Bonding Verification Process Incorporating Test Patterns

Component-to-substrate bonding is verified by a variety of visual inspection techniques that inspect the component-to-substrate connection. The disclosed invention test the component-to-substrate connection by visually inspecting test patterns located on the substrate.

It is well known in the art that substrates experience temperature variations during an automated soldering process due to varying component masses. Specific oven profiles control the temperature variations in high mass areas by increasing the heating time and in low mass areas by decreasing the heating time. If the oven profile drifts, the component mass changes, or the emissivity of the substrate changes, the automated soldering process may yield unacceptable results. To avoid having to rework or scrap product, the soldering process is periodically verified by verification methods such as visual inspections or thermal profile runs.

The visual inspection method verifies the soldering process by periodically sampling the substrates and visually inspecting the solder connections. The visual inspection step is usually performed after soldering process is complete and checks for solder bridges, solder balls and misheated solder connections. This method has limited usefulness because it samples only a small percentage of substrates, requires an extra production step, and does not provide individual substrate test data.

The thermocouple profile run method periodically verifies the soldering process by measuring the oven profile temperatures. This method runs a test substrate instrumented with thermocouples through the automated soldering process. The test substrate records the oven profile temperatures and determines if the oven profile has drifted. This method has limited usefulness because it interrupts the soldering process, requires an extra production step, and does not provide individual substrate test data.

Another verification method employs automated x-ray equipment to inspect the soldering process by inspecting the solder connections. This method inspects the resulting solder connections and detects soldering defects such as porosity, solder bridges and solder balls. This method inspects each substrate without additional production steps; however, it only measures the amount and location of solder, not whether the solder has reflowed properly.

A need therefore exists for a solder verification method that will be an integral part of the soldering process, does not require additional production steps, inspects and produces test results for each substrate, and verifies proper solder reflow.

This need is substantially met by the bonding verification process incorporating test patterns disclosed herein. The disclosed process comprises the steps of: placing test patterns on a substrate; applying a bonding agent to the test patterns; applying the bonding agent to components and the substrate; altering the state of the bonding agent; and inspecting the altered state of the bonding agent applied to the test patterns. The disclosed verification process can be incorporated into an automated bonding process without addtional production steps and provides test data for each substrate.

In one embodiment, the substrate is a printed circuit board (PCB), the test patterns are PCB solder pads, and the bonding agent is solder. By positioning the test patterns proximate to component-to-PCB solder connections, the test patterns will accurately represent the solder connections. After heating, automated visual inspection equipment inspects the test patterns to verify that the solder has properly reflowed. The data is recorded for each substrate and provides a useful aid for determining early life failures.

In another embodiment, the bonding agent is solder and the test patterns are placeable components comprised of solders with different melting temperatures. The placeable components are positioned proximate to component-to-substrate solder connections, such that they will accurately represent the solder connections. After heating, the automated visual inspection equipment inspects the placeable components to determine which solders have melted. By knowning the melting temperatures of the various solders used in the placeable component, the solder connection temperature can be determined.

FIG. 1A illustrates a top and cross-sectional view of the pre-heating condition of a bonding test pattern with a tab, and FIG. 1B illustrates a top and a cross-sectional view of the post-heating condition of the bonding test pattern with a tab.

FIG. 2A illustrates a top and cross-sectional view of the pre-heating condition of a bonding test pattern with the bonding agent extending beyond the bonding test pattern area, and FIG. 2B illustrates a top and cross-sectional view of the post-heating condition of the bonding test pattern with the bonding agent extending beyond the bonding test pattern area.

FIG. 3A illustrates a top and cross-sectional view of the pre-heating condition of a bonding test pattern with a bonding agent area larger than the bonding test pattern area, and FIG. 3B illustrates a top and cross-sectional view of the post-heating condition of the bonding test pattern with the bonding agent area larger than the bonding test pattern area.

