Solder supplying method

Abstract

To provide a method for supplying solder by which the content of Bi contained in a solder alloy in a solder bath is maintained at a prescribed amount even after many printed circuit boards are soldered with a Bi-containing solder alloy.SOLUTION: In the solder supply method, an additionally supplied solder, which is a low-Bi solder alloy having a Bi content lower than that of an Sn and / or Bi-containing solder alloy, is supplied to a solder bath into which the Bi-containing solder alloy has been introduced, and the Bi content in the solder bath is adjusted to the Bi content of the Bi-containing solder alloy at the beginning of being put into the solder bath.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for supplying solder to a solder bath. [Background technology]

[0002] Electronic devices such as televisions, video recorders, radios, computers, copiers, and communication devices use printed circuit boards with electronic components mounted on them. These electronic components must be firmly fixed to the printed circuit board and must have good electrical conductivity with the printed circuit board. For this reason, solder alloys are used to connect the electronic components to the printed circuit board.

[0003] In recent years, the miniaturization of electronic components has progressed, increasing the thermal load during soldering, making it desirable to solder at low temperatures. Lowering the soldering temperature reduces the load on electronic components and circuit boards during soldering, and also leads to a reduction in the environmental load. To perform soldering at low temperatures, a solder alloy with a low melting point can be used.

[0004] As disclosed in JIS Z 3282 (2017), examples of low-melting-point solder alloys include Sn-58% Bi by mass and Sn-52% In by mass. The melting temperatures of these alloys are 139°C and 119°C, respectively, and both are representative alloy compositions of low-melting-point solders. In particular, Sn-58% Bi by mass is widely used as a low-cost solder alloy.

[0005] There are two methods for using solder alloys: the dipping method and the flow method. The dipping method involves melting solder in a solder bath and then bringing a printed circuit board into contact with the molten solder, thereby adhering the solder to the soldered portion. The flow method involves soldering a printed circuit board by jetting the molten solder from a jet nozzle installed in a solder bath that contains the molten solder.

[0006] As described above, each method involves bringing the molten solder in a solder bath into contact with the printed circuit board. However, when soldering a printed circuit board with Cu electrodes or Cu wiring using the dipping or flow method, the Cu content in the molten solder increases because many printed circuit boards are brought into contact with the molten solder in the solder bath, which can cause problems such as an increase in the melting point and poor soldering.

[0007] Therefore, for example, Patent Document 1 proposes a method of additionally supplying a solder alloy with a low Cu content to a solder bath. In the invention described in this document, by additionally supplying a solder alloy with a low Cu content to the solder bath, the Cu content of the molten solder in the solder bath can be returned to a predetermined Cu content. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3312618 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the invention described in Patent Document 1 is a technology for adjusting the Cu content of molten solder. For example, as mentioned above, solder alloys containing Bi as a main component, which are known as low-melting-point solder alloys, have a lower melting point and a lower Sn content than solder alloys containing Cu. Therefore, when using a solder alloy containing Bi as a main component in a dipping or flow process, the amount of Cu eluted into the molten solder is less than that of the Cu-containing solder alloy described in Patent Document 1. This is thought to be because the increase in Cu content in the solder alloy containing Bi as a main component is more gradual than that in the solder alloy described in Patent Document 1. Therefore, considering factors such as maintenance intervals, it is not realistic to apply the technology described in Patent Document 1 to a solder alloy containing Bi as a main component in a dipping or flow process.

[0010] Furthermore, when SnBi solder alloys are used in dipping or flow soldering, there is a concern that changes in the Bi content may affect the desired properties when soldering a large number of printed circuit boards. This is because, for example, as is clear from the equilibrium diagram, in SnBi eutectic solder alloys, changes in the Bi content increase the melting point and may impair solderability.

