The Use of 'INTROMET' Device for the Quality Inspection of the Metallizing of Through-holes of PWBs

摘要

Determination of the thickness of copper coatings in the through-holes is one of necessary steps in the manufacture of PWBs, since considerable part of rejects and later failures are related with inadequate quality of copper coatings inside the through-holes. Well-known method of microsections is suitable for only selective control due to its destructive nature (Fig. 1). Apart from that, it is expensive and laborious. There are also Caviderm CD-8 and CD-10 devices (Veeco Instrument Corp., UPA Technology Division, USA) based on the measurement of electrical resistance (Fig.2). Major disadvantages of these instruments: effects of contact phenomena on the results of the measurements and the impossibility of the measurements prior to etching and the need to get access to both sides of a hole. Thickness-measuring devices "INTROMET" have following advantages in comparison with other types of instruments:- Easy adjustment - it is necessary to introduce a sensor into the hole and to read the numerical values of the indicator expressed directly in micrometers;- The inspection of the coatings is possible at every stage of the process - from the initial stage of copper plating (directly at a plating tank) and also before and after the deposition of metal-resist coatings. This allows to reduce the copper-deposition time and energy consumption under the conditions of guarantied quality of copper deposits;- It is possible to make not only copper-plate thickness measurements, but also to detect failures such as cracks in the copper layer;- There is no need to destroy the board by cutting the specimen for the measurements, which is unavoidable, if electrical resistance is to be measured;- The access to one side of the boards is sufficient. The heart of the device is an eddy current sensor of a transformer type (US Pat. 5, 600, 240) both coils of which are placed inside a steel pipe and are located along with its axis (Fig.3). The principle of its functioning is illustrated in Figs. 4 and S. Technical characteristics of "INTROMET" tester are given in the Table. The instruments are shown in Fig. 6. Most advanced modification ITM-MP has been developed by INTROMETPLUS according to the order of Veeco Instrument Corp., UPA Technology Division and was first exposed at the international exhibition "Control 94" in Chicago in 1994 (Fig. 7). This device is capable to eliminate the interfering effects of changing ambient temperature on the results of the measurements. The effect of the hole diameter is also taken into account automatically, and the values of the board thickness and the conductivity of deposited copper can be introduced into memory of the device. The data obtained may be monitored to the external computer/printer through the interface PS-232. It is also possible to calibrate ITM-MP tester on the basis of the specimens of a customer. It is necessary for this purpose to use a set of PWBs with various thickness of copper layer (4 to 6 specimens representing the whole range of thickness). Some practical experience for the industrial use of the device is described.