FIG. 4A illustrates a top view and cross-sectional view of the pre-heating condition of a bonding test pattern with a bonding agent applied to a nonbonding surface, and FIG. 4B illustrates a top and cross sectional view of the post-heating condition of the bonding test pattern with the bonding agent applied to the nonbonding surface.

FIG. 5A illustrates the pre-heating condition of a placeable component comprised of various bonding agents, and FIG. 5B illustrates the post-heating condition of the placeable component.

FIG. 6 illustrates an automatic solder reflow system that comprises a reflow oven, automatic x-ray inspection equipment, and recording equipment .

The bonding verification process incorporating test patterns is most appicable for testing solder connections created by an automated soldering process. For the automated soldering process, a printed circuit board (PCB) comprises the substrate, PCB test patterns comprises the bonding test patterns, solder or solder paste comprises the bonding agent which is altered by heating and automated x-ray equipment performs the inspection. No additional production steps are required because the PCB test patterns are formed during PCB fabrication and solder is applied to the PCB test patterns during the automated soldering process.

FIGS. 1-4 illustrate a few possible PCB test patterns and solder combinations. The PCB test patterns are positioned proximate to specific solder connections for accurate representation. After heating, the automated x-ray equipment inspects each PCB assembly and produces individual test data.

In FIG. 1, the PCB test pattern (100) consists of a circular PCB pad (101) with a solder tab (102) deposited on a substrate (104). In the pre-heating condition, a solder pattern (103) is positioned in the center of the circular PCB pad (101) but not on the solder tab (102). In the post-heating condition, the solder (103) has reflowed throughout the circular PCB pad (101) and onto the solder tab (102). Automated x-ray equipment inspects the solder tab (102) and can easily determine if the solder (103) has reflowed properly.

FIG. 2 illustrates, in the pre-heating condition, a rectangular PCB test pattern (200), deposited on a substrate (202), with a rectangular solder pattern (201) positioned perpendicular to the PCB test pattern (200). After heating, as shown in the post-heating condition, the solder (201) has reflowed onto the entire PCB test pattern (200). The automated x-ray equipment inspects the PCB test pattern (200) to verify that the solder pattern (201) has reflowed properly.

FIG. 3 illustrates a PCB test pattern (300) which verifies the solder reflow process and the solder's susceptibility to solder balling. In the pre-heating condition, a solder pattern (301) is positioned on the PCB test pattern (300) that is considerably larger than the PCB test pattern (300). After proper heating, the solder (301) beads up on the PCB test pattern (300). If the solder (301) is susceptible to solder balling, solder balls (302) will be formed on the substrate (303). The automated x-ray equipment inspects the PCB test pattern (300) and can easily detect both proper solder reflow and solder balling from this configuration.

FIG. 4 illustrates a solder verifying configuration which verifies the completeness of the solder reflow process. The configuration consists of a PCB test pad (400) and a solder pattern (403) positioned on the PCB test pad (400). The configuration further consists of a nonsolderable circular pad (401) encircled by a solderable ring (402). After heating, the automated x-ray equipment inspects the completeness of the solder reflow process by detecting the amount of solder (403) which has reflowed onto the solderable circular ring (402) and the PCB test pattern (400).

FIG. 5 illustrates a placeable solder verifying component (500) which verifies if the PCB assembly was overheated or underheated. The placeable solder verifying component (500) consists a plurality of solders (501) having different melting temperatures to form a temperature gradient. The placeable solder verifying component (500) can be automatically inserted, or placed, on the PCB and is inspected by the automated x-ray inspection equipment. The x-ray equipment tests which solders have melted such that the temperature of the applied heating is determined.

FIG. 6 illustrates a solder reflow system that comprises a reflow oven (600), automatic x-ray inspection equipment (601), and recording equipment (603). PCB assemblies (602) enter the reflow oven (600) wherein the solder (not shown) is melted to create solder bonds. The automatic x-ray inspection equipment (602) inspects the solder bonds and generates test data for each PCB assembly processed. The recording equipment (603) receives and stores the test data such that each PCB assembly (602) has a permanent record of the solder reflow process.