[0011] Therefore, an object of the present invention is to provide a solder supply method that maintains the Bi content in the solder alloy in a solder bath at a predetermined amount even after soldering a large number of substrates with the Bi-containing solder alloy. [Means for solving the problem]

[0012] The present inventors have conducted extensive research into the cause of changes in the Bi content of molten solder when soldering printed circuit boards using the dipping or flow method, and have found that this is because the molten solder in a solder bath comes into contact with a large number of printed circuit boards during soldering. Accordingly, the present inventors have found that when a SnBi solder alloy is used in a dipping or flow method, continuous soldering is performed, which consumes Sn from the solder alloy in the solder bath, resulting in a relative increase in the Bi content.

[0013] In the dip and flow soldering methods, the time that the molten solder is in contact with the board during soldering is short, and the mass of Sn consumed to join one printed circuit board is very small. However, even if the mass of Sn consumed to join one printed circuit board is small, the number of printed circuit boards soldered in mass production is large. Therefore, the mass of Sn consumed from the molten solder in the solder bath ultimately becomes large. As a result, the mass of Sn contained in the molten solder is significantly reduced from the initial input composition. Thus, when an SnBi solder alloy is used in the dip and flow soldering methods, the Bi content becomes relatively high.

[0014] Furthermore, in the immersion and flow soldering methods, the molten solder in the solder bath adheres to the substrate, so when soldering multiple substrates, the mass of the solder alloy in the solder bath decreases. Furthermore, during molten soldering, dross is generated when the solder alloy reacts with oxygen, and the mass of the solder in the solder bath decreases when the dross is removed. In the immersion and flow soldering methods, the substrate is brought into contact with the molten solder, and proper soldering cannot be performed unless the liquid surface of the molten solder is within a specified range. In other words, if the solder alloy in the solder bath adheres to the substrate and decreases when soldering multiple substrates, and the liquid surface of the molten solder drops, the molten solder cannot contact the substrate at a constant flowing position, resulting in poor soldering.

[0015] For this reason, a level sensor is installed in the solder bath to constantly monitor the level of the molten solder, and when the mass of the molten solder decreases and the level of the molten solder drops, the level sensor sounds an alarm. When this alarm is sounded, an operator or an automatic supply device supplies solder bars or solder wires to the solder bath to maintain the specified level.

[0016] Generally, when the liquid level of the molten solder drops, the Bi-containing solder alloy supplied to the solder bath has the same Bi content as the Bi content of the initial composition. For example, if the Bi content is 58% by mass, the solder alloy supplied has a Bi content of 58% by mass. In addition, in the past, to prevent deviations in the composition after additional supply, all of the solder alloy in the solder bath was removed from the solder bath before the Bi-containing solder alloy of the initial composition was introduced into the solder bath. Alternatively, approximately half of the mass of the Bi-containing solder alloy in the solder bath was replaced with the Bi-containing solder alloy of the initial composition.

[0017] In this way, when a solder alloy of the same composition as the initial composition is added to a molten solder bath with a reduced Sn content, the above-mentioned findings indicate that the Bi content in the molten solder in the solder bath increases as the number of boards to be soldered increases. Furthermore, as the amount of used solder alloy recovered from the solder bath increases, problems arise such as an increase in environmental load, an increase in soldering process time, and even an increase in soldering costs.

[0018] As mentioned above, in the dipping method and the flow method, a large number of substrates are brought into contact with the molten solder in the solder bath, which consumes Sn and increases the relative Bi content. Therefore, the inventors of the present invention realized that if the relatively increased Bi content could be returned to the initial Bi content, the above-mentioned concerns caused by the increase in Bi content could be resolved, and completed the present invention. The present invention has been completed as follows:

[0019] (1) A solder supplying method characterized by supplying additional solder, which is a low-Bi solder alloy having a lower Bi content than the Sn and / or Bi-containing solder alloy, to a solder bath containing a Bi-containing solder alloy, to adjust the Bi content in the solder bath to the Bi content of the Bi-containing solder alloy when it was first placed in the solder bath.

[0020] (2) The solder supplying method according to (1) above, wherein the Bi-containing solder alloy and the additionally supplied solder each contain at least one of In, Zn, Mn, Cr, Co, Fe, Si, Al, Ti, Ag, Cu, Ni, Sb, and rare earth elements.

