Electronic assembly technologies are constantly improved in response to new ideas about the quality of products and reduce their cost requirements. Perhaps one of the last operations that are still carried out manually in the process line remain termoprofilirovanie. This article describes the technology termoprofilirovaniya automatically in real time, and the benefits that it has to manual measurement profile.
LIMITATIONS OF MANUAL PROFILING decades measurement thermoprofile manually enterprise e-assembly was considered the best method to determine the optimum parameters for the reflow oven and wave soldering equipment, and make sure that the thermal process is consistent with the specified parameters.
Before proceeding to the description of the technology of automatic measurement of thermal profile and its advantages over the manual profiling, let’s analyze the advantages and disadvantages of the traditional method.
When profiling manually several thermocouples attached to certain parts of the components on the printed circuit board (PCB), which is sent to a furnace for brazing (see. Fig. 1), after which the obtained measurement recorded recorder to profile. This device receives the data and displays a temperature versus time and provides a sampled data, for example the maximum temperature, melting time, etc.
The advantage of this method is that the rigid connection of thermocouples to the measured areas. Thus, the determination of the time dependence of the temperature of the components is based on the measurement by direct contact.
However, this method has the following number of drawbacks and limitations.
— Mounting thermocouple affects the accuracy of the reading is. When using high-temperature solder or epoxy resin for fixing the welded end of the thermocouple does not measure the surface temperature of the printed circuit board or component, and the temperature of a small area of coverage junction. Using aluminum adhesive tape for fixing a thermocouple significantly increases measurement accuracy due to the fact that the tape only minimally affects them.
— To obtain data characterizing a specific product, you must use a single printed circuit board of the whole party. However, as a result of repeated measurements several times when the board is exposed to high temperature furnace, and its weight decreases due to burnout material and this sample can not be considered representative.
— When re-attaching a thermocouple associated with the rise of / separation from the surface of the thermocouple circuit board / component, these results are not representative. It is well known that these results whenever somewhat different from the previous ones. Also in this case aluminum strip avoids discrepancies.
— Perhaps the biggest drawback to manually measure the thermal profile is a selective method of «blind». In other words, the recording device may not notice changes in the thermal process.
— Profiling hand could hinder the production process. This method is time-consuming and volatile, requiring operator intervention.
AUTOMATIC PROFILING On the market there are a lot of automated systems, including Several systems of KIC — the industry leader in the field of automated tools and systems to monitor thermal processes. The operation of these systems is based on the use of sensors that are embedded in a furnace or other equipment for brazing (see. Fig. 2).
Like most machines, automatic profiling operates in accordance with the installed program. To program the system technician to perform a single manual profiling. As a result generates two separate sets of data streams: a profile of the PCB, as measured by thermocouples are attached to specific parts of PP; Profile intrachamber medium in the path of the printed circuit board, and the speed of movement and the position of the PP during program execution.
The process of heating and cooling an object depends on several thermodynamic variables. By measuring them, you can accurately calculate the thermal profile of the object of interest. In the case of movable furnace board her profile defined by the following variables:
— the temperature difference between the PCB and the ambient air;
— the duration of the impacts experienced by the board;
— weight and thermodynamic parameters of the PP.
Using the temperature sensors 30 installed along the route of the printed circuit board, an automatic system continuously measures environmental parameters that affect the PCB during soldering. The position of the PCB and the time during which it is exposed to the environment are measured in sensor and a belt speed (additionally set in the furnace). Finally, the automated system for studying the thermal parameters of the product PP, tracking the process of heating and cooling during program execution. Thus constructed model accurately calculates the profile of the future on the basis of continuously variable data received. End profile data on each PCB are continuously recorded. These measurements are carried out without human intervention and does not affect the production process.
ADVANTAGES OF AUTOMATIC PROFILING have automated profiling has significant advantages over manual.
The continuous nature of the use of this technique avoids performing thermal process blindly. Measured profile of each separate circuit board. Profile is defined as on the process window to ensure compliance with the requirements specification.
Data is saved in case of re-use, which ensures consistency of measurement process.
Automated profiling allows for efficient data entry into the card SPC. This card is used to inform the responsible specialists of negative trends or the release of thermal process control. As a result, it is possible to adjust the parameters of the manufacturing process or the oven before the appearance of defects.
It reduces the cost of production due to downtime, less waste, eliminating the need to finalize and manual labor.
Termoprofilirovaniya automated method may be used for troubleshooting when problems occur with the yield. Although most engineers believe that only 5-10% of all defects associated with the reflow and wave, they often have to eliminate mistakes, starting with the analysis of the profile. This occurs because the oven or machine for wave soldering, in fact, constitute a «black box» on the background of relatively good access to the data from all other plant and process line technology. For / removal of unplanned operator profile may take up to 30 minutes, sometimes longer, which leads to downtime of the equipment. If the resulting profile indicates that the problem is not related to the thermal process, while costly downtime is used to find the causes of problems on the line. At the same time the automatic profiling immediately informs the operator of the process according to the given parameters.
As this method is gaining popularity in the industry, revealed its additional benefits. One is to take advantage of the automated systems of optical and X-ray inspection. Machines for optical control allow us to consider the hidden solder joints under the components made, for example, using technologies ball pins (BGA) or PoP (package-on-package — body-on-body). Greater control and guarantee the quality of the soldering is achieved by sharing the automatic profiling, which enables the processing of BGA-components and solder joints according to the tolerances of these components and specifications for rations, and a method of optical control. However, the optical control is not to determine the internal microstructure of the solder joint.
X-ray machine control allows you to see the surface of the solder joints under the components, but also unable to determine their internal microstructure to assess the quality of soldering. Such defects as cold soldering and «head on the pillow,» it is difficult to detect. Knowing, however, how these have been carried out properly brazed joints, you can set more control over their quality. Since most factories checks only a small fraction of produced printed circuit boards, an automatic profiling can be used as an aid, identifying those items that were processed in accordance with the desired characteristics of the thermal process.
ACCURACY OF THE METHOD OF AUTOMATIC PROFILING
The question arises of how exactly is automatically measured profile PCB compared to profiling by physical contact between the boards and thermocouples? The accuracy of the automated method is not very high, but higher than the manual method, in view of the major disadvantages of the latter. Furthermore, the method makes it easy to automatically determine the accuracy of measurements. When the automated measurement of the profile is taken into account each board, leaving the soldering equipment. During manual removal profile thermocouples are attached to the board, and receives the two data streams to a single printed circuit board — data measured by automated method and profile data obtained manually. The operator can carry out this procedure on a regular basis in accordance with the established timetable for testing and determining the deviation.
For thousands of factories contract manufacturers around the world use automated method of profiling. Many of these enterprises are equipped with world-class systems, process control and production control, which contributes to the implementation of this method more accurately.
Often there is the question of the appropriateness of control of how the modern furnaces for reflow and wave soldering machines for. However, in some cases, the furnace can not be affected by variations in their control of the process, for example as a result of the unstable operation of the exhaust system of the enterprise. It should be understood that the automatic measurement of the profile only allows to automate the production, to obtain reproducible results, improve product quality and reduce manufacturing costs. But this much.