JPH03109797A - Reflow of double-sided mounting board - Google Patents

Abstract

PURPOSE:To prevent generation of cracks on a mold body of electronic parts by heating a board, on whose surface electronic parts are mounted, at a low temperature to semisolidified paste solder, and mounting the electronic parts on its rear side followed by heating at a high temperature in order to simultaneously melt and solidify paste solder on both sides. CONSTITUTION:A board 10, on whose surface electronic parts P are mounted, is carried into a heating chamber of a reflow device to perform heat treatment on paste solder 15 to adhere an electrode part 14 at above evaporating temperature of a solder solvent and an under melting temperature of solder particles followed by loading electronic parts P on the rear of the board and again carrying this board into the heating chamber in order to perform heat treatment at above melting temperature of solder particles. That is, avoiding heating of the electronic parts mounted on the surface two times, before and after, by a reflow device at a high temperature, they are given heat treatment at high temperature by the second reflow simultaneously with the rear side electronic parts in order to adhere the board. Thereby, deterioration of a mold body due to high-temperature heating and generation of cracks can be prevented.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a reflow method for double-sided mounting boards, in which electronic components are mounted on both the front and back sides of the board, in which paste solder is used to bond electronic components to the board. , relates to a method for heat treating a molded body of an electronic component to prevent cranking.

(Prior Art) Paste solder for bonding electronic components to a board is melted and solidified by heat treatment in a reflow device.

FIG. 5 shows a general temperature profile of this heat treatment, in which the substrate is gradually heated to about 150°C (preheating) while being slowly conveyed by a conveyor in the heating chamber of the beam flow device. Next, the heated state is maintained at about 150° C. (soaking), and then the temperature is suddenly raised to 200° C. or higher to melt the solder paste, and then cooled.

FIG. 6 shows an example of an electronic component that is heat-treated by a beam flow apparatus, and this electronic component P has leads 102 protruding from a molded body 101. Mold body 1
01 is made of synthetic resin such as epoxy resin, and protects the semiconductor chip 103 inside. 104 is Chi.
This is an extremely thin wire that connects the lead 102 to the lead 102.

The electronic component P is mounted with the lead 102 landing on the paste solder 106 spread on the land (electrode) 108 of the board 105, and then as described above, the electronic component P is mounted on the paste solder 106 spread on the land (electrode) 108 of the board 105.
The paste solder 106 is melted and solidified by conveying the solder paste 106 into a heating chamber of a reflow apparatus. paste solder 106
is 10 minute solder particles with a diameter of about 30 to 80 microns.
6a, the internal solvent evaporates while being gradually heated to around 150°C, and then when heated to a solder melting temperature of 150°C or higher, the solder particles 106a are gradually melted and then solidified. , the electronic component P is connected to the board 1
Fixed to 05.

By the way, the synthetic resin that is the material of the molded body 101 is hygroscopic and easily absorbs moisture in the air. When the absorbed moisture vaporizes during the heat treatment, cracks 107 are likely to occur in the molded body 1O1 due to its vapor pressure. Figure 7 shows the relationship between the temperature and strength of the internal vapor pressure of the epoxy resin and the molded body. As the temperature rises, the internal vapor pressure increases; Strength decreases.

Therefore, if the vapor pressure exceeds the resin strength, cranks are likely to occur.

In order to prevent cranks from occurring in the mold body 101 during reflow, the electronic component P is generally subjected to a drying process in advance, and after the dried electronic component is mounted on a board using a mounter, the electronic component P is placed in the mold. The reflow process is quickly performed before the body 101 absorbs moisture from the air.

In recent years, due to demands for higher density and higher integration, double-sided mounting, in which electronic components are mounted on both the front and back sides of a board, has become widespread.

This double-sided mounting first mounts the electronic components on the front surface of the board using a mounter, heats the paste solder using a reflow machine, then mounts the electronic components on the back side of the board using the mounter, and then uses the reflow machine again to mount the electronic components on the back side. The paste solder is heat treated.

(Problems to be Solved by the Invention) However, the reflow method for double-sided mounting boards performed as described above has the following problems.

(i) There is a considerable amount of time between the time when the electronic components mounted on the front surface of the board are heat-treated using a reflow machine and the time when the electronic components are mounted on the back side of the board and heat-treated using the reflow machine. The molded body of electronic components mounted on the front side absorbs moisture in the air, and when the back side glue flow is performed, cranks are likely to occur for the above reasons.

(ii) Since the electronic components mounted on the front surface of the board are also heated when the electronic components mounted on the back surface are heat-treated, they are heated twice before and after at a high temperature of 200"C or more. The mold body deteriorates easily.

Therefore, an object of the present invention is to provide a reflow method for a double-sided mounting board that can solve the above-mentioned problems.

(Means for Solving the Problems) For this purpose, the present invention transports a substrate on which an electronic component having a synthetic resin molded body and an electrode part is mounted inside a heating chamber of a reflow apparatus, After heat-treating the paste solder that bonds the parts to the board at a temperature above the transpiration temperature of the solder solvent and below the melting temperature of the solder particles, electronic components are mounted on the back side of this board, and then this board is placed again in the heating chamber of the reflow equipment. is transported and heat-treated at a temperature higher than the melting temperature of the solder particles.

(Function) In the above configuration, first, the board with electronic components mounted on its surface is sent to the heating chamber of the reflow machine and heated at a relatively low temperature, and the solder solvent evaporates, causing the solder paste to semi-solidify. Electronic components are temporarily fixed to the board. Next, the board with electronic components mounted on the back side is again sent to the heating chamber and heated at a high temperature, and the solder particles of the paste solder on both the front and back sides are melted and solidified at the same time, and the electronic parts are fixed to the board.

