Metal distribution and plating thickness on the PCBs depend upon the process controls of an electrolytic bath. This is done through three methods:
a) Analysis of solution or wet chemical analysis;
b) Physical test on solution; and c) Testing of electrodeposits.
8.7.1 Analysis of Solution (Wet Chemical Analysis)
The wet chemical analysis is carried out to determine the following in the electrolyte:
a concentration of metal ions;
a concentration of acid;
a concentration of base;
a concentration of chlorides; and a metallic impurities.
For carrying out traditional wet chemical methods for metals and non-metal plating solution constituents, it is advisable to study the suppliers’ literature. These methods mostly make use of pH meters, specific ion electrodes, colorimeters, spectrophotometers, etc. The measurement of organic additives in tin, gold and nickel solutions is done by using various techniques which include liquid chromatography, uv/vis (ultraviolet/visible) spectrophotometer, ion-chromatography, polarography and voltametry. The details of these methods can be found in standard texts on instrumental methods in chemical analysis. The details of these methods are, however, outside the scope of this book.
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8.7.2 Physical Tests for Solutions
The following parameters are physically tested in electrolytes:
a Density;
a pH value;
a Surface tension; and a Hull cell test.
Density: The density of an electrolyte depends upon the salt content in the solution. It can be measured by using an instrument called hydrometer. The density measurement gives an accurate value of the metal content and the solution must be maintained within the recommended value.
Density is normally expressed in Twaddell or Baume units. These are derived from the specific gravity. If the density scale is given in Baume gravity, the following formula can be used to calculate the specific gravity:
d = 145 145-∞Be¢ where
d = specific gravity (kg/dm3)
°Be¢ = degree Baume’ gravity
Table 8.2 gives representative values of specific gravity, Baume and Twaddel.
Table 8.2 Conversion Table of Specific Gravity, Baume and Twaddel
Specific Gravity Baume Twaddel
1.000 0 0
1.100 13.0 20
1.200 24.0 40.0
1.300 33.3 60.0
1.400 41.2 80.0
1.500 48.1 100.0
pH Value: The pH value determines the acidity or the alkanity of an electrolyte. For measuring pH in electroplating solutions, normally an electrometric method using pH meter is used. For high accuracy measurements, a digital pH meter is preferred.
Surface Tension: In order to eliminate gas pitting and to improve deposition, surface active agents (surfactants) are added to the electroplating solution, for example, in baths for acid copper electroplating, non-foaming surfactants are added to minimize the foaming during air agitation. It is
necessary to measure the surface tension to the solution for copper maintenance of solutions in process control. Usually, torsion balance is used to measure the surface tension of the solution. This instrument can be used to measure the force required to detach a glass plate of standard area from the surface of the solution under test. The force is indicated by a calibrated dial.
Hull Cell Test: A simple technique to infer when the additive level needs attention and to determine just how much material needs to be added uses a miniature plating cell commonly known as the Hull Cell. The arrangement for Hull Cell test is shown in Figure 8.12. The cell is used to test plate a series of sample boards to determine when the bath needs adjustment and to determine how much of it to add.
Anode
Cathode
Air pump
Hull cell
On ADJ
V A
C V
+ – Regulated power supply 12V/3A
2.5"
17/8"
Cathode Anode
41/16"
267 ml solution level 2.5"
5"
Fig. 8.12 Hull Cell — testing arrangement (NTTF Notes)
The Hull Cell is intended to act as a quick check on the health of the tin/lead plating bath. Using the cell in conjunction with the chemical analyses, it is possible to qualitatively and quantitatively analyse all the major constituents of the bath.
When filled to the line marked on the side of the cell, the volume of the test sample is 267 ml. If we denote:
a V = Volume of main plating tank (litres)
a H = Amount of addition agent added to Hull Cell to produce acceptable test plate (millilitres) a C = Amount of addition agent needed by main tank (millilitres)
The multiplication factor that relates that you add to the Cell and what you will need to add to your plating bath is given by:
C = (V/ 0.267) ¥H
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When run at specified operating conditions in terms of circuit, time and temperature, the Hull test gives indications of pH control, contaminants and the overall bath conditions. It is a valuable aid in solution and process adjustment. It is particularly useful to determine the catalyst ratio (additive ratio) and impurity level in electroplating solution. Therefore, Hull Cell has been the most popular and is the most important tool in the control, and testing of electroplating and related solutions. The apparatus required is simple and inexpensive and the test takes only a few minutes to determine the quality of the electrolyte. However, the main disadvantage of Hull Cell testing is that defects in copper plating such as dull plating, roughness or pitting, etc. are not shown by this test.
8.7.3 Testing of Electrodeposits
If the quality of plating on circuit boards is not satisfactory, it can give rise to problems like corrosion, current-carrying constraints, adhesion, contact resistance and wear resistance. The quality of plating is usually determined by testing the following parameters:
a Metal thickness test [surface and through-holes];
a Porosity test;
a Adhesion test; and a Solderability test.
