JPH02303183A - Mounting of electronic component - Google Patents

Abstract

PURPOSE:To prevent an open defect and a bridge defect and to obtain a bonded part full of reliability by a method wherein, after a solder supplied to a pad part in advance has been melted, a lead terminal is placed on the pad part and, after that, is bonded by cooling the solder. CONSTITUTION:A solder paste 3 is first printed and supplied to a board 1; the board on which the solder paste 3 has been printed is put into an infrared furnace; the solder paste 3 is heated and melted. Then, an electronic component 4 is mounted; the solder paste is cooled; the electronic component 4 is soldered to the board. When the electronic component 4 is mounted, the solder is solderable to a lead terminal 5. Consequently, there is no fear that the solder protrudes to the outside of a pad part 2; the solder is drawn to the lead terminal 5; the solder paste is not hung down. A width of the pad part is 0.40mm; an interval between pad parts is 0.25mm. A pitch between adjacent pad parts and a lead pitch are 0.65mm.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for mounting electronic components having lead terminals.

[Conventional technology]

In recent years, reflow soldering has become increasingly popular for mounting electronic components on printed wiring boards. This reflow soldering method is generally described with reference to FIGS. 3 and 4. FIG. 8 is a plan view showing the printed tell wiring board 111, and FIG. A pad portion tll+ is provided on the substrate Ill. The pad portions (21 are made of, for example, an exposed copper layer. The group of rectangular pad portions arranged in two rows at the bottom left in the figure is the fourth
This is a flat type electronic component that can be cut in two directions as shown in the figure. The group of pads on the upper right side of the illustrated substrate 111 corresponds to seven rat-type electronic components in four directions shown in FIG.

The black flat type electronic component shown in Figures M and +Bl has a large number of lead terminals 5) around it.

Now, what about these flat type electronic components? To mount it on a board, print solder paste on the pad part (2) of the board (11) using a metal mask or screen mask, press the lead terminal of the electronic component onto this, and use the adhesive force of the solder paste to After temporary attachment, the solder base is melted by a heating process using infrared rays, steam, etc., and then solidified. This is a cross-sectional explanatory diagram that mainly shows the state of solder paste (solder) in order, and the bottom represents the state of the vine (final shape).In the figure, I3) solder paste, (5) H electronic components ( 41 lead terminal, 161i lead terminal 1!1) connection part.

The main problem with reflow soldering with lead terminals is the failure of the bridge (7) shown in the perspective view of the pad 14j5+21 and lead terminal 51 after mounting in Figure 6.
These are the occurrence of solder g re-shorting between lead terminals) and the occurrence of open failures (lead terminals are not connected to the pad portion and are in a floating state). - As the degree of integration of the four-chip circuits in the child component increases, the number of lead terminals drawn out from the periphery of the electronic component increases, and the spacing between the lead terminal arrays becomes narrower. As high-density packaging has become possible, generally speaking, too much solder will cause bridging defects, and too little solder current will cause open defects.

The conventional method was to feed the solder paste so that it did not protrude from the pad part in order to prevent bridge defects, or to reduce the solder adhesion.For example, Japanese Patent Publication No. 57-219072 The feature of the flow soldering method shown in the publication is that solder paste is screen printed on the pad portion so that it does not overflow from the pad portion.

Until then, solder plating was applied to the entire pad area and even around the pad area! The amount of solder that was present as being carved out is reduced by screen printing the aforementioned tapping solder paste, thus e.g.
It has been reported that bridge defects no longer occur even with the narrow lead terminal spacing of the 685ATG.

However, with this method, if the amount of solder supplied is extremely small, bridge defects will not occur, but the strength of the joint will decrease, causing problems in terms of reliability. In the case of wet solder supply, bridge defects occur. This is because with the current process, it is difficult to supply solder paste accurately, and even if solder paste is printed so that it does not protrude from the pad area, Then, when the component is pressed, the solder paste may ooze out, or the viscosity of the solder paste decreases during heating, causing it to ooze out from the pad, resulting in a bridge failure.

JP-A-58-182940 discloses that when an electronic component having lead terminals is mounted on a board, a layer of solder paste of a thickness narrower than the length of the connection part of the lead terminal is applied to the surface of the area where the pad part is located on the board. It is characterized by being uniformly coated all over the body. Until then, the solder paste layer t is wider than the length of the connection part of the lead terminal.
It was uniformly coated. In other words, this JP-A-Sho SS
Is 55-1s+o%m also solder acid? The idea is to reduce it.

However, the length of the connection part of the lead terminal is very short, so
With this method of printing solder paste with a width narrower than the length of the connection part, even if the lead terminal is bonded to the pad part, the bond is unstable and does not last long due to the small amount of solder.
-vc quality cannot be guaranteed.

Ten thousand, the thickness of the solder paste? If the amount is increased, the molten solder will remain stable in the adjacent pad portion VC, making it impossible to eliminate bridging defects.

