Method for inspecting the leads of electrical components on surface mount printed circuit boards

• US 5,023,916 A
• Filed: 08/28/1989
• Issued: 06/11/1991
• Est. Priority Date: 08/28/1989
• Status: Expired due to Fees

• Current Assignee: Hewlett-Packard Company (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Breu, Heinz
• Primary Examiner(s): Boudreau, Leo H.
• Assistant Examiner(s): Fox, David
• Application Number:
  US07/399,688
• Time in Patent Office: 652 Days
• Field of Search: 382/8, 382/54, 356/237, 358/106
• US Class Current: 382/150
• CPC Class Codes: B23K 31/12 relating to investigating the properties, e.g. the weldability, of materials
• CPC Class Codes: G01N 21/95684 Patterns showing highly reflecting parts, e.g. metallic elements
• CPC Class Codes: G06T 2207/30152 Solder
• CPC Class Codes: G06T 7/001 using an image reference approach
• CPC Class Codes: G06T 7/181 involving edge growing; involving edge linking
• CPC Class Codes: H05K 13/0817 Monitoring of soldering processes inspection of solder joints or of printed solder paste G01N21/95684

Abstract

A method of inspecting solder joints that connect surface-mount electronic components to printed circuit boards is provided. Preferably, the method includes the steps of obtaining an image of a plurality of the leads of the electronic component, for each obtained image, representing segments of the image that correspond to each of the leads by polygons, projecting pixel intensity values within each polygon each onto an axial dimension to provide a measured waveform, comparing the measured waveform for each lead with a corresponding model waveform, and, classifying each lead as being either satisfactory or unsatisfactory on the basis of said comparisons.

Claims

1. A method for inspecting a joint used to connect leads of a surface mount electronic component onto a pad of a printed circuit board, comprising the steps of:

taking an image of the leads;

chain encoding regions of the image which have intensity values above a predetermined intensity threshold;

computing minimum area bounding boxes for each of the chain encoded regions;

modeling an expected region for each lead and pad combination;

determining those chain encoded bounding boxes which intersect each expected region;

computing a lead bounding box for each lead on the basis of the chain encoded bounding boxes to form a segmented image of the leads;

testing the segmented image consisting of the lead bounding boxes for missing leads and lead bridges;

projecting each lead bounding box as a measured waveform onto an axial dimension of the lead'"'"'s bounding box;

comparing the measured waveform for each lead with a corresponding model waveform to enable inspection; and

,

classifying each lead as being either satisfactory or unsatisfactory on the basis of said comparison.

2. A method for inspecting joints such as the joints between a printed circuit board and a surface-mount electronic component placed onto the board, comprising the steps of:

obtaining image of a plurality of the leads of the electronic component;

for each obtained image, representing segments of the image that correspond to each of the leads by polygons;

projecting pixel intensity values within each polygon each onto an axial dimension to provide a measured waveform;

comparing the measured waveform for each lead with a corresponding model waveform by placing points of each measured waveform into correspondence with data points of the model waveform to return a cost value which indicates goodness of fit;

classifying each lead as being either satisfactory or unsatisfactory by determining the length of the measured projection of a lead bonding box; and

,

using the cost value and the length of the measured projection to discriminate good leads from bad leads.

3. A method for inspecting joints such as the joints between a printed circuit board and a surface-mount electronic component placed onto the board, comprising the steps of:

obtaining an image of a plurality of the leads of the electronic component;

for each obtained image, representing segments of the image that correspond to each of the leads by polygons;

projecting pixel intensity values within each polygon each onto an axial dimension to provide a measured waveform;

comparing the measured waveform for each lead with a corresponding model waveform; and

,

classifying each lead as being either satisfactory or unsatisfactory by determining a matching cost threshold from a statistically significant sample of leads.

4. The method of claim 3, wherein said step of determining a matching cost threshold further includes:

determining a pair of thresholds for discriminating questionable leads which cannot be immediately classified as either good or bad.

5. A method for inspecting joints such as the joints between a printed circuit board and a surface-mount electronic component placed onto the board, comprising the steps of:

obtaining an image of a plurality of the leads of the electronic component;

for each obtained image, representing segments of the image that correspond to each of the leads by polygons;

projecting pixel intensity values within each polygon each onto an axial dimension to provide a measured waveform;

comparing the measured waveform for each lead with a corresponding model waveform by placing points of each measured waveform into correspondence with data points of the model waveform to return a cost value which indicates goodness of fit;

classifying each lead as being either satisfactory or unsatisfactory on the basis of said comparisons; and

,

testing the segmented images for missing leads and solder bridges by clustering bounding boxes into expected locations representing leads as follows;

a) comparing a minimum area bounding box for each image of a lead with all of the expected locations;

b) for those minimum area bounding boxes that intersect or overlap an expected location, designating those bounding boxes as being part of the lead associated with that expected location;

c) if a minimum area bounding box intersects more than one expected location, adding the minimum area bounding box to the set representing each of the intersected expected locations;

d) rejecting the remaining minimum area bounding boxes and;

e) for each accepted bounding box and each expected location, calculating a lead bounding box that represents the sum of the minimum area bounding boxes included in each of the sets.