JP5056333B2 - Plating substrate with lead-free plating layer - Google Patents

Description

  The present invention relates to a plating substrate having a lead-free plating layer.

  In an electronic component such as a semiconductor device, copper, a copper alloy, 42 alloy, or the like is used as a base material for an external terminal. However, if the substrate is left as it is, the terminal surface may be oxidized to cause poor conduction due to poor soldering or the like. is there. For this purpose, a protective film (plating layer) is usually formed on the terminal surface by plating or the like to prevent oxidation.

  When using an Sn alloy as a material for the plating layer, an alloy containing lead (Pb) has been used conventionally. In recent years, there has been a demand for lead-free from the viewpoint of reducing the environmental burden, and the plating layer material of the terminal is made of, for example, pure Sn or Sn alloy such as Sn—Cu, Sn—Bi, Sn—Ag. As such, materials that do not contain lead are used. However, when the surface of the terminal of the electronic component is plated with a lead-free material, Sn whisker having a length of several hundred μm or more is generated from the plating layer.

  In recent years, for example, electronic components such as a semiconductor device in which an IC chip is mounted on a lead frame have been required to be further reduced in size, and as a result, the distance between the terminals has been reduced to about several hundred μm. The whisker may grow to a length of several hundred μm, and if the distance between the terminals is as narrow as several hundred μm as described above, there is a possibility that a short circuit between the terminals may occur due to the generated needle whisker. is there. Therefore, measures for suppressing the occurrence of acicular whiskers are required.

  Although the mechanism of whisker generation and growth has not been fully elucidated, it is thought that the increase in internal stress of the plating layer is one of the causes. Examples include changes in the structure and structure of plating particles due to the above, residual stress during processing of the lead frame, residual stress during plating, and a difference in coefficient of linear expansion between the lead frame and the plating layer. From such a point of view, Patent Document 1 discloses a substrate having an insulating base member and a conductor layer formed on the base member, and a Pb-free solder formed on the conductor layer. In the conductive connection member provided with a layer, the conductive connection member is configured such that the surface of the base member has irregularities so as to absorb internal stress generated when a load is applied to the Pb-free solder layer from the outside. Has been.

Further, in Patent Document 2, in the terminal of an electronic component having a metal base and a tin or tin alloy electroplating film formed on the base, the surface of the base and the surface of the electroplating film are not provided. It is described that whisker generation is suppressed by forming a rough surface having a directional uneven shape and having an Rz value of approximately the same value within a range of 0.5 to 5 μm.
JP 2005-353948 A JP 2006-124788 A

  The present inventors have continued many experiments and research on the generation of whiskers in the lead-free plating layer, but the lead-free plating layer is formed by forming irregularities on the surface of the base material or making it rough. It has been experienced that none of the conventional techniques for suppressing the generation of whiskers in the solder layer (or solder layer) has achieved satisfactory results.

  The present invention has been made in consideration of the above circumstances, and in a plating base material having a lead-free plating layer, lead that can more completely suppress the occurrence of whiskers while basically using a technique for relaxing internal stress. It is an object to provide a plating base material having a free plating layer.

  By conducting more experiments and research on the generation of whiskers in the lead-free plating layer, the present inventors have found that the conventional method has the residual stress on the base material side generated when processing the base material, or the actual machine. As a result, there is not enough mitigation (reduction) measures against the residual stress on the base metal side caused by the temperature history, etc. that is incurred during use. It was found that this was one of the causes of whiskers.

  The present invention is based on the above knowledge, and the plating base material having the lead-free plating layer on the surface of the base material according to the present invention has a plurality of recesses having a depth of 50 μm or more on the surface of the base material. It is characterized by. A recess having a depth of less than 50 μm has an insufficient residual stress reducing effect, and as a result, a sufficient whisker generation suppressing effect cannot be obtained. The recess having a depth of 50 μm or more includes a through hole penetrating the base material.

  In the plating base material having a lead-free plating layer according to the present invention, preferably, the diameter of the recess is in the range of 0.5 μm to 10 μm, and preferably, the total area of the plating layer forming portion of the base material is The ratio occupied by the plurality of recesses is in the range of 10% to 20%. Note that a recess having a diameter of less than 0.5 μm is difficult to process, and a sufficient residual stress reduction effect cannot be obtained. A recess having a diameter exceeding 10 μm can reduce the residual stress, but is not preferable because it affects the formation of a uniform plating layer. Further, when the ratio of the plurality of recesses to the total area of the plating layer forming portion of the base material is less than 10%, a sufficient residual stress reduction effect cannot be obtained, and whiskers are easily generated, and 20% In the case where it exceeds 1, the strength of the base material may be lowered, which is not preferable.

  In the plating base material having the lead-free plating layer according to the present invention, preferably, the base material is Cu, Cu alloy or 42 alloy, and 42 alloy is particularly preferable. The lead-free plating layer can be exemplified by a plating layer of pure Sn or Sn alloy such as Sn—Cu, Sn—Bi, Sn—Ag.

  As shown in the following examples, in the plating base material having the lead-free plating layer according to the present invention, the residual stress on the base material side is reliably reduced by forming unevenness in a numerical range limited to the base material surface. As a result, it is possible to reliably suppress the occurrence of whiskers in the lead-free plating layer. In this case, the strength of the plating base material (base material) is not lowered to such an extent that it is insufficient for use as an actual machine. When forming a recess outside the numerical range specified in each claim, the effect of reducing the residual stress on the base metal side becomes insufficient, and it is not possible to suppress the occurrence of whiskers on the plating layer, or As a result, the strength of the plating base material is greatly reduced and it cannot be used as an actual machine.

