JPH01189153A - Lead frame material comprising copper or copper alloy - Google Patents

Abstract

PURPOSE:To bond a piece of wire to a lead frame directly without presence of a plated layer, by setting the average roughness of 0.15mum or less for the central line of the surface of a lead frame material comprising copper or copper alloy, and flattening the head of the minute projection of the surface. CONSTITUTION:A conventional plate material comprising oxygen-free copper and the like is rolled to a thickness of about 0.26mm. Light finish rolling is performed with a roll, whose surface roughness Ra is 0.12mum or less. At this time, the amount of deformation of a load is about 1-5%. The Ra of the surface of a lead frame material is made to be Ra<0.15mum by said light finish rolling. At the same time, the head of a minute projection is crushed and flattened. When a piece of wire comprising Au and the like is bonded to the surface, the bonded part is pushed into the deep part of a minute recess part without a gap. The bonded part is strongly compressed to the falt head of the minute projection and rigidly bonded. Since there is no gap, bonding strength is not decreased due to the intrusion of moisture and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial Application Field" The present invention relates to lead frame materials used in the manufacture of integrated circuit devices, and in particular to lead frame materials to which wires can be directly bonded without intervening plating layers. Regarding.

``Prior Art'' A typical lead frame is manufactured by stamping and forming a plate made of copper or copper alloy, and then plating gold or silver at the wire bonding area. The reason why gold (silver) plating is applied in this manner is that gold or aluminum wires cannot be bonded with sufficient strength if they are punched and formed from a metal plate.

However, since the quality of the plating has a very large effect on wire bonding, extremely high precision is required when plating lead frame materials, resulting in low productivity and high costs due to the use of precious metals even in minute quantities. It had the disadvantage of being high.

Therefore, from the viewpoint that it would be possible to significantly reduce costs if wire bonding could be performed directly on the lead frame material without gold (silver) plating, for example,
In Publication No. 071, the surface roughness of the lead frame material made of copper or prepared alloy is set to 0.5μ in maximum height (Rwax).
It has been proposed that direct bonding of gold wires is possible by setting the bonding temperature to less than 100%.

"Problem to be solved by the invention" However, as a result of actual experiments conducted by the present inventors, Rn
+ax: It has been found that wire bonding may not always be successful even with lead frame materials of 0.5 μm or less. Therefore, the present inventors conducted a more detailed study on the cause and came to the following findings.

In other words, even though it is said that Rmax is 0.5 μm or less, the actual surface properties vary, and lead frame materials that have only been made Rmax: 0.5 μm or less by ordinary high-precision finish rolling cannot be used. As shown in FIG. 3, many sharp microprotrusions remain on the surface. For this reason, when a wire is bonded to this lead frame, a gap is created between the valley of the microprotrusions and the wire bonding part, as shown in Figure 4, which reduces the bonding area and stabilizes the bonding strength of the wire. It is difficult. Furthermore, in this case, impurities such as moisture may enter the gap, which may cause a decrease in bonding strength.

"Means for Solving the Problems" The present invention has been made to solve the above problems, and the center line average roughness (Ra: JIS standard) of the surface of a lead frame material made of copper or copper alloy is 0.15 μm. In addition to the following, it is characterized in that the tops of the microprotrusions on the surface are flat surfaces.

The present invention will be explained in detail below.

As for the lead frame material used in the present invention, materials similar to those conventionally used such as oxygen-free steel, iron copper, and tinned copper are used. Then, this plate material is first formed into a thickness of, for example, about 0.3 mm by normal rolling processing, then subjected to normal finish rolling to a thickness of about 0.26 mm, and further subjected to puff polishing, pickling treatment, etc. as necessary. give Next, this plate material is subjected to finish light rolling by applying a light load using a rolling roll having a pole-smooth surface. Here, the surface roughness required for the rolling roll is Ra: 0.12 μm or less, and the load during light finishing rolling may be set so that the amount of deformation is about 1 to 5%. desirable. If the surface of the rolling roll is rougher than the above value, the desired surface roughness cannot be obtained. Also 1
If the amount of machining is less than 5%, the tops of the minute protrusions existing on the surface of the lead frame material cannot be flattened, while if it exceeds 5%, new protrusions will be generated and the surface roughness of the lead frame material will deteriorate.

