhesiveness.
Further, the solder paste has an excellent feature that the solderability is good, which is not found in the conventional solder paste for preventing residue cracking.

Continuation of front page (56) References JP-A-4-200992 (JP, A) JP-A-5-42388 (JP, A) JP-A-8-3322591 (JP, A) JP-A-4-351287 (JP) JP-A-7-1184 (JP, A) JP-A-3-57589 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 35/363

Claims (1)

(57) [Claims] 1. A solder paste obtained by mixing a powdery solder and a paste-like flux, wherein the flux of the solder paste contains a resin and an activator, and further comprises tri-2-ethylhexyl trimellitate, which is a trimet acid ester.
Or tri-normal octyl trimellitate
Solder paste characterized by adding 30% by weight.