However, there is a reflow soldering method that adjusts the amount of supplied solder to ensure a firm and stable bond between the pad and lead terminal connection that can be maintained for many years, while also suppressing the occurrence of bridging defects. There was a need for the development of

[Problem to be solved by the invention] In other words, the conventional reflow soldering method used in Dahui production, which is used to mount all VC electronic components on printed wiring boards, is a method that causes bridge defects when the number of solder particles increases, and the sensitivity of the solder deteriorates. When this decreases, there are problems in that open defects occur and long-term bonding stability cannot be obtained.

This invention has been made to solve these problems, and it is possible to firmly mount electronic components having lead terminals on a printed wiring board with a sufficient amount of solder, and to prevent bridge defects that cause short-circuits between adjacent lead terminals. The purpose of the flow soldering method is to provide a method for mounting electronic components that provides a highly reliable mounting base.

Current solder paste? In printing technology.

Due to the thixotropic properties of solder paste (shear stress; the property that the viscosity decreases when it is applied, that is, the property that the viscosity decreases during printing), the amount of printing glJ and printing precision K can be managed with high precision. During printing, the solder paste drips and the solder paste protrudes from the pad.

This dripping is transparent and impossible for mass production.

Historically, sag occurs when magnetic components are mounted and when heated. And the current process is ridiculous = rllll
It is.

Here, if the absolute amount of solder is small, there will be less dripping.
A method that applies uniformly to the entire surface eliminates mold ridge defects.
6, but on the contrary, poor opening is observed, and joint strength and epicondylarity are reduced.

Open defects occur due to slight variations in the height of the connection portions of lead terminals. Open defects are difficult to detect because when the probe bottle R is applied during a continuity test, the VC#i connection part is pushed down and the connection part appears to be OK. On the other hand, bridging defects are easy to detect. Therefore, a predetermined amount of solder is determined to avoid open failures and to maintain joint strength and reliability of 7 mils.

The solder paste is printed for this predetermined amount, but
Manage print volume and printing accuracy with high precision.

This will be briefly explained in detail with reference to FIG. Figure 7 1A and B) is the relationship between the volume of solder (supplied acid) and the volume that can be filled using the conventional method? In the explanatory diagram shown in FIG.
The state after the paste is melted is shown. Total volume of metal components of solder paste (solder paste melting surface 11fJ
When Va is larger than the fillable volume 1V1), that is, when /Va < 1, a short circuit occurs across the pads K, forming a bridge. If 1≦cloth<A (1 ho number), it may or may not form a bridge depending on the amount of solder paste that protrudes from the pad, i.e., the degree of wrinkling -vb//va≦
In case A, it does not become a bridge. Here, %A is a constant greater than l.

1st, due to poor printing accuracy, solder supply ML
, t・vb/va<1, or on individual pads [Even with the solder paste printing method, sagging occurs up to the point of heating reflow, which is impossible with the current process. 7 va/A, and a bridge occurs.

Regarding vb, the larger the value, the more advantageous it is to bridgeless, but in order to make wire connection a joke,
Is the lead material or surface treatment that makes it easy to consider solder? Therefore, when the solder melts, it does not spread on the pad and moves toward the lead, so vb has a small value.

Now, the first problem is how to absorb the VC in the solder metal pad protruding from the pad.

[Means for solving problems]

The actual length method for electronic components of the present invention is to mount electronic components by reflow soldering on pads corresponding to the lead terminals of a printed wiring board having a plurality of lead terminals protruding at predetermined intervals. After melting the solder supplied to the above pad part, the above lead terminal? It was placed on the above-mentioned pad, then cooled and bonded.

[Effect]

According to this invention, when there is no lead terminal when the solder starts melting, the melt-resistant solder spreads on the pad portion, and the tolerance t that can be filled with solder can be made larger than when there is a lead terminal. As a result, too much solder is supplied, some dripping occurs, and the solder that hangs over the pads and shorts between pads can be absorbed onto the pads.
Once the shoulder is cut off, even if sub-components are mounted, no bridge failure occurs and the condition is stable? Can be maintained.

FA FIIJ) Hereinafter, one embodiment of the present invention will be described with reference to the drawings. 1st
Figures (4) to (p) are main part cross-sectional configuration diagrams showing the electronic component mounting method according to an embodiment of the present invention in the order of steps, where tAl is after supplying solder paste, tBl is CIV during solder melting, and
'i When electronic components are mounted, (Dl represents the state after electronic components are joined (final shape).

Figure 2 IAI tBl are respectively Figure 1 [Al tBl
FIG. 3 is an explanatory diagram of the state seen from the top surface of the substrate.

In the figure, 111 is a printed wiring board, 12+ is a pad part, +31i1 solder paste, +411"r is a flat type electronic component, [61 is an electronic component (solder connection part that comes out from 41), and (6 ) Halid terminal, tip -i of 61 ◇ Connection part (the inclination angle of 61 is 8 to 4 degrees. Pad part; 2) is copper. Solder paste 31 with 11 lead eutectic solder ( g point 188°C) 10-150 μm particle powder (approximately 90% by weight) and flux component (a) 10
It is a paste-like product made by kneading together (polydispersed percent) and special additives. The flux consists of 50 weight percent solids and 50 weight percent solvent.・The main components are rosin, activator, thixotropic agent, and stabilizer. The solvent component is glycol or the like. Special additives are used in consideration of printability and viscosity at room temperature. The solder paste used in this example has a material temperature of 100,000 to 907JC poise and a temperature of 2 to 5C poise at 183°C.The lead terminal 51 is 42 alloy (F-nickel alloy), and 908N -1 opb solder plating (melting point 215°C) is applied.The width of one pad part is 0.40=m, and the pad part spacing is 0.25 ms.Therefore, the adjacent pad part pitch and lead pitch are 0.61 It is $fl.

