Measuring and testing – Testing of material
Reexamination Certificate
2002-02-21
2004-07-06
Raevis, Robert (Department: 2856)
Measuring and testing
Testing of material
C228S103000, C073S053010
Reexamination Certificate
active
06758108
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
The present invention claims priority to priority document no. 2001-051611 filed in Japan on Feb. 27, 2001, and incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solderability testing apparatus and a solderability testing method.
2. Description of the Related Art
To achieve a desirable soldering, enough metallic bonding should be formed between a metal composing a base material (e.g. copper foil (or land portion) on printed circuit boards, and electrode portion of surface mounted devices) and solder (which is generally made of an alloy of tin and lead. It is thus necessary to ensure wetting of the surface of the base material with the solder (more specifically, tin). The wetting solder (tin) diffuses into the base material and forms therein an alloy layer formed with such base material through metallic bond, which is a final form of the soldering. So that testing the wetting balance between the base material and the solder can provide a quantitative evaluation of solderability.
Known apparatuses for evaluating wetting balances of solder, flux, solder alloy, solder paste (also referred to as cream solder) and so forth provided to lead portions of lead parts, lead and electrode portions of surface mounted parts, or land portions on printed circuit boards include an apparatus disclosed in Japanese Laid-Open Patent Publication No. H7-72064, and an apparatus specified based thereon by Standards of Electronic Industries Association of Japan (EIAJ) ET-7404, “Method for Testing Solderability of Surface Mounted Parts Using Solder Paste (Equilibrium Method)”. The solderability testing apparatuses disclosed in these documents are suitable for solderability testing based on the equilibrium method.
As schematically shown in
FIG. 1
, the solderability testing apparatus specifically comprises a sample parts holding means
20
, an external force detection means
10
for supporting such sample parts holding means
20
, a solder paste container
30
, and a heating means
40
. The external force detection means
10
has a load cell (high-sensitivity load sensor). The solder paste container
30
contains a solder paste
31
which is internally added with a flux. The sample parts holding means
20
comprises a sample parts holding member
23
for holding a sample (or standard test piece)
50
, an expansion sliding portion
21
for supporting such sample parts holding member
23
, and an electromagnetic clutch
22
for locking such sliding portion
21
. The sliding portion
21
is suspended at the upper end thereof from the external force detection means
10
.
The solder paste container
30
is supported by a holder
32
, and such holder
32
can ascend or descend, together with the solder paste container
30
, with the aid of a stepping motor
33
. The heating means
40
has a solder bath
41
which serves as a heat source. Solder
42
contained in the solder bath
41
is heated by a heater
43
to be brought into a molten state. The temperature of the solder
42
is monitored with a temperature sensor (e.g., thermocouple), not shown, and results of the measurement are fed back to control the heater
43
. This allows the molten solder
42
in the solder bath
41
to be kept at a predetermined temperature. By dipping the solder paste container
30
in the solder bath
41
containing the molten solder
42
then successfully heats the solder paste
31
contained in such solder paste container
30
to thereby keep the molten state thereof at a predetermined temperature. The solder bath
41
can be ascend or descend with the aid of the stepping motor
44
provided thereunder.
FIG. 6A
is a partial schematic view of the sample parts holding member
23
in a state of holding a sample
50
(e.g., surface mounted parts). The sample parts holding member
23
of the conventional solderability testing apparatus has been made of all sort of metals which can form structural member (except for those having a melting point of 500° C. or lower, or those possibly act as a solder poison such as zinc and aluminum), which can be typified by steel and stainless steel material.
In the solderability testing, the sample
50
is held by the sample parts holding member
23
, the stepping motor
33
is activated so as to raise the holder
32
together with the solder paste container
30
containing the solder paste
31
, and the lower end of the sample
50
goes into the solder paste
31
and finally reaches the bottom plane of the solder paste container
30
. Thereafter the solder paste container
30
pushes the sample
50
upward while being raised by the ascending holder
32
. Thus the sample parts holding member
23
moves upward within the sliding portion
21
of the sample parts holding means
20
as much as the length of ascending path of the sample
50
. After the ascending of the holder
32
together with the solder paste container
30
comes to the end, the sliding portion
21
is locked by means of the electromagnetic clutch
22
. The lower end of the sample
50
is now in contact with the bottom plane (upper bottom) of the solder paste container
30
. The stepping motor
33
is then activated to descend the holder
32
together with the solder paste container
30
. Thus the sample
50
is held so that the lower end thereof is dipped in the solder paste
31
to a predetermined depth so as to keep a predetermined gap between such lower end and the upper bottom of the solder paste container
30
.
Since the external force detection means
10
composing the load cell is applied with a load which is ascribable to the weights of the sample parts holding means
20
and the sample
50
, so that such load is canceled as a tare so as to attain a load-zero status.
Then the stepping motor
44
is activated to raise the solder bath
41
. This allows the solder paste
31
contained in the solder paste container
30
to be quickly heated to a temperature of the molten solder
42
and brought into a molten state. In the melting process of such solder paste
31
, acting forces exerted on the sample
50
, which are typified by buoyancy attributable to the solder paste
31
and surface tension of the molten solder paste
31
, are detected by the load cell which composes the external force detection means
10
, and then output as electric signals.
In the process of soldering, acting forces effecting between the molten solder paste
31
and the sample
50
are considered as two ways; that are acting force f
1
ascribable to the surface tension of the solder paste
31
; and buoyancy f
2
from the molten solder paste
31
, while ignoring the weight of adhered solder. The acting force f
1
and buoyancy f
2
can be expressed by the following equations (1) and (2), respectively, where force (tension) directed downward is defined as positive force.
f
1
=&ggr;l cos &THgr;) (1)
f
2
=−&rgr;vg (2)
, where, meanings of &ggr;, &THgr;, l, &rgr;, v and g are respectively as follows:
&ggr;=boundary tension between molten solder paste and flux;
&THgr;=contact angle of molten solder paste with sample;
l=outer peripheral length of a sample measured at contact plane with molten solder paste;
&rgr;=density of molten solder paste;
v=volume of displaced molten solder paste; and
g=gravitational acceleration.
When heating of the solder paste
31
starts, the surface of the sample
50
starts to be wet with the flux preliminarily mixed into the solder paste
31
, where the flux is responsible for removing oxide film or foreign matters from the surface of the sample
50
to thereby clean such surface of the sample
50
.
Then the solder paste
31
starts to melt, buoyancy ascribable to such molten solder paste
31
starts to effect, and wetting with such solder paste
31
also starts when the temperature of the sample reaches a predetermined level. The force F exerted on the sample
50
herein is expressed as an equation below.
F=f
1
+f
2
=&ggr
Masatoki Tamiharu
Shinozaki Yutaka
Raevis Robert
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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