JPH02201997A - Soldering of double-sided packaging circuit board - Google Patents

Abstract

PURPOSE:To perform solvent volatilization in the case of preheating from vacancies when reflow-heating is performed by drying creamy solder at the faces of a board after loading components and decreasing the quantity of a volatile matter in solder and then providing the vacancies among solder granules of creamy solder. CONSTITUTION:Each creamy solder 2 is printed on each electrode land 7 on the first faces of a board 1 and each solder 2 is equipped with each component 3A and is heated and dried. In such a case, the surroundings of creamy solder granules in creamy solder are filled with a flux 5 because of solvent volatilization from the inside of creamy solder. After removing a part of its solvent by drying, creamy solder reaches a state that there are a large number of vacancies 6. After the board 1 is reversed, the first and second faces are treated in a lump with reflow-heating and double sided soldering of loaded components is performed. Solvent volatilization is thus performed through a vacancy part and falling and the like due to sudden gasification of a volatile matter is prevented.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to soldering chip components and a wiring board for the purpose of manufacturing a hybrid IC or the like in which surface-mounting chip components are mounted on both sides of a wiring board. Concerning how to connect by attaching.

[Prior Art] Conventionally, in a hybrid IC in which surface-mounting chip components (hereinafter referred to as components) are mounted on both sides of a wiring board and these are soldered to the board, there are After printing cream solder and applying adhesive between the lands, the component is mounted, and then the adhesive is cured to fix the component. Next, cream solder is printed on the electrode land on the second surface and components are mounted thereon, and then the first and second surfaces are soldered all at once through a reflow oven. In the conventional method described above, soldering tends to be defective due to adhesive extrusion during application. To prevent this, the soldering shown in Figure 3 a - f is performed by soldering components on one side at a time, as shown in the process diagram. The most common method was mounting and soldering. That is, the cream solder 2 is printed on the first surface of the board 1, the component 3A is mounted on that surface (a and b in the same figure), and then heated in a reflow oven (c in the same figure) and soldered. After inverting and then printing solder on the second side in the same way as the first side and mounting component 3B (d and e in the same figure), solder component 3B on the second side through the reflow oven again (f in the same figure). It is. At this time, if solder having the same melting point temperature is used on the first and second surfaces, the shape of the solder on the first surface (lower side) is often distorted, as shown in f' in the figure.

[Problem to be solved by the invention] As mentioned in the previous explanation, in the conventional soldering method, reflow heating for soldering components is performed on each of the front and back sides of the board. from,
Regarding the first reflow surface, the solder melted again due to heating after the substrate was turned over, resulting in the following problems.

(1) By remelting the solder, the solder ~
Interdiffusion between electrode land materials progresses, resulting in reduced reliability of the finished product due to thermal stress. This phenomenon is particularly noticeable in the case of thick film electrode materials formed on ceramic substrates when thermal stress is applied due to temperature cycles.

(n) Solder remelting causes the solder surface to lose its smoothness, resulting in a decrease in commercial value from a visual standpoint.

As a remedy for the above problems, published patent publications No. 80-192389, No. 81-194797,
Soldering methods described in Japanese Patent No. 62-31193, No. 82-35695 and No. 63-177584 have been proposed.

These proposals can be summarized as follows: (A) using solder with a different melting temperature on each reflow surface to prevent the solder used on the first reflow surface from remelting during the second reflow; (C) A method for temporarily fixing components to be mounted on the bottom side of the board by using flux as an adhesion agent. (Figures 4a-e are cross-sectional views illustrating the process of double-sided soldering by this method, in which solder 2 is printed on the first surface of the board 1, and the chip component 3A is
Apply a paste-like solder flux 5 on the board to temporarily fix it when mounting the board (a and b in the same figure), turn the board over (c in the same figure), print solder on the second side, and mount component 3B (in the same figure). d and e), and then solder the chip components mounted on both sides of the board using the reflow method), but these methods also have the following limitations and constraints: matter exists.

Regarding method (A): It is necessary that there is a temperature difference of several tens of degrees Celsius in the melting point of the solder used for the 11th and 2nd reflow surfaces, and on the other hand, the upper limit of the heat resistance temperature of various surface mount components and the Soldering is subject to restrictions on the solder and parts that can be used, in terms of the heating temperature in subsequent steps or the limit heat-resistant temperature that guarantees reliability.

Regarding method CB): A jig is required for each product, and measures must be taken to avoid heat absorption by the jig during reflow.

Regarding method (C): Since flux is essentially not an adhesive (it softens at temperatures below 100°C), it cannot be expected to have a good function of supporting parts, and when preheating the board, cream or volatile components in the flux Due to sudden removal, a force is applied to pull the component away from the board, so if it is a heavy component, there is a risk of the component falling during reflow. During reflow, the flux applied to the lower part of the chip component is not able to support the chip component sufficiently and the component falls. Figure f' shows how the above-mentioned force is applied to the solder to separate the component from the board. (shown schematically).

[Means and effects for solving the problem] The present inventors
In order to solve the above-mentioned problems in the double-sided soldering method, as a result of careful observation and analysis of the conventional soldering process, we decided to print cream solder on the first side of the board and apply cream solder after mounting the chip components. Dry at a predetermined temperature, then dry at a second temperature.
The present invention was achieved by discovering that the above-mentioned problems can be solved by printing cream solder on the surface and subjecting the chip parts mounted thereto to reflow heat treatment, thereby making it possible to perform highly reliable double-sided soldering.