The surface of the lead frame material is rounded by the above finishing light rolling.
a: 0.15 μm or less, and at the same time crush the tops of the microprotrusions to flatten them as shown in FIG.

If Ra is larger than 0.15 μm, a minute gap will be created between the bonding portion of the wire and the surface of the lead frame, reducing the bonding strength. In addition, the diameter of the flattened top of the head is 0.1~
It is desirable that the thickness be 5 μm; if it is less than 0.1 μm, the bonding strength will be insufficient as in conventional products, and if it exceeds 5 μm, the rate at which the wire will get caught in the minute recesses will decrease, and the bonding strength will also decrease.

When a wire made of gold or aluminum is bonded to the surface of the lead frame material having the above structure,
As shown in FIG. 2, the bonded portion of the wire fits into the micro-recesses on the surface almost without gaps, and is strongly pressed against the flat tops of the micro-protrusions to be firmly bonded to the tops. As a result, it is possible to obtain extremely strong bonding strength as a whole, and it is possible to significantly improve the reliability of direct wire bonding. Furthermore, since a gap is unlikely to be formed between the surface of the lead frame and the bonded portion of the wire, there is no risk of impurities such as moisture entering there and reducing the bonding strength, and reliability can be improved from this point as well.

Although the fine protrusions on the upper side were flattened by finishing light rolling, the lead frame material of the present invention is not limited to the above method, and can also be manufactured by, for example, polishing or etching.

"Example" Next, examples will be given to demonstrate the effects of the present invention.

Six lead frame materials each having a thickness of 0.25 mm, a width of 25 mm, and a length of 30 mm were formed from copper alloy plates of C19400 (CDA number) by normal rolling.

Next, three of the sheets were pickled with a 1svo1% sulfuric acid aqueous solution, and the surface roughness was R8 = 0.09.
Finish light rolling was performed using a μm surface smoothing roll with a deformation amount of about 2%.

Then, the surface condition of the six lead frame materials was measured using a surface roughness meter and a microscope, and these were set in an automatic wire bonding device to perform wire bonding. The conditions are shown below.

Joining method: Thermosonic method Wire material: gold Wire diameter: 25μm Lead frame temperature = 225℃ Bonding load = 35g Ultrasonic Crab 0.2W Ultrasonic generation time: 20m5ec.

Next, the joined wire was pulled up using a hook, and the lifting load required to cut the wire was measured. At the same time, the bonding strength was also evaluated.

This evaluation is ``○'' if the joint part withstands the wire pulling load and no change is seen until the wire breaks, and ``Δ'' if the wire breaks after the joint part peels off. Completely peeled off is marked "×".

The results will be presented to the deputy director. In addition, in the table, Examples 1 to 3 are examples,
Ratios 1 to 3 are comparative examples. Moreover, the flat length indicates the average diameter of the crown/part of the head.

As is clear from the above table, the lead frame materials of Examples 1 to 3 had high bonding strengths that could withstand actual use.

"Effects of the Invention" As explained above, the lead frame material made of copper or copper alloy according to the present invention has a center line average roughness (R
a) is 0.15 μm or less, and the tops of the microprotrusions on the surface are flattened, so when a wire is bonded to the surface, the bonded part of the wire will bite deep into the microscopic recesses on the surface, and at the same time It is strongly pressed against each flat top of the protrusion, and is firmly joined to this top of the head. Therefore, extremely strong bonding strength can be obtained for the entire bonding portion, and the reliability of wire bonding can be significantly improved. Furthermore, since no gap is created between the surface of the lead frame material and the wire joint, there is no risk of impurities such as moisture entering there and reducing the joint strength.

[Brief explanation of the drawing]

FIG. 1 is an enlarged cross-sectional view of a lead frame material made of copper or copper alloy according to the present invention, and FIG. 2 is an enlarged cross-sectional view of the same lead frame material in a wire-bonded state. On the other hand, Figure 3 is an enlarged cross-sectional view of a conventional lead frame material.
FIG. 4 is an enlarged cross-sectional view of the wire bonded state.

Claims (1)

[Claims] A lead frame material made of copper or copper alloy, characterized in that the centerline average roughness (Ra) of the surface is 0.15 μm or less, and the tops of the microprotrusions on the surface are flattened.