[0021] (3) The solder supplying method according to (1) or (2) above, wherein the Bi-containing solder alloy is a Sn-Bi solder alloy or a Sn-Bi-Ag solder alloy.

[0022] (4-5) A solder supplying method according to any one of (1) to (3) above, wherein the Bi-containing solder alloy initially introduced into the solder bath is introduced at predetermined time intervals or at predetermined timings during the time period from when the Bi-containing solder alloy is first introduced into the solder bath until immediately before additional solder is supplied.

[0023] (6-9) A solder supplying method according to any one of (1) to (5) above, in which additional solder is supplied based on the Bi content constituting the solder alloy in the solder bath immediately before the additional solder is supplied.

[0024] (10-13) The solder supplying method according to any one of (6) to (9), wherein a Bi-containing solder alloy having a mass approximately equal to the amount of additionally supplied solder is drawn out of the solder bath from the Bi-containing solder alloy in the solder bath, and then the additionally supplied solder is supplied into the solder bath. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in more detail below. In this specification, "%" relating to the solder alloy composition means "% by mass" unless otherwise specified.

[0026] 1. Solder supply method The solder supplying method according to the present invention is a method for supplying solder to a solder bath containing a Bi-containing solder alloy. The present invention is applicable to soldering using a soldering apparatus equipped with a solder bath.

[0027] The capacity of the solder bath used in the solder supplying method according to the present invention may be any capacity that can accommodate the amount of molten solder required to solder a printed circuit board, and may be, for example, several grams to several hundred kilograms.

[0028] The initial composition of the solder alloy initially charged into the solder bath is not particularly limited as long as it is a Bi-containing solder alloy, and examples thereof include Sn-Bi solder alloy, Sn-Bi-Ag solder alloy, etc. In the present invention, unless otherwise specified, the "Bi-containing solder alloy" refers to the initial composition initially charged into the solder bath.

[0029] The Bi content in the Bi-containing solder alloy may be, for example, 10 to 60%, or may be 30 to 60%. Specific examples of the Bi content in the Bi-containing solder alloy include 58%, 57%, 40%, 35%, and 30%.

[0030] The Bi-containing solder alloy used in the present invention may also contain at least one of In, Zn, Mn, Cr, Co, Fe, Si, Al, Ti, Ag, Cu, Ni, Sb, and rare earth elements as optional elements. The content of each element is preferably 5% or less in total for at least one of In, Sb, and Zn, and 1% or less in total for at least one of Mn, Cr, Co, Fe, Si, Al, Ti, and rare earth elements. In the present invention, rare earth elements refer to 17 elements, including Sc and Y, which belong to Group 3 of the periodic table, and 15 elements of the lanthanum group with atomic numbers 57 to 71.

[0031] Examples of alloy compositions for Bi-containing solder alloys include Sn-57Bi-1Ag (melting temperature: 138-145°C), Sn-58Bi-0.5Sb-0.015Ni (melting temperature: 140-145°C), Sn-57Bi-0.4Ag (melting temperature: 138-141°C), Sn-40Bi-0.5Cu-0.03Ni (melting temperature: 139-174°C), and Sn-35Bi-0.5Cu-0.03Ni (melting temperature: 140-182°C). The melting points in parentheses indicate the temperature range from the solidus temperature to the liquidus temperature.

[0032] The solder alloy may contain unavoidable impurities as long as the properties of the solder alloy of the initial composition are not impaired. The melting temperature of the Bi-containing solder alloy is 190°C or lower.

[0033] The additional solder supplied to the solder bath is a low-Bi solder alloy with a lower Bi content than the Sn- and / or Bi-containing solder alloy. The Bi-containing solder alloy in the solder bath consumes Sn during soldering of the board, resulting in a relative increase in Bi content. For this reason, the Bi content of the additional solder supplied to adjust the Bi content of the solder alloy in the solder bath to the Bi content of the initial composition must be lower than that of the Bi-containing solder alloy having the initial composition.

