What are the causes and solutions for component offsets that occur during reflow soldering?

- Sep 23, 2019-

What are the causes and solutions for component offsets that occur during reflow soldering?

What causes the transfer of SMT components?

We used two identical reflow ovens, both of which were properly maintained.

We also run the exact same board through two ovens, but we only use one oven for component shifting.

What are the causes and solutions for the component transitions we encountered during reflow?

A good question will raise other questions and hope to get an answer. Start:

First, we need to make sure the oven is really "completely the same." The best way to do this is to use a heat map. If two ovens do produce exactly the same contour on the same test vehicle, they can be considered identical. Many ovens may differ and may cause performance differences. Some examples:

One. The thermal controller in the oven can have tolerances up to +/- 1 degree Celsius

Thermocouple position within the Bay Tunnel - oven controlled t / c relative to product height may affect board level results

C. Thermocouple tolerances are up to +/- 1.8 degrees - so control t / c can have such a range, and the t / c you use on the test vehicle is subject to the same tolerance. Tolerances can be superimposed, so a range of up to 4-5 degrees Celsius can be seen only due to tolerance buildup.

There are some matching t/c available that can alleviate some of the buildup, usually the oven controller can be calibrated or offset to create a match between the ovens, so you can "call out" some. The bottom line is to make sure the configuration file is really the first one.

Then there will be some questions: Is the same part moving all the time? Is it a random location and a random time?

Let's take a look at both:

A) The same part has been changing:

We need to verify that the location of the paste and components is accurate and identical. My guess is that the two ovens are provided by two separate printers and the Pick and Place system.

In some cases, if the paste is recorded incorrectly or if the assembly is placed on a production line slightly wrong, it is due to wetting imbalance from one terminal to the other or due to lack of stickiness.

The parts are perfectly placed and visually inspected on the P&P machine, but the Z direction is high. This will cause the oven to move later. So sometimes you only need to make a slight printer adjustment or Z-axis adjustment on the P&P.

In rare cases, airflow in the oven can be the culprit. If a small portion or MELF is moving, high velocity air or local jets can cause movement. In some ovens, air is transferred to the product through small diameter holes in the heater module. These small jets produce a locally high airflow that moves parts in the right environment.

This is rare, but an easy way to confirm this is to reduce the fan speed in the oven and see if the problem disappears. If the oven does not have fan speed control, you can use Kapton tape to block the hole directly above the problematic part for a very primitive and quick inspection.

If there is a local current or vortex, the conveyor track can usually be moved in a simple manner. This is not to say that another problem may not be generated elsewhere, so fan speed control may also be ideal for process control and repeatability.

B) Different part shifts or shifts are random:

Usually the main reason for component movement is mechanical components. In other words, a force is acting on the part. There are some forces that can work on components:

1) During the heating process, the solvent in the flux will boil. If the heating rate is too high, small bubbles will flow out and physically move the small parts. As a result, personal data may have an impact - but as mentioned above, we have excluded personal data - I just want to point out that for those who might see mobile in a single-line environment.

2) Processing - An important reason for component movement is processing. Not so much operator handling, but automation. The intersection between the upstream conveyor and the oven may be a rough road. If the two conveyors are not properly aligned, then perfect printing and perfect placement can be futile if the board encounters the edge of the oven from time to time.

Or sometimes the conveyor belt can place the board on a chain (not a pin) in the oven conveyor chain. When the board passes through the oven, it can be turned back onto the pin, which may be just a mechanical force that causes the component to move.

3) Oven Conveyor - If the oven's rail system is not properly installed, the board may be caught in the oven tunnel. The mechanical forces associated with pinching, bending and sometimes ejecting plates can easily move parts. Areas to check:

a) Set the rails - Sometimes the operator will set the conveyor track too tight. The board expands with increasing temperature and the rail needs to accommodate this expansion. We usually recommend leaving a gap of 1.5 - 2mm between the edge of the PCB and the links to allow for expansion.

In many cases, the operator will make up a 1.5mm thick feeler gauge (or use a piece of 0.062 FR-4) to insert between the PCB and the chain for easy setup.


b) Guide rail warping or parallelism - If the rails are bent inward or not parallel, they will also clamp the PCB when expanding in the PCB heating tunnel. The problem with pinching is that the board may be pinched or bent for a short time, or ejected and returned to the conveyor chain, so when it leaves the oven, it does not show evidence of the human operation it has experienced.

An easy way to test this is to set the oven belt's track to be 0 - 5 mm larger than the PCB width. With the oven open (cold), slide the PCB manually through the oven to see if there are any restraints or rough spots on the road.

Shenzhen Peng Zexiang smt patch processing: I hope this is helpful. If you have any questions, or if we can help, please feel free to contact our team and we will be happy to help.

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