From PCBA to product development and design solutions, what if the ground stress is more reasonably prevented?

- Dec 12, 2019-

From PCBA to product development and design solutions, what if the ground stress is more reasonably prevented?

1. Problems on the PCB, such as cracked solder balls; circuit damage; pads lifted; substrate cracked.

2. Problems on PCBA, such as broken components and poor function test; “popcorn” appears on the device

First, you need to understand the potential sources of stress

When you understand that the ground stress comes from the key mechanical and mechanical forces and temperature gradients, what potential ground stress risks will you experience in the entire process from new product development and design to mature PCBA? Let's follow Mr. X to come together Take a look.

a, mechanical stress of component molding

b, mechanical stress of component placement

c, Thermal stress of reflow soldering

d, mechanical stress of ICT test

e, mechanical stress of the split (split)

f, Thermal stress of wave soldering

h, Thermal stress of manual welding

Most of the ground stressors are generated in the whole process from PCB to PCBA. Then people analyze the connection points that will be generated by the whole process of PCBA assembly.

First step: board loading, PCB cleaning, low risk

The second step: printing solder paste, there is a risk that the PCB stress will be deformed by the blade stress

Third step: solder paste inspection, non-contact, low risk

The fourth step: Chip component placement, IC component placement. Pick and place components with a chip head, which is risky, such as crystal oscillators, porcelain electronics, RF connectors, etc.

Step 5: Pre-furnace inspection, low risk

Step 6: Reflow soldering, high risk, PCB thermal deformation, component thermal deformation

Seventh step: low risk

Step 8: AOI Non-Contact Low Risk

Ninth step: plug-in device molding Mechanical damage to the device

The tenth step: plug-in line, low risk, pick up the device and insert it into the PCB

Step 11: Feeder

Step 12: Wave Soldering Thermal Stress Shock

Thirteenth step: unloading machine, automatic angle cutting machine mechanical stress

Step 14: Circuit board cleaning Mechanical cleaning stress

The fifteenth step: plate stress

Step 16: ICT and FT test mechanical stress

The seventeenth step: assembly, transportation, pull-out, hand-held stress, stress during transportation.

For example, when the PCBA is disassembled from a clamp that is too tight, it will cause a chip capacitor to cause a crack; the second side of the printed side of the two-sided packaging is not properly adjusted, causing cracks or damage to the top-mounted electronic device. Condition; Hand-made sub-boards have broken boards or damaged electronic components.

For example: in the process of reflow soldering, wave soldering, and manual soldering of PCBA, when the temperature difference is too large, it may cause the PCB to warp, and the curing of the solder will cause mechanical stress on the components on the PCB, resulting in stress on the ceramic and glass parts of the components. Cracks, stress cracks are unfavorable factors that affect the long-term reliability of solder joints.

For example: PCBA can withstand the impact of mechanical shock due to improper collision or fall. Solder joints are generally not damaged when subjected to mechanical shock. However, other parts of the electric welding structure will be invalid. For example, the large inertial force caused by the impact of large and heavy leaded electronic devices by mechanical equipment can cause the copper on the PCB board to detach or the board to break, so that the electronic device itself Will destroy.

For example: PCBA packaging is improperly protected, and its resistance to vibration during transportation. For PCBA packaging protection, we should consider the design of packaging materials. For PCBA transportation, use a special anti-static box with a built-in knife card.

Second, know some common stress testing methods

The important content and means of eliminating risks is to perform stress tests.

Strain testing provides an objective analysis of the strain and strain rate levels experienced during PCBA assembly, testing, and operation. Through the identification of sensitive areas of manufacturing variation, strain testing indicates the direction of the increase in yield, which has become the benchmark for future process improvement, and the effect of adjustment can be quantified. It has been proven that the use of strain measurement to control printed board warpage is very beneficial to the electronics industry, and it has gradually become available as a method to identify and improve detrimental manufacturing processes. Many printed board assembly plants are now required to operate at strain levels specified by their customers or component suppliers.

Common methods of strain testing

1. Electrical measurement method: resistive, capacitive, inductive

2. Optical measurement method: moire method, holographic interference

3. Optical fiber sensing technology

4, digital image processing technology: CCD camera to measure strain, high-speed camera technology

5. X-ray technology: welding residual stress test

We can analyze the root cause of strain failure based on the strain test data, and determine the thresholds. The stress thresholds are different for different materials, failure types (ceramic cracking, BGA solder joint fracture, etc.) and process types (split plate, wave soldering). Parameters such as materials, processes, and operations can also be improved so that the stress value is within the threshold range. By controlling the strain on the critical chips / components in each process, the incidence of client product failure can be reduced.

IPC-9704 gives the method of stress testing, which will not be detailed here. After all, the key point is the fourth point.

Third, know how to improve stress from the design side and the machining side

Design side:

In the PCB design process, it is often easy to ignore the stress design. On the one hand, everyone has less contact, on the other hand, because everyone thinks that this is more related to production. Is this really the case?

1. For example: during the V-cut design process, how to define the parameters of the V-cut so as to minimize the stress on the device. Another example is to place the device away from the V-cut edge to increase the stress failure of the device.

2. For example, the parts that are easy to crack on the product should be placed vertically on the long side.

3, such as: thermal balance design of small resistance capacitor size to avoid the monument effect

4, such as: reasonable choice of board, reasonable way of puzzle, etc.

Production side:

1, to avoid rough or irregular operations, such as unreasonable pulling lines, sub-boards, etc.

2. Set important parameters of the production equipment, such as the slope of the temperature rise / fall.

3. Pay attention to the stress generated during the test of the needle bed fixture.

Needle bed fixtures for testing circuit boards (ICT, MDA, FVT) are directly tied to the test points on the circuit board with a probe. If the needle bed fixture is not well designed, it may easily cause the probe to exert uneven force on the PCBA. This will easily cause bending stress to cause tin cracking.

4. Pay attention to the stress during the assembly of the machine, especially the locking screw operation

When assembling bare metal, most products lock the screws on the circuit board or the front, rear, left and right shells. From the perspective of structural mechanics, usually a screw is locked first, and then the screws on the corners are locked. Lock other screws in the future. Only the first screw you choose and the order of screw locking are different. I think it will cause different stress hazards to the board.

5, other such as when the thickness of the board is too thin to use the auxiliary side, pay attention to the use of jigs, etc.

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