Now, the difference from the conventional method shown in Fig. 5 is that the electronic component 1
Before the lead terminals in 4) and 5) are placed, the solder is removed.
The lead terminal +51 is placed after the lead terminal is melted by heat.

In other words, in this embodiment, first the solder is applied to the substrate + tli.
Pace) 41 f Printing supply Figure 1 A), solder
The board 111 printed with the paste 131 is placed in an infrared furnace and the solder paste 31 is heated and melted (Fig. 1B).Then, electronic components 4) are mounted (Fig. 1C), and the solder paste 31 is heated and melted. Component 141 is soldered to the board (FIG. 1D).

Now, when soldering is carried out in this way, even if solder is supplied all over the pad part (21) as shown in Figure 2, when the solder melts in the bath, the solder (3
Since there is no lead terminal +51 where 1 is easily considered, the old figure 2 (
As shown in the figure, the fillable volume of the solder (31) is larger than that of the conventional method, and the amount of solder (31) that extends over the pad part 31 as shown by the arrow is larger than that of the conventional method. Therefore, all the solder 3j is absorbed onto the pad part (2).Also, when the electronic component 141 is mounted, the lead terminal (5) is easily covered with solder 31, so the solder does not protrude outside the pad part (2). It is attracted to the lead terminal 16) without any worries, and there is no dripping of solder paste as in the conventional case. Also, once solder is applied to the pad part (2)
Since the lead terminal (51) is placed on the lead terminal (51), the volume that can be filled with solder becomes Vb'(Vb+'>Vb), which is larger than the conventional vb.

In other words, all the excess solder that was bridging between the pad parts is absorbed onto the pad part because the solder filling capacity Vtn of the pad part is large, and when electronic components are mounted, the excess solder is absorbed by the outside of the pad part 17i! : Because there is no protruding solder,
If Vb'/Va≧1, bridging can be inhibited.

Conventionally, a bridgeless structure could not be achieved unless Vb/va≧A, or 1≦vb/Va<A, where the degree of droop was small, and when Va was large, y-1vb was small, resulting in a bridge. .

In addition, in the above embodiment, when printing on the solder paste pad part, the solder paste was printed all over the pad part, but it may be printed on each pad part individually. In that case, It does not matter if the solder paste protrudes from the pad.

Also, there is no problem even if the thickness of the solder paste increases to some extent.

In the above embodiments, infrared 4iA was used as a heating source, but it may also be used for a VPS device that heats using the latent heat of condensation of steam, or hot air or a hot plate may be applied. In addition, although the solder paste material VC--lead eutectic solder was used, it is also possible to use a solder with a different compounding ratio, or with a different composition of I.
An n-based low melting point solder or the like may also be used. The pad portion (2) is not limited to @, and the surface may be coated with eutectic solder, for example. The lead terminal 151 is also 42 alloy and 90 Sn −
It does not have to be plated with 10 pb solder, but may be made of a copper alloy, for example.

Further, in the above embodiments, parts with a pitch lead of 0.65 pitch were described, but it goes without saying that the present invention can also be applied to parts with a narrower pitch such as 0.50 pitch or 0.401 pitch.

〔Effect of the invention〕

As described above, according to the present invention, the predetermined interval? In a method of mounting an electronic component having a plurality of lead terminals protruding apart from each other by reflow soldering on pad portions V corresponding to the lead terminals of a printed wiring board, solder is supplied in advance to the pad portions. After melting, all of the lead terminals are placed on the pads, and then cooled and bonded. This has the effect of preventing open defects and bridging defects, and providing highly reliable bonding. .

[Brief explanation of drawings]

Figures 1~p) are surface groove diagrams of important parts showing the mounting method of electronic components according to an embodiment of the present invention in the order of steps, Figure 2 +Bl
are explanatory diagrams for Figure 1 + Bl, respectively, and Figure 8 is a printed wiring board? The plan view shown in FIG. 4, IAI tBl, is the plan view showing the electronic components, and FIG.
FIG. 6 is a perspective view showing an example of a defective bridge, and FIG. In the figure, 11 is a printed wiring board, 2) is a pad part, 31 is solder, (4 is an electronic component, and 151 is a lead terminal. In 1 figure, the same reference numerals indicate the same or equivalent parts. Show◎

Claims (1)

[Claims] In a method of mounting an electronic component having a plurality of lead terminals protruding at predetermined intervals on pads corresponding to the lead terminals of a printed wiring board by reflow soldering, the solder previously supplied to the pads is melted. rear,
A method for mounting an electronic component, characterized in that the lead terminal is placed on the pad portion, and then cooled and bonded.