JPS6021534A - Circuit mounting structure - Google Patents

Abstract

PURPOSE:To realize reduction in thickness of a wrist watch while keeping the reliability level of a circuit block thereof by face down bonding of integrated circuit element having a solder bump to a flexible circuit substrate with copper foil and keeping an interval between substrate conductor pattern surface and integrated circuit element surface to the particular range. CONSTITUTION:An IC chip with a side of 2-4mm. having a solder bump 3 comprising a nickel core 2 in the thickness of about 15mum is used and a flexible circuit substrate with copper foil is used as a substrate 5. The IC chip 1 and substrate lead is aligned in position and are heated for joining. On the occasion of such heating, the IC chip is heated from the rear surface. Thereby, the surface of nickel core 2 in the solder bump 3 and substrate lead surface are forced to join in such a condition as they are in contact and clearance between the IC chip and substrate is always kept at 10-20mum. The solder perfectly surrounds the substrate lead up to the side surface by setting amount of solder of bump to the thickness of about 40mum and the joining condition where edge short-circuit by flow of solder is not generated can be obtained.

Description

[Detailed description of the invention] The present invention relates to a mounting structure of a circuit block for a wristwatch.

Generally, in wristwatches, the wire bonding method, tape carrier method, and face-down bonding method are used as methods for mounting an integrated circuit element (hereinafter referred to as "C chip") on a circuit board. Among these, the tape carrier method or the face-down bonding method is the mainstream for strong Ln analog watches that are particularly oriented toward thinness and miniaturization. The mounting space for these methods is, for example, the size of the IC chip on one side asaab and the thickness as 0.
．． In the case of 3w, in the case of tape carrier method,
In the thickness direction, approximately 420μ from the surface of the board conductor pattern to the back surface of the IC chip is required, and in the plane direction, 3.2 x 3.2m'' is required.Furthermore, in the case of the face-down bonding method, approximately 400 force is required in the thickness direction, and approximately 400μ in the plane direction. 3×3−
The current situation is what we need.

On the other hand, the need for wristwatches to be thinner and more compact has been increasing in recent years, and the current mounting space has reached a level where these demands cannot be fully met.

However, in the tape carrier method, in order to prevent edge shorts, it is necessary to bend the finger leads in the direction of the IC chip by about 100μ, and from the viewpoint of wristwatch design, the IC chip must be placed on the copper foil side of the board. It is difficult to reduce the thickness beyond the width of the legs, as it is often necessary to do so.

Also, in the face-down bonding method, from the viewpoint of joint reliability against thermal stress due to the difference in thermal expansion coefficient between the substrate and the IC chip, the amount of solder after bonding, that is, the IC
The larger the gap between the chip surface and the substrate conductor pattern surface, the more desirable it is, and generally H is 100μ as shown in FIG.
However, it is difficult to further reduce the bonding thickness to an even greater extent.

The present invention eliminates such drawbacks, and its purpose is to reduce the gap between the IC surface and the substrate conductor pattern surface to about 15μ, about 1/6 of the conventional one, while maintaining the reliability level required for a wristwatch circuit block. An object of the present invention is to provide an improved face-down bonding method that enables a thickness reduction of approximately 3-μ.

The present invention will be described in detail below based on examples. In the present invention, an IC chip having 2 to 4 sides on each side and having solder bumps with a nickel core of about 15 .mu.m thick inside is used, and a flexible circuit board with copper foil is used as the substrate. After aligning the IC chip and the substrate leads, they are heated and bonded.

During this heating, the IC chip is pressurized from the back side to forcibly bring the surface of the nickel core within the solder bump into contact with the surface of the substrate lead, thereby performing the bonding. This allows the gap between the C-chip and the substrate to be kept constant. Also, the thickness of the bump is approximately 40 mm as the amount of solder.
By setting μ, as shown in FIG. 1, it is possible to achieve a state after bonding in which the solder completely wraps up to the side surfaces of the substrate leads, and a bonded state in which edge shorts due to solder outflow do not occur.

