JPS5942981B2 - Reflow method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solder reflow method, and more particularly to a reflow method in which solder is heated in liquid flux.

The prior art will be explained with reference to FIGS. 1 and 2.

That is, lead-tin solder 2 is deposited by vapor deposition on the soldering pattern 4 on the silicon wafer 1 shown in FIGS. 1a and 1b. A liquid flux 3 is applied onto this with a brush, heated in an inert gas atmosphere to a temperature higher than the melting point of the solder 2, and then cooled to form a solder ball 5 on the pattern 4 as shown in c. However, liquid flux 3
The solder reacts with oxides on the surface of the solder 2, oxygen or moisture contained in the inert gas, and becomes a reactant 6 that is very difficult to remove and is fixed on the silicon wafer 1. The most effective way to avoid this is to apply a large amount of flux 3 as shown in FIG. 2d. In this method, the reactants 6 are suspended in the flux 3 or on the surface thereof, and are hardly attached to the silicon wafer 1. (See e) However, with this method, as shown in f, the viscosity of the flux 3 decreases during heating, and the flux 3 flows from the silicon wafer 1 onto the heater 7, resulting in an insufficient amount of effective flux, as shown in c. The situation is often the same. Furthermore, in the state f, flux 3 has entered between the heater T and the silicon wafer 1.
This also has the disadvantage that the silicon wafer 1 adheres to the heater 7 and becomes difficult to remove. The present invention eliminates the above-mentioned drawbacks of the prior art and provides a reflow method in which flux reactants do not stick.

As a result of investigating how the rate at which flux reactants adhere to the surface of a silicon wafer changes depending on the thickness of the flux, the result is as shown by the solid line in FIG. 3.

That is, as shown in Fig. 4, the thickness of the flux reactant is approximately 0.17llR, and the thickness of the flux decreases by approximately 0.3x due to the disappearance of volatile components in the flux. It was found that if the thickness was not set to 0.4 mm or more, all the flux would react and become stuck.

Furthermore, in order to remove the flux reactants to an extent that is not a problem for practical use, it is necessary to remove 0.0% as shown by the solid line in Fig. 3.
A flux thickness of 5- or more is required. On the other hand, when the flux thickness is decreased from 0.4 mm, the amount of reactants fixed on the flux decreases, as is clear from the solid line in FIG.

Therefore, similar results can be obtained in terms of fixation of reactants by reducing the amount of flux, but the flux action (cleaning the solder surface, preventing oxygen from entering the atmosphere, etc.) may be incomplete. As a result, the reflow performance deteriorates as shown by the broken line in FIG. From the above results, the thickness of the flux is 0.51! It has been found that more than IR is required, but in order to reliably achieve this value, a method must be adopted in which the silicon wafer is immersed in a container filled with flux and the entire container is heated.

An example is shown in FIG.

In order to achieve a flux thickness of 0.5 centimeters or more, a reflow jig 8 was placed on the heater 7, a silicon wafer 1 was placed therein, and the periphery of the silicon wafer 1 was completely covered with the flux 3.

In this embodiment, the silicon wafer 1 is supported by the tips of the plurality of protrusions 9, which has the following effects.

(1) Since the layer of flux 3 surrounding the lower part of the silicon wafer 1 is thick, it is easy to remove the flux with a solvent after cooling.

(2) Since the flux 3 reliably wraps around the bottom of the silicon wafer 1, more complete submerged heating can be achieved;
Good temperature uniformity.

When reflowing is performed using the method according to the present invention, the flux thickness is always 0.5? Because of the above, the state shown in FIG. 2e, that is, the state in which the flux reactant 6 does not stick to the silicon wafer 1 can be reliably obtained.

[Brief explanation of the drawing]

FIG. 1 is a plan view and an enlarged sectional view showing an example of the conventional method;
FIG. 2 is a sectional view and an enlarged sectional view showing an example of the conventional method,
FIG. 3 is a graph showing the relationship between flux thickness and fixation, FIG. 4 is an enlarged sectional view showing the flux fixation mechanism, and FIG. 5 is a sectional view showing an example. 1...Silicon wafer, 2...Vapour-deposited solder, 3...Liquid flux, 4...
...Soldering pattern, 5...Solder ball, 6...Flux reactant, 7...
Heater, 8... Reflow jig, 9...
protrusion.

Claims (1)

[Claims] 1. Applying a soft brazing material to a pattern made of a solderable substance on a semiconductor wafer using vapor deposition, plating, or printing methods, and then heating the pattern to a temperature higher than the melting temperature of the soft brazing material. In the reflow process in which the soft solder metal is brought into close contact with the pattern, the soft solder metal and the pattern are covered with a liquid flux suitable for the soft solder metal to a thickness of at least 0.5 mm. A reflow method characterized by heating the soft filler metal above its melting temperature and cooling it to room temperature.