[0034] The Bi content of the low Bi solder alloy may be approximately 0 to 90% of the Bi content contained in the Bi-containing solder alloy, and may be changed depending on the time of supply. The solder alloy may also be Bi-free. Note that the additionally supplied solder may contain unavoidable impurities within a range that does not impair the properties of the Bi-containing solder alloy. Furthermore, the additionally supplied solder may contain the optional elements described above, just like the Bi-containing solder alloy.

[0035] When using additional solder whose Bi content is slightly lower than that of the initial composition, it may be necessary to increase the amount of additional solder supplied to restore the Bi content to that of the initial composition. Therefore, the composition of the additional solder supplied can be determined arbitrarily, taking into account the balance between the amount of additional solder supplied and the Bi content of the additional solder supplied. However, as mentioned above, the Bi content of the additional solder supplied must be at least lower than that of the initial composition.

[0036] In the present invention, the Bi content increased in the solder bath can be adjusted to the Bi content of the Bi-containing solder alloy having the initial composition by using the additional solder supply described above. This is possible because the Bi content of the additional solder supply is less than the Bi content of the initial composition. A preferred embodiment will be described later.

[0037] As mentioned above, in soldering using the immersion method or the flow method, dross must be removed periodically. Furthermore, the greater the number of substrates to be soldered, the greater the amount of solder required for soldering. Therefore, in the present invention, the Bi-containing solder alloy initially introduced into the solder bath may be introduced at predetermined intervals or at predetermined timings during the time period from when the Bi-containing solder alloy is first introduced into the solder bath until immediately before the additional solder is supplied.

[0038] The predetermined timing is, for example, the timing for removing dross. The predetermined time is, for example, when the number of boards soldered per unit time remains constant over a long period of time, and the amount of molten solder in the solder bath decreases with the soldering time. Under such circumstances, additional solder can be supplied at predetermined time intervals.

[0039] The timing for additionally supplying solder may be, for example, when the liquid level of the Bi-containing solder alloy has dropped by about 1 to 20 mm from the initial liquid level when the Bi-containing solder alloy was poured into the solder bath, or may be 5 to 8 mm. Depending on the liquid level at the time of supply, a low Bi solder alloy with an adjusted Sn or Bi content as described above may be supplied.

[0040] In the present invention, another timing for supplying additional solder may be based on the Bi content of the solder alloy in the solder bath immediately before the additional solder is supplied. For example, additional solder may be supplied when the increase in Bi content in the solder bath immediately before the additional supply is 2% or more, or when it is 2.4% or more. This increase rate can be calculated by [{(Bi content immediately before additional supply) - (Bi content in the initial composition)} / (Bi content in the initial composition)] × 100 (%). The Bi content in the solder bath immediately before the additional supply may be determined, for example, by ICP mass spectrometry.

[0041] In the present invention, the amount of additional solder supplied can be calculated as follows: (Bi content of the initial composition) - (Bi content of the additional solder) = X, and (Bi content immediately before the additional supply) - (Bi content of the initial composition) = Y. Using these, the amount can be calculated by (initial amount) / {(X / Y) + 1}. For example, if the initial amount is 420 kg, the Bi content of the initial composition is 58 mass %, the Bi content of the additional solder is 0% (e.g., Sn), and the Bi content immediately before the additional supply is 59.77 mass %, then X = 58, Y = 1.77, and therefore (amount of additional solder supplied) = 420 / {(58 / 1.77) + 1} ≈ 12.44 kg.

[0042] In the present invention, the additional supply amount of additional solder thus determined is added to the solder bath. However, as described above, if a Bi-containing solder alloy having the same composition as the initial composition is periodically supplied, the amount of solder in the solder bath is always nearly full. In such a case, before supplying the additional solder, molten solder of approximately the same mass as the additional supply amount of additional solder may be drawn out of the solder bath. If a Bi-containing solder alloy having the same composition as the initial composition is not periodically supplied, additional solder may be supplied when the amount of solder in the solder bath has decreased by the additional supply amount. The decrease in the amount of solder in the solder bath can be determined based on the change in the liquid level of the molten solder in the solder bath.

[0043] It is preferable that the difference between the Bi content in the solder bath after the additional solder alloy is added and the Bi content in the initial composition of the Bi-containing solder alloy is small. For example, the ratio of the difference between the Bi content in the initial composition and the Bi content after the additional addition [{(Bi content after additional addition) - (Bi content in the initial composition)} / (Bi content in the initial composition)] × 100 (%) should be within ±1.0% of the Bi content in the initial composition, and preferably within ±0.57%. If it is within this range, the properties of the initially added Bi-containing solder alloy are maintained.

[0044] In this embodiment, the supply method has been described focusing on Bi, but it can also be applied to elements whose content relatively increases due to soldering. In particular, the content of Ag can also be adjusted to the initial input composition by the same method as for Bi.

[0045] According to this embodiment, when soldering is performed using a Bi-containing solder alloy by the immersion or flow method, even if Sn is consumed from the molten solder in the solder bath, increasing the Bi content, the additional solder is a low-Bi solder alloy with a lower Bi content than the initial composition, so the Bi content in the solder bath can be made approximately the same as the initial composition. Therefore, by adding additional solder to the solder bath using the solder supply method of the present invention, deterioration of solderability can be suppressed, and the occurrence of soldering defects can be greatly reduced. Furthermore, the amount of used solder alloy removed from the solder bath can be reduced, thereby reducing environmental impact, shortening the soldering process time, and suppressing increases in soldering costs. [Example]

[0046] The present invention will be described by the following examples, but the present invention is not limited to the following examples. Example 1: Using a flow soldering machine (MTF-300) manufactured by Senju Metal Industry Co., Ltd., 420 kg of SnBi solder alloy (Bi content: 58 mass %, with the remainder being Sn and unavoidable impurities) with the initial composition shown in Table 1 below was added to a solder bath. After that, until additional solder was added, a Bi-containing solder alloy (additional feed composition 1) with the same composition as the initial composition was continuously introduced into the solder bath every time dross was removed. Using this solder bath, soldering of Cu-clad substrates (Cu area: 200 mm × 350 mm) was continued using the flow method.

[0047] Then, after the Bi content in the solder bath reached 59.77% by mass, Sn (the remainder being unavoidable impurities) was used as additional solder, and the amount of additional solder supplied was determined to be 12.44 kg. 12.44 kg of molten solder was withdrawn from the solder bath, and 12.44 kg of additional Sn (the remainder being unavoidable impurities) (additional supply composition 2) was introduced. The Bi content in the solder bath was measured by ICP analysis using an Agilent Technologies ICP mass spectrometer (ICP-MS) (RF power: 1600 W).

[0048] The Bi content in the molten solder after additional solder having additional supply composition 2 was added to the SnBi solder alloy in the solder bath was measured using the ICP mass spectrometer described above. The result showed that the Bi content was 58.25 mass%, which was close to the composition of the solder alloy initially added.

[0049] Examples 2 to 4, Examples 7 to 10 In Example 1, additional solder was supplied in the same manner as in Example 1, except that the initial input composition, additional supply compositions 1 and 2, additional supply timing, and additional supply amount were as shown in Table 1. As shown in Table 1, in all cases, the composition was close to that of the solder alloy of the initial input composition.

[0050] Examples 5, 6, 11, and 12 Using a small flow device (Flow Simulator, manufactured by Malcom), soldering was carried out on a Cu-clad substrate measuring 70 mm x 100 mm, and additional supply was carried out under the conditions shown in Table 1 for the initial charge amount, initial charge composition, additional supply compositions 1 and 2, additional supply timing, and additional supply amount. As shown in Table 1, in all cases, the composition was close to that of the solder alloy of the initial charge composition.

[0051] Example 13 In Example 13, a Bi-containing solder alloy (additional supply composition 1) having the same composition as the initial input composition was not introduced into the solder bath, and soldering of a Cu-clad substrate (Cu area: 200 mm × 350 mm) was carried out in the same manner as in Example 1 until the liquid level of the molten solder in the solder bath dropped by 5 mm from the initial liquid level. After that, additional supply solder (additional supply composition 2) was supplied until the liquid level returned to the initial level.

[0052] Comparative Example 1 In Example 1, additionally supplied compositions 1 and 2 were the same as the initially charged compositions, and soldering of a Cu-clad substrate (Cu area: 200 mm × 350 mm) was carried out in the same manner as in Example 1, with additional solder supply under the conditions shown in Table 1. As shown in Table 1, the Bi content was significantly increased compared to the initially charged compositions. All of the solder compositions in Table 1 contain unavoidable impurities.

[0053] [Table 1]

[0054] As described above, it was found that in all Examples, the Bi content in the solder bath after supplying solder of additional supply composition 2 was adjusted to a composition close to the initial charge composition. On the other hand, in Comparative Example 1, after discharging 210 kg, which is half the mass of the solder bath, additional supply solder of the same composition as the initial charge composition was supplied, so it was not possible to adjust to the initial charge composition.

Claims

1. A solder supplying method characterized by supplying additional solder, which is a low-Bi solder alloy having a lower Bi content than Sn and / or the Bi-containing solder alloy, to a solder tank containing a Bi-containing solder alloy, thereby adjusting the Bi content in the solder tank to the Bi content of the Bi-containing solder alloy when it was first placed in the solder tank.

2. 2. The solder supply method of claim 1, wherein the Bi-containing solder alloy and the additionally supplied solder each contain at least one of In, Zn, Mn, Cr, Co, Fe, Si, Al, Ti, Ag, Cu, Ni, Sb, and rare earth elements.

3. 3. The solder supplying method according to claim 1, wherein the Bi-containing solder alloy is a Sn--Bi solder alloy or a Sn--Bi--Ag solder alloy.

4. 3. The solder supply method according to claim 1, wherein the Bi-containing solder alloy initially introduced into the solder bath is introduced at predetermined time intervals or at predetermined timings during the time period from when the Bi-containing solder alloy is initially introduced into the solder bath until just before the additional solder is supplied.

5. 4. The solder supply method according to claim 3, wherein the Bi-containing solder alloy initially introduced into the solder bath is introduced at predetermined time intervals or at predetermined timings during the time period from when the Bi-containing solder alloy is first introduced into the solder bath until just before the additional solder is supplied.

6. 3. The solder supplying method according to claim 1, wherein the additional solder is supplied based on a Bi content constituting the solder alloy in the solder bath immediately before the additional solder is supplied.

7. 4. The solder supplying method according to claim 3, wherein the additional solder is supplied based on the Bi content of the solder alloy in the solder bath immediately before the additional solder is supplied.

8. 5. The solder supplying method according to claim 4, wherein the additional solder is supplied based on the Bi content of the solder alloy in the solder bath immediately before the additional solder is supplied.

9. 6. The solder supplying method according to claim 5, wherein the additional solder is supplied based on a Bi content constituting the solder alloy in the solder bath immediately before the additional solder is supplied.

10. 7. The solder supply method according to claim 6, wherein the Bi-containing solder alloy is extracted from the solder bath in an amount substantially equal to the amount of the additional solder supplied, and then the additional solder is supplied into the solder bath.

11. 8. The solder supply method according to claim 7, wherein the Bi-containing solder alloy is extracted from the solder bath in an amount substantially equal to the amount of the additional solder supplied, and then the additional solder is supplied into the solder bath.

12. 9. The solder supply method according to claim 8, wherein the Bi-containing solder alloy is extracted from the solder bath in an amount substantially equal to the amount of the additional solder supplied, and then the additional solder is supplied into the solder bath.

13. 10. The solder supply method according to claim 9, wherein the Bi-containing solder alloy is extracted from the solder bath in an amount substantially equal to the amount of the additionally supplied solder, and then the additionally supplied solder is supplied into the solder bath.

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