By adopting the above-mentioned mounting structure, the I shown in FIG.
The gap H4- between the C chip surface and the substrate conductor surface can be made extremely small to about 15 μm, and a sufficient level of bonding reliability for a wristwatch can be ensured. This is due to the following reason.

First, since a flexible board is used as the board, the thermal stress on the solder joint due to the difference in thermal expansion coefficient between the board and the XC chip is smaller than in the case of a rigid board.

Secondly, IC chips for analog electronic watches are usually as small as 3 wJ on a side, so the degree of thermal stress described above is relatively small.

Thirdly, at the joint between the board and the IC chip, by sufficiently encircling the solder to the side of the board conductor button, the p1 cylindrical solder wraps around the diameter formed by the nickel core and the conductor pattern. Since the structure is joined at
The height of the solder bulb after bonding is equivalent to approximately 50μ by adding 35μ for the thickness of the board conductor to the 15μ for the nickel core, and the aforementioned thermal stress does not become extremely large.

Fourth, in wristwatches, the IC chip used is tl
Since in most cases there is only one IC chip, the reliability required for each IC chip is not relatively high compared to the case of multiple chips such as a computer.

As a result of the combination of the above factors, a bond with a reliability level that can be guaranteed for more than 10 years can be achieved with this structure.

Specific examples are shown below.

As an IC, chip size is 2*3 m x 3. Oru chip thickness 3
00μ Bump structure Nickel core Thickness 15μ On top of that, half 1) Thickness 40μ Bump dimensions Mouth 180μ Thickness 55μ Solder composition 10% Bn 90% Pb Base material for substrate Polyimide tape (thickness 125μ) Conductor 35μ
Thick silver foil surface Gold plated joint lead dimensions φ110μ 6 - As a molding agent, soft epoxy resin After bonding, gap between IC chip surface and board conductor surface H15μ Joint bump outer dimension Approximately φ230μ In addition, other substrate materials are available. Gafebo tape.

Alternatively, BT resin tape, polyester tape, etc. may also be used. Copper may also be used as the bump core, and the solder composition may be 5% 8s9a%Pb or 60%.
% Sn 40 tit F6 or other compositions may be used.As described above, the present invention has the following advantages: while maintaining sufficient reliability as a wristwatch, it is possible to reduce the thickness of the mounting space by about 90μ compared to the conventional one; By forcibly applying pressure during bonding and greatly deforming the solder bump, the oxide film on the surface can be destroyed, making it possible to create solder bonds with sufficient strength without using flux. It is effective.

7-

[Brief explanation of drawings]

FIG. 1 is a diagram of a circuit IC chip mounting structure for a wristwatch according to the present invention, fα) is a plan view, and [b) is a sectional view. Figure 2 is a diagram of the conventional wristwatch circuit IC chip mounting structure.
α1 is a plan view, and 1M+ is a cross-sectional view. 1. C-chip 2. Bump metal core 3. Φ Bump solder portion 4 in the present invention. Circuit board copper foil pattern 5e@Flexible circuit board 3-@. Bump solder portion 5' in conventional method. Circuit Substrate and above Applicant Suwa Seikosha Co., Ltd. Agent Patent Attorney Mogami Tsumu 8- (α) 2nd year of age

Claims (2)

[Claims] (1) In the circuit block, a flexible circuit board with copper foil is used on the board, an integrated circuit element having solder bumps is bonded face-down to it, and the gap between the surface of the board conductor pattern and the surface of the integrated circuit element is set to 10~10.
A circuit mounting structure characterized by keeping the thickness to 20μ. (2) The structure of the joint is such that a nickel or copper metal core with a thickness of 10 to 20 μm provided in the solder bump is in contact with a copper foil pattern of 18 to 35 μm in thickness on the circuit board, and
The circuit mounting structure according to claim 1, wherein the metal core and the outer peripheral side surface of the conductor pattern are joined by solder or in a cylindrical shape. −】−