GB2185813A

GB2185813A - Assessing soldered joints

Info

Publication number: GB2185813A
Application number: GB08700512A
Authority: GB
Inventors: Hans-Jurgen Albrecht; Hendrik Bohme; Matthias Nordhausen; Uwe Dolling; Jurgen Tanner; Wolfgang Scheel
Original assignee: Elektro Apparate Werke VEB
Current assignee: Elektro Apparate Werke VEB
Priority date: 1986-01-27
Filing date: 1987-01-09
Publication date: 1987-07-29
Anticipated expiration: 2007-01-09
Also published as: GB2185813B; DE3636607A1; GB8700512D0; FR2593605A1; DD258658A1

Abstract

To assess a soldered joint, on a printed circuit board for example, the joint 1 is exposed alternately to the light from two differently inclined sources of illumination 2,3, and its optical reflection profile detected by a CCD line or matrix sensor 4 via optics 7. A subsequent analogue to digital conversion 5 of the sensor output signal enables a mathematically modelled evaluation or intensity evaluation of the profile of the joint to be effected by a high speed digital computer 6. The evaluation is effected by rapid Fourier conversion, exponential approximation, numerical approximation, Young-Laplace regression or analysis of interfacial tensions of fusible solders on solid bodies. <IMAGE>

Description

SPECIFICATION Testing solder joints in electronic assemblies This invention relates to the automatic, optoelectronic non-destructive and contactless testing for detecting and assessing curved surfaces and is particularly for the joined surfaces of visible solder joints between electronic components carried on a wide variety of flat assemblies. The invention is suitable for diagnosing faults in visible joint connections, such as soldered and adhesive connections, on conductor plates or other substrate carriers fitted with components and joined together.

Systems are known in which the quality control and fault diagnosis of solder joints between component, solder and soldered connection are effected visually, using the naked eye, and with the aid of optical means enabling an enlargement to be produced of one portion or the whole of the object being tested.

Examples of such means are magnifying glasses, microscopes, ground-glass plates, video cameras with colour monitor (see, No.

25/26, 17.12.85--lnteraction of Components) and projectors with high resolving power (S.

Sommer: Optical, computer-assisted solderjoint testing on conductor plates, Feinwerktechnik & Messtechnik, 92 (1984) 7, pp. 373, 374). The method of operation of the testing stations described in the aforementioned sources is partly defined and optimised by the use of micro-computers. A characteristic common to these testing methods or systems is the fact that the actual evaluation and assessment of the quality of the solder joint is carried out by an inspector so that the visual and/or partly projected image of the soldered joint is governed by subjective factors such as sharpness of vision, eyesight, condition and experience and also by the angle from which the solder joint is viewed.

An automatic testing apparatus has also become known (Lea, C; Howie, F. H., Seah, M.P: Automated inspection of PCB solder joints--an assessment of the capability of the Vanzetti LT 6000 infra-red laser inspection instrument, Brazing & Soldering, Wela publications, Ltd., England, No. 8, Spring 1985), which automatically detects the solder joint of the specimen, supplies thermal energy to the soldered connection with a long-wave laser beam and analyses the time characteristics of the heating and cooling process (T-f(t1) by means of a highly sensitive deep-cooled (77K) infra-red sensor. The comparison of these temperature characteristic ("thermo-profiles") with those obtained previously from an adequate and satisfactory sample assessed visually provides information on the location of defective or inadequate joints.This information is conveyed to a re-soldering station for the repairs required for the purpose.

This method suffers from the disadvantage that for a clear disgnosis each joint has to be selectively scanned, the testing process being rendered uneconomical by the high cost of the apparatus involved. Furthermore, standard calibrations of satisfactory joints are required for every conceivable configuration of connections and thus for the entire assembly to be tested, in order to record the comparison between the reference thermoprofiles and the required thermoprofiles.

An opto-electronic process is also known for distinguishing different structures on surfaces without making contact therewith (DE A 3212190). This is a two-dimensional surface analysis of any desired structures on surfaces which, with regard to the assessment of joints, only provides information on the twodimensions expansion but none concerning the actual quality of joint.

