Method for detecting a coating material on a substrate

A method for detecting the presence of a coating material, particularly an organic solderability preservative, such as imidazole, on a copper substrate includes placing a test droplet on the surface of the substrate, forming an image of the droplet and measuring the contact angle between the wall of the droplet and the surface of the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for detection of a coating material on a 
substrate and more particularly to a method for detecting and/or 
distinguishing the presence of an organic solderability preservative on a 
copper substrate. 
2. Description of the Related Art 
Bare copper surfaces oxidize on exposure to air and develop a patina 
containing copper hydroxide and various other copper compounds. Even 
invisibly thin coatings of this patina are sufficient to interfere with 
the ability of solder to wet and form a bond with the copper surface. 
Surface oxidation renders a copper substrate non-solderable, and unusable. 
Hence, newly etched copper used in electronic circuit boards is coated 
with a solderability preservative. 
Use of imidazole as an organic solderability preservative ("hereinafter, 
"OSP") is described in U.S. Pat. No. 4,373,656, herein incorporated by 
reference. Other OSP's are also known and include various azole 
derivatives such as benzimidazole, benzotriazole, and alkyl imidazoles. 
OSP's generally work by forming a complex with copper, which prevents 
further surface oxidation without diminishing the solderability of the 
surface. 
Detection of the OSP should be performed immediately after the coating 
process as a process control step, or in the field prior to the soldering 
process. One problem arises from the fact that the OSP coating is present 
in very low quantities. Very often, the OSP coating has a thickness of 
only one monomolecular layer of OSP-copper complex. 
A large percentage of the printed circuit boards currently being used are 
coated with imidazole. Imidazole deposition is a delicate process which 
would be dramatically improved with a detection technique used as a 
process monitoring tool. Such a technique should meet three criteria: it 
should be non-destructive, it should be sensitive enough to detect minute 
levels of the OSP, and it should be simple to perform with minimal 
training of the personnel performing the inspection procedure. 
Various attempts have been made to achieve such a method: FT-IR reflective 
techniques, use of a quartz crystal oscillator microbalance, fluorimetry, 
viscometry, spectroscopy, ESR techniques, XPS, IR, and electrochemical 
methods such as voltammetry, tensammetry, and SERS. Chemically, imidazole 
has been detected by its reaction with epoxy compounds. Absence of OSP has 
also been detected by nitric acid and silver nitrate drop tests. 
Many of these methods do not meet the above criteria for a non-destructive, 
sensitive method which is easily learned by new personnel. Some of the 
methods require years of advanced study to properly perform the tests and 
accurately interpret the results with acceptable efficiency. Also, some of 
the methods mentioned above are for use in aqueous solutions which require 
a significant area of sample preparation relative to the surface area of 
the printed wiring product. 
We have discovered an improved technique for detecting an OSP coating 
especially imidazole on copper. The method is non-destructive, sensitive 
enough to detect monomolecular layers of coating, and employs relatively 
simple instrumentation with which a high level of operator skill can be 
acquired in a matter of hours. The technique is both simple and accurate 
and may be broadly applied to various types of coating materials and 
substrates. 
SUMMARY OF THE INVENTION 
A method for detecting the presence of a coating material, especially an 
organic solderability preservative, on a substrate is disclosed herein. 
The method includes the steps of placing a droplet of test liquid on the 
surface of the substrate (e.g. a copper surface), and comparing a surface 
feature of the droplet against a predetermined standard. Generally, an 
image is formed of the droplet and the contact angle defined by the wall 
of the droplet and the surface of the metal substrate is measured and 
compared with known values of contact angles for the particular coating 
material and bare substrate surface. The method is particularly applicable 
to imidazole coated copper substrates used for printed circuits. Other 
organic solderability preservatives may also be detected by the method 
described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) 
The method of the present invention relies on the fact that the surface 
features of a droplet of liquid vary depending on the surface of the 
substrate upon which the droplet is placed. The method includes the steps 
of placing a droplet of test liquid on the surface of the substrate (e.g. 
a copper surface), and comparing a surface feature of the droplet against 
a predetermined standard. Generally, an image is formed of the droplet and 
the contact angle defined by the wall of the droplet and the surface of 
the metal substrate is measured and compared with known values of contact 
angles for the particular coating material and bare substrate surface. The 
contact angle is a convenient surface feature to examine to achieve a 
quantifiable determination. 
Because this method is especially advantageous for measuring OSP, 
especially imidazole, on copper substrates used in printed circuit boards, 
the method will be described in terms of imidazole coatings on copper. 
However, it should be understood that the present method can be used to 
detect other OSP's and should not be limited to copper substrates. Other 
azole derivatives useful as OSPs include benzotriazole, benzimidazole, 
2-methyl benzimidazole, 2-phenylimidazole, and alkyl imidazoles such as 
2-undecyl imidazole and 2-heptadecyl imidazole. Other metal substrates can 
be iron or aluminum. 
