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Why is it necessary to make SMT fixtures for PCB assembly?
When using SMT dedicated fixtures, reinforce the support on the fixture, avoiding the location of SMT components. Place the PCB board on the fixture so that the overall support strength of the PCB board (pcb assembly) can be uniform and reliable (the fixture can be used for printing, placement, and IR-Reflow).
Why use SMT fixtures for PCB assembly?
Why does the PCB (PCB assembly) need to be very flat during the welding production process? Why do PCBs (PCB assembly) with a thickness of less than 1.0 mm require SMT fixture support? Let me take you to find out!
With the development of the electronics industry, the development of PCBs is moving towards thinner substrates, multi-layer circuits, refined circuit geometry, miniaturization and precision of supporting components, increased component density, and high reliability. Through-hole components are converting to SMD, promoting the widespread application of SMT (Surface Mount Technology) in electronic products. Key equipment in SMT production—the placement machine—has also been improved and developed. During the process, there are often phenomena of poor soldering of thin boards below 1.0 mm, especially with the emergence and intensive application of CSP devices, the flatness requirements for PCBs are getting higher and higher.
The welding production of each product must go through multiple verifications. Sometimes, even if we have done a very good job in all aspects and considered it very carefully, there is still a possibility of encountering some problems in actual production, leading to welding failure. Even with a comprehensive process flow, exceptions can occur. Whether it's management errors, process loopholes, personnel operation errors, or equipment malfunctions, please see the following case analysis:
A customer's PCBA project with self-designed boards. The customer's product layout is compact, with multiple BGA chips and 0201-size resistors on both sides. The finished PCB thickness is 0.80 mm, the PCB is puzzle-shaped, and has process edges.
According to our conventional understanding, the PCB (PCB assembly) has process advantages and a well-designed layout, and the wiring can be smoothly completed during welding production. However, during our pre-production assessment, the process engineer prescribed the use of an SMT fixture for welding. The customer was momentarily confused, wondering why an SMT fixture was needed.
Next, let's talk about the hazards of uneven PCB surface flatness for thin boards.
On the SMT production line, if the PCB is not flat, it will cause inaccurate equipment positioning.
A. Printing—The PCB and the stencil are not on the same plane, which will cause uneven solder paste printing, resulting in thin, sharp, uneven, and non-uniform solder deposition.
B. SMD—This will cause SMD component flying, offset, bridging, and component damage.
C. IR-Reflow—This will cause chip cold solder joints, and standing-up resistors and capacitors.
If the solder paste is sharpened, it may also cause short circuits and other problems after welding.
Why do these defects occur?
When the PCB (pcb assembly) thickness is less than 1.0 mm, adding connection points or V-cut slots will greatly reduce (weaken) the strength of the entire panel because the V-cut depth is 1/3 of the board thickness. Strengthening the middle of the PCB, supporting frame - fiberglass cloth V-break, resulting in significantly softer strength. Without fixture support, it will affect the subsequent processes of the PCBA.
1. Printing stage—Whether the support of the pins can withstand the pressure of the squeegee printing, considering whether there is space to place the pins under the BGA chip. In addition, if there are SMT devices within 1.5-3 mm around the pins, there is also a risk of the pins damaging the devices.
2. SMD—Whether the top pins can support the deformation compensation of the PCB (PCB assembly).
3. IR-Reflow reflow soldering—When the furnace temperature reaches the Tg temperature during the PCB (PCB component) oven process, warping will occur. If the support strength is insufficient during the reshaping process, it will cause quality problems and the root cause of potential failure risks.
A1 Printing—Compact layout. Printing pins cannot be stably distributed to support the top surface. PCB thickness is less than 1.0 mm. After reflow soldering on the BOT side, the PCB assembly will have some deformation before TOP production.
Uneven PCB surface will cause printing difficulties and affect quality risks, such as missing printing, thick solder, warping, bridging, and insufficient solder.
B1 SMD—Uneven PCB, deformation, insufficient support strength, and mounting problems such as offset, cold solder joints, and component damage.
C1 Reflow soldering—Uneven PCB, deformation, insufficient support strength, and welding quality problems after IR-Reflow. Cold solder joints, standing components, solder deposition, and chip pillow effect.
When using a dedicated SMT fixture, strengthen the support on the fixture and avoid the location of the SMT components. Place the PCB on the fixture, so that the overall support strength of the PCB (pcb assembly) can be uniform and reliable (fixture—printing + placement + IR-Reflow can be universal).
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