This invention seeks to obtain an automatic optoelectronic representation and characterization of the reflecting surface geometry of visible joints of micro- and/or optico-electronic components, particularly SMD (surface mounted devices) constructions on any desired flat assemblies, such as PCB's (printedcircuit boards) or ceramic substrates, without recourse to subjective factors and without pattern comparisons, in order to determine the quality of the joint from the evaluation of its surface topology.

According to this invention the joints (soldered and/or adhesive connections) are alternately illuminated with orthogonally and angularly defined associated light sources of which the image occurs on sensors with local resolution as a reflected image and that the measuring signals of the photosensitive sensors, of which the intensity characteristic is a mathematical indication of the three-dimensional topology of the joint, are allocated via a computer in accordance with the location and/or joint and conveyed to an evaluation unit and combined together via a mathematical function (exponential approximation, Young-Laplace regression etc.) to form a three dimensional image which is compared within the computer with the image or mathematically prepared information of a standard joint, a control magnitude for an automatic refinishing operation being derived therefrom.

This invention thus provides a process for the analysis and diagnosis of the surface topology of joints, particularly reflective visible solder joints between electronic components which can be mounted or plugged in and flat assemblies on substrate carriers (PCB, ceramics, etc.) without recourse to subjective factors and without comparison between patterns in order to assess the quality of the solder joint, for example, from the evaluation of its topology, and also, if necessary, to enable refinishing stations to be computer-actuated in a coordinate compatible manner.

The output signal of the sensor is processed by a circuit technique in such a way that it can be evaluated by a micro-computer.

The method for the evaluation of the optical reflection profile of the joint is based on the principle that the optical path of the rays of the light which is incident at a known angle and with known intensity distribution on the surface of the solder joint is changed in accordance with the perpendicular meniscus function in the boundary surface between the component, the solder and the conductor connection and thus with a definite " solderability criterion" and forms the basis for the diagnosis.

Computer evaluation via mathematical approximations and/or intensity distributions of the reflection plane provides clear analyses of the joints, and when these analyses are evaluated in accordance with the coordinates then computer-assisted stations for example, can be connected up for the purpose of refinishing SMD assemblies on PCB.

The invention is further described by way of an example and with reference to the drawings.

EXAMPLE The invention as described in the foregoing was adapted, from the point of view of apparatus, to an incident-light microscope, for objective assessment and automation of opticoelectronic soider-joint diagnosis and analysis of SMD structural shapes and/or plugable assemblies on universal carrier substrates, such as PCB, ceramic and glass substrates.

A solder joint 1 was exposed to the light from two illumination sources 2, 3 and their optical reflection profile detected by a CCD sensor 4 (CCD line and/or CCD matrix) via a microscope 7. A subsequent analogue to digital conversion 5 of the sensor output signal enables a mathematically modelled evaluation or intensity evaluation of the reflection profile to be effected by a high speed digital computer 6. The evaluation is effected by rapid Fourier conversion, exponential approximation, numerical approximation, Young-Laplace regression or analysis of interfacial tensions of fusible solders on solid bodies. When the course taken by the perpendicular meniscus on the solder joint is reflected in the reflection profile, of maximum reflection values of characteristic shapes at characteristic points, an objective, unambiguous and reproducible conclusion can be drawn as to the quality of the solder joint and thus the reliability of the process.

The foregoing testing station operates fully automatically, provides objective test results, is of advantage for miniaturized assemblies and enables the computer to be coupled to the x-y coordinate control system of any necessary refinishing station in SMD technique.

Claims

1. Method for the automatic testing and assessment of joints in electronic assemblies using opto-electronic aids in which method the joints are alternatively illuminated with orthogonally and angularly defined associated light sources from which an image is produced on sensors with local resolution as a reflected image, the signals from the photo-sensitive sensors, of which the intensity characteristic is a mathematical indication of the three-dimensional topology of the joint, being allocated by a computer in accordance with the location and/or joint and conveyed to an evaluation unit and combined together using a mathematical function of approximation to form a three-dimensional image which is compared within the computer with the image or mathematically prepared information of a standard joint, a control signal for an automatic refinishing operation being derived therefrom.

2. Method for the automatic testing and assessment of joints in electronic assemblies as described herein and exemplified.

3. Apparatus for the automatic testing and assessment of joints in electronic assemblies substantially as described herein and exemplified with reference to the drawings.