7 Fine pitch pin assembly process
The fine pitch pin assembly process is increasingly important in the current SMT technology. We combine the fine pitch pin printing process to talk about the overall assembly process.
7.1 Fine Pitch Printing Process
The printing press has a printing accuracy of ±0.02mm, and contact printing can be used to prevent solder paste from flowing onto the backside of the board or PCB outside the pad.
The PCB holders, stencils and print head squeegees must be kept level.
With a metal scraper, the blade head pressure is (1.6 to 2) Mpa.
The printing speed is slower than the normal printing speed, the speed of the blade lifted from the template is appropriate, and the speed of releasing the template from the printed board is reduced.
Scrubbing the template when the graphic is not clear, but it should not be cleaned too often, generally according to the specific circumstances.
Each addition of solder paste should be moderate.
It is necessary to adjust the printing press before printing solder paste, adjust the machine parameters and printing parameters so as to achieve the most ideal state.
7.2 Fine Pitch Mounter Features
Mounting accuracy ≤±0.03mm, with basic optical positioning and component optical correction double correction function
Because fine-pitch QFP leads are fine, the spacing is small, and they are easily deformed and brittle, the coplanarity of individual pins tends to occur during transportation and transfer. Therefore, pin coplanar inspection is required before mounting.
When mounting, according to different devices, the stroke of the working axis of the suction nozzle can be changed slightly to realize intelligent control of the working shaft so that the device can be fitted properly, and the contact with the substrate can be achieved to achieve a soft landing of the patch.
7.3 fine pitch device welding technology
Because fine pitch soldering devices are small, the amount of solder paste is small, the solder content is low, volatile, and its activity is lower than ordinary solder paste, resulting in easy oxidation of the solder powder, so the soldering temperature is not too high and the soldering time is not too long.
The temperature distribution is uniform and controlled accurately. When welding, the temperature curve should be carefully tested to establish an ideal temperature curve. When measuring the temperature, it is necessary to take into consideration the characteristics of fine pitch, large area, and large heat capacity of the device. Generally, the probe is placed around the device for testing.
Inert atmosphere full hot air convection heating method, especially suitable for fine pitch device welding.
8 Major Printing Failures and Countermeasures
8.1 Solder Paste Bridges
Reason: In the printing, the solder paste leaks from the gap between the substrate and the screen, and adheres to the back of the screen.
Countermeasures:
a. In the design, the opening of the printing screen is made smaller than the width of the substrate pad.
b. In order to eliminate the gap, the contact amount between the substrate and the printing screen is set larger on the printing apparatus.
c. Increase the scraper angle and reduce the pressure of the solder paste.
d. The diameter of the solder particle including the solder paste is selected to be an appropriate value (general purpose is not to use a high-density solder paste with a small particle diameter).
8.2 printing shape askew
Reason: When the scraper enters the opening, the solder paste in the opening is scraped and often occurs in a wider printing section.
Countermeasures:
a. Reduce the printing pressure applied to the blade section.
b. Replace the polyurethane rubber blade with a metal blade. In addition, if it is polyurethane rubber, increase its hardness, making it hard.
8.3 Residual Solder Paste on Printing Screens
Reason: After the scraper moves on the printing screen surface, the solder paste still remains on the screen surface. This is because the screen is not in close contact with the blade, there is a gap, resulting in leakage of solder. If this happens, the shape will be blurred after printing.
Countermeasures:
a. Adjust the substrate fixing mechanism of the printing apparatus to correct the bow warpage of the substrate.
b. Increase the printing pressure exerted on the blade
c. Replace metal scraper with polyurethane rubber scraper. In addition, if it is polyurethane rubber, reduce its hardness, making it soft.
8.4 Solder paste remains in the opening of the printing screen
Reason: From the opening portion of the printing screen, the solder paste is poorly detached, and the solder paste remains in the opening portion.
Countermeasures:
a. The printed screen opening is machined into the previously described figure shape, or an additional screen plate is used.
b. Decrease the speed of the small substrate.
c. Increase the thixotropy of the solder paste.
Author/Sheng Jianwei Yuan Heping
(1 State Key Laboratory of Modern Welding Production Technology, Harbin Institute of Technology
2 Day East Electronic Technology (Shenzhen) Co., Ltd.
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