8.7.3.1 Metal Thickness Test
The plating thickness on the circuit pattern and in the through-hole is measured by two methods:
a) Non-destructive method — Beta back scattering method; and b) Destructive methods — micro-section or cross-section method.
Non-destructive Method: The most commonly used non-destructive method for testing PCB finishes is the b-ray backscatter method. This method is particularly useful for measuring the plating thickness on the pattern and plated through-hole during or after the plating and before the etching operation. The method makes use of a radio isotope emitting b-rays, which is mounted on a suitable probe. The probe comprises both the beta-ray source and a receiver. The probe is placed on the surface whose thickness is to be measured. A part of the b-rays which impinge on the surface get reflected. The amount of reflected rays decrease in number as the thickness of the coating increases. With suitable calibration with a standard, a b-ray electronic counter directly gives the thickness of the coating in microns. The instrument gives the average thickness. It is mainly used to measure gold, tin, tin-lead on copper and copper on epoxy and photo-resist on copper. Several b-emitting particles and interchangeable probe operations are necessary to cover a wide range of thicknesses. The technique is a quick, accurate and non-destructive method of measuring thickness. Therefore, it is very popular in the field of quality control of PCB manufacture, as the test can be conducted even by an unskilled operator.
Destructive Methods: The most popular destructive technique which gives a direct measurement of the thickness of the deposit is micro-sectioning, which involves the preparation of a metallographic specimen and examining it under the microscope.
The plated PCB is cut vertically or horizontally and seen under a microscope to observe the visible layer structure and is photographed. The variation of plating thickness on the pattern and the hole walls can thus be determined. Normally, the boards are cross-cut horizontally by using a diamond circular saw and polished with diamond paste or emery paper with rotating wheel polisher. The dried samples are put under a microscope at 30 to 1000 times magnification, examined and photographed at three locations on each plated through-hole wall. The results are generally reported as an average.
The micro-sectioning technique, when applied to thin coatings, may introduce a large error. For example, for a 1-micron deposit, the error may be as high as 50 per cent. However, for thick coatings, the error is considerably less. The error is 2 per cent for a 5-micron deposit.
This technique has an advantage in that it can be applied to any geometry, including hole walls.
Additional information on the coatings such as number and nature, uniformity, presence of voids and undercut of conductors, etc., can also be easily obtained. However, the method is time-consuming and requires a skilled operator.
8.7.3.2 Porosity Test
The porosity test is used to detect discontinuities such as pores and cracks in deposits in the plated surface. The test is important especially on contact tabs, as corrosion of the base metal through the pores may have a detrimental effect on any electrical contact. Therefore, the pores and cracks are mainly tested on precious metal deposits.
Porosity on the deposits can be tested by various techniques. However, the electrographic test and some gaseous reagent porosity tests are more common. The gas test entails exposing the plated PCB to a twenty-four hours exposure to sulphur dioxide. The specimen is kept in a closed vessel of 10 litre capacity to which 0.5 cm3 of water and 100 cm3 gaseous sulphur dioxide are added. After twenty-four hours., the vessel is opened, 100 cm3 of hydrogen sulphide are injected into it and the vessel is re-sealed. Porosity is indicated by corrosion of the base metal which may be observed either with the naked eye or with a microscope. The gaseous porosity test is mostly used for coating of gold, platinum metals and tin-nickel alloys on substrates of copper and its alloys.
8.7.3.3 Electrographic Test
This test entails using cadmium sulphide paper by interposing it wet between the circuit copper as an anode and an aluminium plate as cathode under pressure, and passing a certain current through the sandwich. The current flows from copper through pores in the plated metal through cadmium sulphide to the aluminium. The black spots formed on the cadmium sulphide paper indicate the presence of pores in plating, whereas pore-free plating does not show up such black spots. Nickel- plated PCBs are mostly tested with this method. Micro-cracks on the plated surface can also be found out by these methods.
8.7.3.4 Adhesion test
Adhesion represents bonding between the metals. When the adhesion of coating is poor, it could result in cracking or peeling which determine the quality of adhesion.
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In order to carry out this test, a piece of adhesive tape is placed on the coated surface and pressed uniformly (without air bubble). The tape is then pulled from the edge with force. The metal surface is examined after the pull.
8.7.3.5 Solderability Test
Solderability is a measure of the ability of a surface to be completely wetted by molten solder. It is usually tested for finished PCBs. The test is performed by subjecting the fluxed specimen of usually one inch square to a contact with dross or oxide free solder at the recommended temperature (240 °C/s). The cleaned boards are observed through a 10-times magnifier for the following:
a Wetting: formation of uniform solder coating;
a Dewetting: irregular solder cover; and
a Non-wetting: irregular solder covered with exposed base metal.
The specimen must exhibit 95 per cent complete wetting.