(Example) Next, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a side view of the beam flow device, and 1 is a heating chamber, inside of which are a preheating heater 2 and a fan 3, a soaking heater 4 and a fan 5, a high-temperature heating heater 6 and a fan 7, An air cooling fan 8 is provided. 9 is the substrate 10
11 and 12 are conveyors for carrying the substrate 10 into the heating chamber 1 and carrying it out from there.

Pl is an electronic component mounted on the surface of this board 10 by a mounter. FIG. 2 shows the state immediately after mounting (that is, before reflow), and a glue 14 as an electrode portion protrudes from the molded body 13 of this electronic component PL.

15 is a paste solder applied on the land (electrode) 17 of the substrate 10 by a screen printer, and the lead 1
4 has landed on this paste solder 15. This paste solder 15 consists of solder particles 15a having a diameter of about 30 to 80 microns, and is mixed with a solvent. 16 is a temporary fixing bond, which is used for the second gluing flow (described later)
This pad 16 is applied as a precaution to prevent the electronic component P1 from falling off the bottom surface of the board 10 during the process, and the pad 16 is not necessarily necessary. Next, a reflow method using the above-mentioned reflow apparatus will be explained.

First, the mounter places the electronic component P1 on the surface of the board 10.
Equipped with Next, this substrate 10 is transferred to a carry-in conveyor 11.
is transferred to the conveyor 9 in the heating chamber 1, and
transported inside. The inside of this heating chamber 1 is controlled at a relatively low temperature that is above the transpiration temperature of the solder solvent and below the melting temperature of the solder particles 15a (for example, the maximum temperature is 130 to 140°C). Handa 150
Although the solder particles 15a are not melted, the solvent evaporates, the paste solder 15 is semi-solidified, and the electronic component P1 is temporarily fixed to the substrate 10.

The electronic components Pl mounted on the surface in this way are transferred to the substrate 1.
0, the electronic component P2 is also mounted on the back surface of this board 10 by a mounter, and this board 10 is again transported inside the heating chamber 1 (see FIGS. 3 and 4). The temporary fixing force of the electronic component P1 applied in the first mounting flow is caused by the impact when the electronic component P2 lands on the board when the electronic component P2 is mounted on the back side by the mounter.
It is sufficient that the size is such that the electronic component P1 on the lower surface side of 0 does not fall from the board 10. Further, the bond 16 prevents the electronic component P1 from falling off from the substrate 10 during the second folding process, which is performed by turning the substrate 10 upside down.

The temperature profile of this second flow is similar to the conventional temperature profile shown in Fig. 5, in which the temperature is first preheated to about 150°C, then soaked, and then the temperature is suddenly raised to 200°C or more. After simultaneously melting the solder particles 15a of the paste solder 15 on both the front and back surfaces, the melted solder 15 is gradually cooled to solidify, and the electronic components Pi and P2 on both the front and back surfaces are fixed to the substrate 10.

As described above, in this method, for the electronic component P1 on the surface of the board that will be sent to the flow device twice before and after the first
The second time, the electronic component P1 is processed at a relatively low temperature that allows the solder solvent to evaporate, without melting the solder particles 15a.
The solder particles 15a are temporarily fixed to the substrate 10, and then heated to a high temperature of 200° C. or more in the subsequent second gluing flow to melt the solder particles 15a on both the front and back surfaces at the same time. Therefore, the molded body 13 of the electronic component P1 on the front side is heated to a high temperature of 200° C. or higher twice, and the molded body 13 is cranked due to steam pressure. It is possible to prevent the mold body 13 from deteriorating.

(Effects of the Invention) As explained above, the present invention provides a substrate on which an electronic component having a synthetic resin molded body and an electrode portion is mounted.
The solder paste used to bond the electrode to the board is transported through the heating chamber of the reflow machine and heated at a temperature above the transpiration temperature of the solder solvent and below the melting temperature of the solder particles, and then electronic components are mounted on the back side of the board. Then, this board is transferred again into the heating chamber of the reflow machine and heat-treated at a temperature higher than the melting temperature of the solder particles. By the second glue flow, the electronic components on the back side can be heated at high temperature and fixed to the board at the same time, avoiding the mold body from deteriorating due to high temperature heating and absorbing this. Due to the vapor pressure of the water
It is possible to prevent cracks from occurring in the molded body.

[Brief explanation of drawings]

The figures show embodiments of the present invention, in which Figure 1 is a side view of a beam flow device, Figure 2 is a side view of electronic components mounted on a board, and Figure 3 is a partial side view of the beam flow device. FIG. 4 is a side view, FIG. 5 is a temperature profile diagram, FIG. 6 is a side view of the electronic component, and FIG. 7 is a temperature characteristic diagram. 1...Heating chamber 9...Conveyor 10...Substrate 13...Mold body 14...Electrode part 15...Paste solder P...Electronic component

Claims (1)

[Claims] A board on which an electronic component having a synthetic resin mold body and an electrode part is mounted is transported into the heating chamber of a reflow machine, and the paste solder that bonds the electrode part to the board is heated to a temperature higher than the transpiration temperature of the solder solvent. After heat treatment at a temperature below the melting temperature of the solder particles, electronic components are mounted on the back side of this board, and then this board is transported again into the heating chamber of the reflow equipment and heat treated at a temperature above the melting temperature of the solder particles. A reflow method for a double-sided mounting board, characterized by: