JP2010514932A - Sn-B plating solution and plating method using the same - Google Patents

Abstract

Provided is a Sn-B plating method which does not contain lead and suppresses whisker generation in a plating layer.
An Sn-B plating solution that does not contain Pb ions and contains tin sulfate as a Sn ion source and dimethylamine borane or trimethylamine borane as a B ion source, and Sn-B plating method using the same .
[Selection] Figure 1

Description

  The present invention relates to a Sn-B plating solution containing no lead component (hereinafter referred to as “lead-free”) and a plating method using the same, and more specifically, lead-free Sn— that can suppress whisker generation. The present invention relates to a B plating solution and a plating method using the same.

The semiconductor lead frame has various shapes depending on the method of increasing the density and integration of the semiconductor chip and mounting the substrate.

  Basically, a semiconductor lead frame is mounted with a chip which is a semiconductor memory element, and a pad portion for maintaining a static state, an inner lead portion connected by wire bonding, and a connection with an external circuit. It consists of an outer lead for the purpose. A semiconductor lead frame having such a structure is generally manufactured by a stamping method or an etching method.

  The semiconductor lead frame is packaged through an assembling process with a chip as a memory element, and the assembling process of the packaging includes a chip attaching process, a wire bonding process, a molding process, a marking process, a separating process, and the like.

  In the manufacturing process of such a semiconductor package, in order to maintain the bonding property of the lead wire connecting the chip and the internal lead part and the good characteristic of the pad part, the end part of the pad part and the internal lead part is made of silver. Plating using a metal material such as (Ag). In addition, after the resin protective film molding, in order to improve the soldering performance for mounting on the substrate, plating using a solder material is applied to a predetermined region of the external lead portion, that is, Sn (tin) -Pb (lead) Perform plating. However, it is difficult to carry out such a plating method, and in the plating process, the plating solution permeates between the surface of the lead frame and the epoxy molding, which frequently causes a failure of the semiconductor chip. There is a problem that an additional step is required to remove the non-uniformity.

  In order to solve such problems, a pre-plated frame (PPF) method has been proposed. This is because a material with good lead wettability is applied to the lead frame before the semiconductor assembly process. In this method, the upper surface is pre-plated. Currently, as an example of such PPF plating, a Ni (nickel) layer is formed as an intermediate layer on a lead base metal base material such as Cu (copper), and Pd (having a good lead wettability as an uppermost layer). a structure in which a palladium layer or the like is entirely or partially applied on an intermediate layer (two-layer structure), a Ni layer on a base material, a Pd layer, and an Au (gold) flash layer as a top layer (three layers) Structure), and a structure having a Ni strike layer, a Pd—Ni alloy layer, a Ni layer and a Pd layer (four-layer structure) on the base material are commercially available. However, such a method is unpredictable due to severe corrosion and unstable Pd price when the base material is Cu or an alloy such as alloy 42 containing no Cu component. There arises a problem that an increase in cost may be caused.

  Recently, a two-tone pre-plated frame (two-tone pre-plated frame) that uses metal that is different from each other for the part corresponding to the internal lead and the part corresponding to the external lead in the metal base material. A plated frame method has been proposed. In such a two-color pre-plating frame method, for example, a metal base material is plated with silver or the like on the portion corresponding to the internal lead, and Sn—Pb plating is performed on the portion corresponding to the external lead. ing.

  However, the plating method used in the pre-plated frame or the two-color pre-plated frame method has many problems due to problems such as environmental pollution caused by lead. Various matters and related laws have been established in the direction to regulate the use of lead in products, and the industry has continued research to replace solder paste and Sn-Pb plating that often use lead. Is going.

  As an alternative to Sn—Pb plating to solve such problems, pure Sn plating may be the best method. However, in the case of pure Sn plating, whisker generation is severe. There is a problem in that the risk of a short circuit occurring in the circuit increases.

  The whisker is a protruding crystal that is generated on the surface of the plating layer when different substances are joined and spread to each other. Such whiskers are sensitive to heat and moisture. If such whiskers are formed on the surface of the plated layer of the lead frame, there is a problem that an electrical short circuit occurs between the lead frames, causing a circuit failure.

  As countermeasures against whisker generation, methods such as post-Sn plating heat treatment, Ni plating, Sn particle size adjustment, and an alloy of Sn and a dissimilar metal are considered. Of these, representative alloys of dissimilar metals include Sn. -Bi (bismuth) alloys are widely used.

  However, the Sn-Bi alloy does not suppress the occurrence of whiskers to a satisfactory level, and the eutectoid is difficult to eutectoid because the difference between Sn and Bi is significant at the precipitation potential. is there. Further, if the Bi concentration in the solution is high, there is a risk that Bi will precipitate in the form of particles on the cathode surface and fall off after soldering. If the Bi content in the plating layer is high, cracks will occur when bending. There is a problem that (crack) occurs.

  An object of the present invention is to provide a lead-free Sn-B plating solution that can suppress whisker generation in a plating layer and a plating method using the same without containing lead.

  In order to achieve the above-described object, the present invention does not include Pb ions, tin sulfate as a tin ion source, and dimethyl amine borane as a B ion source. An Sn-B plating solution containing trimethylamine borane is provided.

  The Sn ion source may include 15g / L to 50g / L.

  The B ion source may be included in an amount of 0.1 g / L to 3.0 g / L.

  The Sn-B plating solution includes 30 ml / L to 70 ml / L of sulfuric acid, 10 g / L to 40 g / L of cresolsulfonic acid or phenolsulfonic acid, and β-naphthol. Of 0.1 g / L to 0.5 g / L and gelatin of 0.1 g / L to 3 g / L.

  The present invention also provides a Sn-B plating method for plating using the above plating solution.

The current density can be 0.5 to 5 A / dm ² during plating.

  And it can be plated at room temperature.

  According to the present invention, it is possible to provide a Sn-B alloy plating layer in which whiskers are not generated.

  That is, according to the present invention, since the size of atoms is relatively smaller than Sn, B enters and dissolves in an interstitial site of Sn as described later. Therefore, if the plating solution containing Sn and B forms a plating layer on the lead frame made of Cu, Cu is prevented from diffusing into the Sn layer, and this phenomenon occurs in the plating layer. The generation of whiskers is suppressed. As a result, an electrical short circuit between the lead frames is suppressed, and the life of the electronic device to which the lead frame having a plating region containing Sn and B is applied is improved.

  In addition, the plating solution having the above composition can generate a relatively smooth surface, which is more ductile to external impact than a relatively rough surface, and has no surface damage. It is suitable for providing a plating layer that wakes up and protects the outer surface of the external lead frame.

  And since the plating solution of this invention does not contain lead (Pb), it is harmless to a human body and is environmentally friendly.

  In addition, by plating using the plating solution of the present invention, plating can be performed with a relatively low current density, and plating can be performed at room temperature without the need to heat the plating solution separately, further improving productivity and economy. sell.

(A) thru | or (c) are the SEM (scanning electron microscope) photographs which respectively showed the surface state of Experimental example 1 thru | or Experimental example 3 after a normal temperature leaving test. (A) thru | or (c) are the SEM photographs which respectively showed the surface state of Experimental example 4 thru | or Experimental example 6 after a normal temperature leaving test. (A) thru | or (c) are the SEM photographs which respectively showed the surface state of Experimental example 7 thru | or Experimental example 9 after a normal temperature leaving test. (A) thru | or (c) are the SEM photographs which respectively showed the surface state of Experimental example 10 thru | or Experimental example 12 after a normal temperature leaving test. (A) thru | or (c) are the SEM photographs which respectively showed the surface state of Experimental example 13 thru | or Experimental example 15 after a normal temperature leaving test. (A) is the SEM photograph which image | photographed the surface of Experimental example 16 immediately after plating, (b) is the SEM photograph image | photographed after performing the normal temperature leaving test with respect to Experimental example 16, (c). These are the SEM photographs which showed the surface state of Experimental example 17 after a normal temperature leaving test. (A) thru | or (e) are the SEM photographs which respectively showed the surface state of the comparative example 1 thru | or the comparative example 5 after a normal temperature leaving test.

  As described above, the conventional Sn-based plating layer has a problem of whisker on the surface. By the way, the cause of the occurrence of such whiskers has not been clearly clarified so far.

  If the Sn-based plating solution is to form a plating layer on a lead frame made of Cu, the present inventors are that the diffusion rate of Cu is faster than Sn at the bonding interface between Sn and Cu. noticed.

That is, at the junction interface between Sn and Cu, the Cu diffusion rate is faster than the Sn diffusion rate, so that the Cu component of the lead frame diffuses to the Sn grain boundary side of the plating layer. Become. Thereafter, an intermetallic compound is formed on the plating layer with a composition of Cu ₆ Sn ₅ .

  When the intermetallic compound applies compressive stress inside the Sn of the plating layer, Sn grows whiskers that are whisker-shaped single crystals on the surface of the plating layer, and this compressive stress is eliminated. Thought.

  Therefore, the present inventors have reduced the generation of compressive stress inside Sn by inserting a metal having a small atomic size into an interstitial site in the crystal structure of Sn to suppress intermetallic diffusion. In this way, we tried to suppress the occurrence of whiskers themselves. For this purpose, boron (B) is preferably used as the metal having a small atomic size.

  The plating solution according to the embodiment of the present invention described below does not contain lead (Pb) ions, but includes tin sulfate as a tin ion source and dimethyl amine borane as a B ion source. borane).

The Sn ion source tin sulfate (SnSO ₄ ) is contained in an amount of 15 g / L to 50 g / L, and the B ion source dimethyl amine borane (DMAB) is 0.1 g / L to 3.0 g / L may be included. Trimethylamine borane (TMAB) can also be used as the B ion supply source.

  When the boron (B) ion supply source is contained in an amount of 0.1 g / L or more, boron (B) is present at the interstitial site of tin (Sn) in the plating layer as compared with the case where the boron (B) ion supply source is less than 0.1 g / L. Since the amount to be inserted is sufficient, the effect of suppressing the growth of the intermetallic compound between tin and the base material as described above is great, and whiskers are not generated. On the other hand, when the boron (B) ion source exceeds 3.0 g / L, the boron entering the interstitial site of tin (Sn) is saturated, so the cost increases and the cost is inferior. Rather, the excess boron can cause the surface of the plating layer to become non-uniform and make the plating bath itself uneasy.

To the plating solution, 30 ml / L to 70 ml / L of sulfuric acid (H ₂ SO ₄ ) is added in order to adjust the conductivity of the electrolyte and the ion mobility of Sn ^{2+ and the} like. When sulfuric acid is less than 30 ml / L, the electric conductivity of the plating solution is lowered, that is, the electric resistance of the plating solution is increased, the plating speed is lowered, thereby reducing the productivity and unevenness in the object to be plated. May be plated. When sulfuric acid exceeds 70 ml / L, a large amount of slime is generated at the anode, the plating solution becomes unstable, and defects may occur in the plating layer.

  Then, 10 g / L to 40 g / L of cresolsulfonic acid or phenolsulfonic acid is added so as to delay the oxidation of Sn. If the cresol sulfonic acid or the phenol sulfonic acid is less than 10 g / L, the oxidation will easily occur, and if it exceeds 40 g / L, the plating solution may become unstable.

  In addition, the plating solution may contain 0.1 g / L to 0.5 g / L of β-naphthol so that the roughness of the surface of the plating layer can be adjusted. When (beta) -naphthol is less than 0.1 g / L, a bad crystal grain may be produced | generated, and when it exceeds 0.5 g / L, the surface may become very rough.

  The plating solution may further include 0.1 g / L to 3 g / L of gelatin. When gelatin is less than 0.1 g / L, the crystal grains become poor, and when it exceeds 3 g / L, many acicular forms or protrusions may be generated.

Using such a plating solution, electrolytic plating is performed on a copper (Cu) plate that can be the main material of the lead frame. This copper plate is used as the cathode, and soluble tin is used as the anode. At the time of plating, the current density is 0.5 A / dm ² to 5 A / dm ² , but 1 A / dm ² to 3 A / dm ² is desirable, and in the following examples, 1 A / dm ² was used. If the current density during plating exceeds 5 A / dm ² , the plating surface becomes very rough, crystal growth is irregular, and unstable plating may occur. Therefore, the reliability of the plated film can be reduced. When the current density at the time of plating is less than 0.5 A / dm ² , the plating time becomes too long, and the productivity can be lowered.

  The temperature of the plating solution was normal temperature (25 ± 3 ° C.). When the temperature of the plating solution is increased, for example, when the plating solution is set to a condition of 50 ± 3 ° C., the additives and the like are decomposed and normal plating is not performed, and as a result, whiskers can occur.

  Hereinafter, preferred embodiments will be presented to help understanding of the present invention.

<Example 1>
Tin sulfate 15 g / L, sulfuric acid (H ₂ SO ₄ ) 30 ml / L, cresolsulfonic acid 10 g / L, β-naphthol 0.1 g / L, gelatin A plating solution containing 0.1 g / L was produced.

  Experimental Example 1 is obtained by adding 0.1 g / L of DMAB to the plating solution. Experimental Example 2 is obtained by adding 0.5 g / L of DMAB to the plating solution. This is a solution obtained by adding 3 g / L of DMAB to the plating solution.

Using such a plating solution, plating was performed under the above-described plating conditions. That is, a copper plate was used as the cathode, soluble tin was used as the anode, the current density was 1 A / dm ^2, and room temperature.

  The experimental example plated in this way was allowed to stand at room temperature for 12 months, and then the occurrence of whisker on the surface was confirmed.

  FIGS. 1A to 1C are SEM (scanning electron microscope) photographs showing the surface states of Experimental Examples 1 to 3, respectively, after the room-temperature standing test as described above.

<Example 2>
Tin sulfate 30 g / L, sulfuric acid (H ₂ SO ₄ ) 50 ml / L, cresolsulfonic acid 20 g / L, β-naphthol 0.3 g / L, gelatin A plating solution containing 0.5 g / L was produced.

  Experimental Example 4 is obtained by adding 0.1 g / L of DMAB to the plating solution. Experimental Example 5 is obtained by adding 0.5 g / L of DMAB to the plating solution. This is a solution obtained by adding 3 g / L of DMAB to the plating solution.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  FIGS. 2A to 2C are SEM photographs showing the surface states of Experimental Examples 4 to 6 after the room temperature standing test as described above.

<Example 3>
Tin sulfate 50 g / L, sulfuric acid (H ₂ SO ₄ ) 70 ml / L, cresolsulfonic acid 40 g / L, β-naphthol 0.5 g / L, gelatin A plating solution containing 1.0 g / L was produced.

  Experimental Example 7 is obtained by adding 0.1 g / L of DMAB to the plating solution, Experimental Example 8 is obtained by adding 0.5 g / L of DMAB to the plating solution, This is a solution obtained by adding 3 g / L of DMAB to the plating solution.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  FIGS. 3A to 3C are SEM photographs showing the surface states of Experimental Examples 7 to 9 after the room temperature standing test as described above.

<Example 4>
Tin sulfate 50 g / L, sulfuric acid (H ₂ SO ₄ ) 70 ml / L, cresolsulfonic acid 40 g / L, β-naphthol 0.5 g / L, gelatin A plating solution containing 3.0 g / L was produced.

  Experimental Example 10 is obtained by adding 0.1 g / L of DMAB to the plating solution. Experimental Example 11 is obtained by adding 0.5 g / L of DMAB to the plating solution. This is a solution obtained by adding 3 g / L of DMAB to the plating solution.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  4 (a) to 4 (c) are SEM photographs showing the surface states of Experimental Examples 10 to 12 after the room temperature standing test as described above.

<Example 5>
Tin sulfate 30 g / L, sulfuric acid (H ₂ SO ₄ ) 50 ml / L, cresolsulfonic acid 20 g / L, β-naphthol 0.5 g / L, gelatin A plating solution containing 3.0 g / L was produced.

  Experimental Example 13 is obtained by adding DMAB to the plating solution at 0.1 g / L, Experimental Example 14 is obtained by adding DMAB at 0.5 g / L to the plating solution, This is a solution obtained by adding 3 g / L of DMAB to the plating solution.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  FIGS. 5A to 5C are SEM photographs showing the surface states of Experimental Examples 13 to 15 after the room temperature standing test as described above.

<Example 6>
Tin sulfate 50 g / L, sulfuric acid (H ₂ SO ₄ ) 70 ml / L, cresolsulfonic acid 40 g / L, β-naphthol 0.5 g / L, gelatin A plating solution containing 1.0 g / L was produced.

  In Experimental Example 16, 30 ppm of DMAB was added to the plating solution, and in Experimental Example 17, 4 g / L of DMAB was added to the plating solution.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  6A is an SEM photograph taken immediately after plating the surface of Experimental Example 16, and FIG. 6B is an SEM photograph taken after a room temperature standing test on Experimental Example 16. FIG. . FIG.6 (c) is the SEM photograph which showed the surface state of Experimental example 17 after a normal temperature leaving test.

  As can be seen from FIGS. 1 (a) to 5 (c), whiskers were not generated on the surface of the plating layers according to Examples 1 to 5 of the present invention even after a long time had passed.

  As can be seen from FIG. 6 (a) and FIG. 6 (b), when the content of DMAB is very small, whisker was not generated immediately after production, but whisker was generated after a long time passed. I was able to confirm that.

  As can be seen from FIG. 6C, whisker was not generated when the content of DMAB was large, but it was confirmed that the surface of the plating layer was uneven and rough.

<Comparative example>
Comparative Examples 1 to 5 are the same as those in Examples 1 to 5, respectively, using a plating solution having a composition excluding DMAB.

  Using such a plating solution, plating was performed under the same plating conditions as in Example 1, and then the plated experimental example was allowed to stand for 12 months at room temperature, and then the occurrence of whisker on the surface was confirmed.

  FIGS. 7A to 7E are SEM photographs showing the surface states of Comparative Examples 1 to 5 after the room temperature standing test as described above.

  As can be seen from FIGS. 7A to 7E, when plating is performed using a plating bath that does not include a boron (B) ion supply source, it can be seen that whiskers are generated on the surface.

  Thus, according to the present invention, it is possible to provide a plating layer that does not contain lead (Pb) and can simultaneously suppress the generation of whiskers.

  The present invention has been described in detail with reference to preferred embodiments. However, the present invention is not limited to the above-described embodiments, and various modifications can be made by those skilled in the art within the scope of the technical idea of the present invention. Various modifications would be possible.

Claims (10)

  A Sn-B plating solution that does not contain Pb ions and contains tin sulfate as a Sn ion source and dimethylamine borane or trimethylamine borane as a B ion source.   The Sn-B alloy plating solution according to claim 1, wherein the Sn ion supply source is contained in an amount of 15 g / L to 50 g / L.   The Sn-B plating solution according to claim 1, wherein the B ion supply source is contained in an amount of 0.1 g / L to 3.0 g / L.   The Sn-B plating solution according to claim 1, further comprising 30 ml / L to 70 ml / L of sulfuric acid.   The Sn-B plating solution according to claim 1, further comprising 10 g / L to 40 g / L of cresol sulfonic acid or phenol sulfonic acid.   The Sn-B plating solution according to claim 1, further comprising β-naphthol in an amount of 0.1 g / L to 0.5 g / L.   The Sn-B plating solution according to claim 1, further comprising 0.1 g / L to 3 g / L of gelatin.   An Sn-B plating method that uses an Sn-B plating solution that does not contain lead ions and contains tin sulfate as a Sn ion source and dimethylamine borane or trimethylamine borane as a B ion source. The Sn-B plating method according to claim 8, wherein plating is performed at a current density of 0.5 A / dm ² to 5 A / dm ² .   In the Sn-B plating solution, the Sn ion source includes 15 g / L to 50 g / L, the B ion source includes 0.1 g / L to 3.0 g / L, and the Sn-B The plating solution is sulfuric acid 30 ml / L to 70 ml / L, cresolsulfonic acid or phenolsulfonic acid 10 g / L to 40 g / L, β-naphthol 0.1 g / L to 0.5 g / L, gelatin 0.1 g / L to The Sn-B plating method according to claim 8 or 9 containing 3 g / L.