The first step of the method is to dispense a droplet of water on the 
copper surface to be tested. The droplet preferably has a volume of from 
about 0.1 to about 30 microliters, more preferably from about 0.5 to about 
10 microliters, and most preferably from about 1.5 to about 2.5 
microliters. Such a droplet can be dispensed by means of a microliter 
syringe. Typically a 100 microliter syringe is used, but other sizes of 
syringe may also be employed. The needle preferably has a 90.degree. bevel 
at the tip to allow good contact between the droplet and the surface to be 
tested, and the barrel of the needle is held perpendicular to the sample 
surface. The volume of the dispensed droplet may be easily controlled 
using a repeater dispenser, which allows the rotation of a screw to 
activate the syringe plunger. Approximately 1.5 microliters of fluid is 
dispensed from the syringe while the sample surface is not in proximity of 
the droplet. The droplet will hang from the syringe at this point. The 
surface is then slowly brought into contact with the bottom of the droplet 
surface by actuating a positioner on which the sample rests. An additional 
0.5 microliters of fluid is then dispensed, after which the sample is 
lowered. 
Preferably, deionized water is used, although ordinary tap water may also 
be used. Optionally, other liquids such as glycerol may be used instead of 
water. Use of liquids other than water, while still yielding a detectable 
difference between a bare clean copper and OSP-coated copper, will result 
in different absolute contact angles. 
Detection of imidazole on the copper surface is performed by measuring the 
contact angle of the droplet with the copper surface. This is preferably 
accomplished through visual inspection by forming an image of the droplet 
with any appropriate combination of optics and visually measuring the 
contact angles of the droplet as depicted in the image. The measurements 
are preferably made within 30 seconds of dispensing the test droplet. 
Referring to FIG. 1, a diagrammatic illustration of the imaged test droplet 
D is shown deposited on the surface of substrate S. Angles A and B are the 
contact angles as defined by the angle between the surface of the 
substrate and a line tangent to the surface of the droplet at the point 
where the droplet contacts the surface of the substrate. Under ideal 
conditions angles A and B should be equal. However, lack of droplet 
symmetry may be caused by, for example, deviation of the surface from a 
true horizontal position or from true flatness. Under such conditions the 
shape of the droplet may be skewed, thereby resulting in dissimilar 
angles. Accordingly, it is advisable to measure the contact angles on 
opposite sides. 
EXAMPLES 
In the examples set forth below the samples of imidazole coated copper and 
bare copper were prepared shortly before performing the test. The longer 
the time interval between sample preparation and subsequent testing, the 
more inaccuracy is introduced. Freshly prepared samples avoid 
complications caused by oxidation of the bare copper and modification of 
the coating by complexing or other processes. Accordingly, to achieve the 
most accurate and consistent results testing should be performed within 
about 30 minutes from sample preparation and more preferably within 5 to 
20 minutes. 
EXAMPLE 1 
A freshly etched copper surface was coated with imidazole in a manner 
similar to that described in U.S. Pat. No. 4,373,656. This test was 
performed within 5-10 minutes after imidazole coating and was carried out 
under ambient laboratory conditions 25.degree. C. and 50% relative 
humidity. A droplet of deionized water approximately two microliters in 
size was placed on the imidazole coated copper surface and an image of the 
droplet was obtained by using a video contact angle system designated as 
VCA 2000, which is available from Advanced Surface Technology, Inc. 
located at Nine Linnell Circle, Billerica, Mass. 01821-3902. Standard 
operating parameters were used. Left and right contact angles were 
observed to insure symmetry of the droplet. This testing procedure was 
performed a number of times sufficient to obtain a statistically relevant 
average measurement at 95% confidence levels. The mean contact angle is 
set forth below in Table I. 
EXAMPLE 2 
Bare uncoated samples of copper were prepared for comparison purposes. The 
uncoated copper surfaces were prepared by a standard etching treatment 
comprising dipping of the copper sample in the following solutions, in the 
order given, for one minute in each bath: 
1. Dipping in 5 wt % sodium hydroxide (NaOH) in deionized water. 
2. Dipping in deionized water. 
3. Dipping in a mild aqueous etch solution comprising 1.5 lbs of sodium 
persulfate per gallon of water held at about 90.degree. F. with mild 
agitation. 
4. Dipping in deionized water. 
5. Stabilization by dipping in a bath comprising 50% deionized water and 
50% stock solution of 68.6 grams of 85% phosphoric acid solution, 7.5 
grams ethylene glycol, and 23.9 grams distilled water. 
6. Dipping in deionizcd water. 
7. Drying in warm air. 
The uncoated copper surfaces were tested within 5-10 minutes of their 
preparation using deionized water with the technique and equipment and 
under the same laboratory conditions as set forth above with respect to 
Example 1. A sufficient number of samples were prepared and tested to 
obtain a statistically relevant average measurement at 95% confidence 
level. The mean contact angle is set forth below in Table I. 
TABLE I 
______________________________________ 
Contact Angle Measurements 
Mean Contact Angles 
______________________________________ 
Example 1 54.5 .+-. 5.4.degree. 
(imidazole coated 
copper) 
Example 2 25.8 .+-. 4.5.degree. 
(uncoated copper) 
______________________________________ 
As can be seen from the above Examples, the imidazole coated copper surface 
results in a substantially different contact angle than bare copper with 
the use of deionized water as the test droplet. As mentioned above, 
different test liquids and OSP's produce different characteristic contact 
angles. These characteristic angles can be determined beforehand by 
standardized measurements, then used to detect and distinguish OSPs on 
copper surfaces by a simple measuring procedure. 
Although the subject invention has been described with respect to preferred 
embodiments, it will be readily apparent to those having ordinary skill in 
the art to which it appertains that changes and modifications may be made 
thereto without departing from the spirit or scope of the subject 
invention as defined by the appended claims.