In other words, the double-sided soldering method of the present invention is to print cream solder on the first side of the circuit board, mount the knob parts, and then reduce the volatile components in the cream solder on the first side by heating and drying. Then, after printing cream solder on the second side of the board and mounting the chip/knob parts, the first and second sides of the board are collectively subjected to reflow heating.

In the soldering method of the present invention, after printing cream solder on the first surface of the board and mounting the components, the cream solder is incompletely dried to reduce the proportion of volatile components in the cream solder, thereby bonding the board and the components. Temporarily fix firmly.

Next, the board is turned over, cream solder is printed on the second side, components are mounted, and the first and second sides are subjected to a batch reflow heat treatment to solder both sides at once.

During soldering, on the first side (the bottom side after flipping the board), the volatile components in the cream solder have already been reduced due to the above drying process and voids have been created in the solder, so during the subsequent reflow heating, The solvent from the flux contained in the solder cream is evaporated through the pores.

Therefore, as seen during reflow heating in the conventional method, the rapid gasification of volatile components from the flux contained in cream solder does not generate a force that attempts to separate the component from the board. There is no risk of falling even in the case of heavy parts.

Note that the drying conditions in the above drying process should be determined depending on the type of cream solder used.
A combination of drying temperature and time can be appropriately selected such that at least about 50% of the solvent normally contained is volatilized by drying. 150℃ for Sn/Pb eutectic solder
, 5 minutes is an example of preferable drying conditions.

Next, the present invention will be explained in detail based on examples.

[Example] FIGS. 1A to 1F are cross-sectional views for explaining the steps in the screen soldering method of the present invention. First, as shown in FIGS. Print cream solder (Sn-Pb eutectic solder) 2 on land 7, and place part 3A on top of it.
is installed to heat and dry cream solder 2. This drying was carried out at 150°C for 5 minutes, and the amount of solvent evaporated from the solder cream at this time was approximately 50% of the amount of solvent originally contained in the solder (Figure 2 shows the amount of solvent evaporated from the cream solder). (It is a graph showing the relationship between the percentage of solvent volatilization in the sample and the heating temperature.)

As can be seen in Figure b', which is an enlarged view of the circled area in Figure 1, the cream solder 2, in which the surroundings of the cream solder grains 4 were filled with flux 5 (consisting of solid content and volatile matter), After a portion of the solvent is removed by drying, a large number of pores 6 can be seen in the flux 5, as shown in Figure C', which is an enlarged view of the circled area in Figure C.

Next, after reversing the substrate 1, cream solder (Sn-Pb eutectic solder) 2 is printed on the electrode land 7 on the second surface of the substrate in the same way as on the first surface, and the component 3B is mounted (d and e in the same figure). Thereafter, the mounted components were soldered on both sides by performing a reflow heat treatment on the first and second sides (f in the same figure).

In addition, multilayer capacitor, QFP-IC, and 5OP-I
The falling limit of various parts such as C was investigated through experiments, but no falling parts were observed in any of the parts used in the experiment, confirming that normal soldering was performed. .

Additionally, the weight of the chip components used in the experiment was up to 665 tons.
ng, the areal load of the terminal electrode is about loomg/-, and those skilled in the art are well aware that these values are weights that can accommodate most of the surface mount components currently in use. By the way.

[Effects of the Invention] As explained above, according to the soldering method of the present invention, the cream solder on the first side of the board is dried after mounting the components, the amount of volatile components in the solder is reduced, and the solder of the cream solder is dried. Since pores were provided between the grains, when the first surface was directed downward and reflow heating was performed, it was possible to volatilize the solvent during preheating through the pores.

Therefore, the force that separates the board from the parts that occurs when the solvent evaporates is reduced, parts are prevented from falling, and reflow heating can be performed with heavy parts placed on the bottom side of the board.
The method of the present invention has excellent workability and does not cause troubles due to remelting of solder as in conventional methods.
Reliable soldering is a method that can provide a finished product.

[Brief explanation of the drawing]

FIGS. 1a-f are cross-sectional views illustrating steps in the double-sided soldering method of the present invention. FIG. 2 is a graph showing the relationship between the amount of solvent volatilized in the solder and the temperature for heating and drying cream solder in the method of the present invention. Figures 3 and 4 are cross-sectional views illustrating the process of conventional double-sided soldering; the former is a commonly used method, and the latter is a temporary fixation of parts using flux as an adhesion agent. This is an example of how this method is applied. Explanation of symbols 1... Board 2... Cream solder 3A, 3B... Chip parts 4... Solder grains 5...
Flux 7...electrode land

Claims (2)

[Claims] (1) After printing cream solder on the first side of the circuit board and mounting the chip components, the volatile components in the cream solder on the first side are reduced by heating and drying, and then the cream solder is printed on the second side of the circuit board. A method for soldering a double-sided circuit board, which is characterized in that after printing solder and mounting chip components, the first and second sides of the board are subjected to reflow heating at once. (2) As the heating and drying conditions for the cream solder on the first side, the heating temperature and time are selected so that the amount of volatile components volatilized from the solder before the batch reflow heating is at least 50%. The method described in Section 1.