  Furthermore, as shown in the following examples, the plating substrate having the lead-free plating layer according to the present invention is allowed to stand for 30 minutes on the low temperature side −40 ° C. and left for 30 minutes on the high temperature side 80 ° C. for one cycle. Whisker generation was not observed even when a cooling cycle history was repeated 2000 times. The temperature range of this thermal cycle is in the range experienced by electronic components mounted on automobiles. Therefore, the plating base material having a lead-free plating layer according to the present invention is used as a plating base material for in-vehicle electronic components. Can be used particularly preferably.

  According to the present invention, in the plating base material having the lead-free plating layer on the surface of the base material, the influence of the residual stress generated on the base material side can be eliminated, and it is ensured that whiskers are generated in the lead-free plating layer. Can be suppressed.

[Example 1]
As shown in FIG. 1, 42 alloy was used for the base material 1, and drilling was performed to form a plurality of recesses 2 having a diameter of 1 μm on one surface thereof. The size of the base material 1 is 16 mm × 44 mm × 0.5 mm (thickness). 4 types of depths of the recess 2 of 10 μm, 50 μm, 100 μm, and through-holes are prepared, and a lead-free plating layer made of Sn so that each test piece has an average thickness of 7 μm to 9 μm by electrolytic plating To form a plating substrate. After the plating layer was formed, the plating layer was subjected to a cooling cycle in which a cooling cycle was performed 2000 times, which was left for 30 minutes on the low temperature side −40 ° C. and left for 30 minutes on the high temperature side 85 ° C.

The residual stress (MPa) on the base material side of each plating base material after the cooling cycle history was measured. The results are shown in FIG. Residual stress was measured by the sin ² ψ method. Moreover, the generation | occurrence | production state of the whisker in the plating layer after a thermal cycle history was observed using the electron microscope (SEM). The results are shown in Table 1.
[Comparative Example 1]
A lead-free plating layer made of Sn was formed on the surface of the base material 1 having the same dimensions as in Example 1 without drilling to form a plating base material. On the other hand, the same thermal cycle as Example 1 was given, and the residual stress (MPa) on the base material 1 side in the plating base material after the thermal cycle history was measured. The result is shown in FIG. Further, the whisker generation state in the plating layer was observed in the same manner as in Example 1. The results are shown in Table 1 with the depth of the recess being 0.

[Example 2]
A plurality of base materials 1 identical to those in Example 1 were test pieced, and a hole was formed on one surface thereof to form a plurality of recesses 2 having a diameter of 1 μm and a depth of 50 μm. However, the ratio (working area) occupied by the recess 2 with respect to one surface area of the base material 1 is three types of 10%, 20%, and 30%, and the same Sn as in Example 1 is prepared for each test piece. A lead-free plating layer was formed as a plating substrate. After the plating layer was formed, the same cooling cycle as in Example 1 was performed.

As the strength of each plating base material after the cooling cycle history, the breaking strength measured by a tensile test was measured. The strength of each plated member of Comparative Example 2 (that is, a plated member that is not subjected to drilling), which will be described later, was set to 100, and the respective strengths were compared. This is shown as strength retention in FIG. Further, in the same manner as in Example 1, the state of whisker generation in the plated layer after the cooling cycle history was observed. The results are shown in Table 2.
[Comparative Example 2]
A lead-free plating layer made of Sn was formed on the surface of the base material having the same dimensions as in Example 2 without drilling in the same manner as in Example 2 to produce a plating substrate. On the other hand, the same cooling cycle as in Example 1 was applied, and the strength at break by a tensile test was measured as the strength of the plating base material after the history of the cooling cycle. The results are shown in FIG. 3 with a processed area of 0%. Moreover, the generation | occurrence | production state of the whisker in a plating layer was observed similarly. The results are shown in Table 2 as a processed area of 0%.

[Discussion]
As shown in FIG. 2, when the depth of the recess is 50 μm or more, the residual stress is greatly reduced. As a result, as shown in Table 1, the occurrence of whiskers in the plating layer is suppressed. However, as shown in FIG. 3, when the ratio of the processing area of the recess is about 30%, even if whisker generation can be suppressed, the strength retention rate of the base material becomes 80% or less, It can be seen that insufficient strength may occur.

The figure explaining the state which bored the base material used in the Example. 6 is a graph showing the relationship between the depth of the recess and the residual stress of the base material in Example 1. The graph which shows the relationship between the recess processing area in Example 2, and an intensity | strength retention rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Base material (base material), 2 ... Recess formed in base material surface

Claims (3)

A plating base material having a plating layer of lead-free pure Sn or Sn alloy on the surface of a base material made of Cu, Cu alloy or 42 alloy, having a depth of 50 μm or more and a diameter of 0.5 μm on the surface of the base material A plating substrate having a lead-free plating layer, wherein a plurality of circular recesses in the range of 10 μm to 10 μm are formed.   2. The plating base material having a lead-free plating layer according to claim 1, wherein a ratio of the plurality of recesses to a total area of the plating layer forming portion of the base material is in a range of 10% to 20%. . The plating base material having a lead-free plating layer according to claim 1 or 2 , wherein the plating base material having the lead-free plating layer is a plating base material in an in-vehicle electronic component.

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