Analysis of six Common Causes of Defects in SMT Solder paste Printing

Analysis of six Common Causes of Defects in SMT Solder paste Printing

I. Tin Ball:
Before printing, the solder paste was not fully thawed and stirred evenly.

2. If the ink is not refluxed for too long after printing, the solvent will evaporate and the paste will turn into dry powder and fall onto the ink.

3. The printing is too thick, and the excess solder paste overflows when the components are pressed down.

4. When REFLOW occurs, the temperature rises too quickly (SLOPE>3), causing a boiling over.

5. The pressure on the surface mount is too high, and the downward pressure causes the solder paste to collapse onto the ink.

6. Environmental impact: Excessive humidity. The normal temperature is 25+/-5 ° C, and the humidity is 40-60%. During rain, it can reach 95%, and dehumidification is required.

7. The shape of the pad opening is not good and no anti-solder bead treatment has been done.

8. The solder paste has poor activity, dries too quickly, or there are too many small tin powder particles.

9. The solder paste was exposed to an oxidizing environment for too long and absorbed moisture from the air.

10. Insufficient preheating and slow, uneven heating.

11. Printing offset caused some solder paste to stick to the PCB.

12. If the scraper speed is too fast, it will cause poor edge collapse and lead to the formation of tin balls after reflux.

P.S. : The diameter of the tin balls should be less than 0.13MM, or less than 5 for 600 square millimeters.

Ii. Erection of a Monument:
Uneven printing or excessive deviation, with thick tin on one side and greater tensile force, and thin tin on the other side with less tensile force, causes one end of the component to be pulled to one side, resulting in an empty solder joint, and the other end to be lifted up, forming a monument.

2. The patch is offset, causing uneven force distribution on both sides.

3. One end of the electrode is oxidized, or the size difference of the electrodes is too large, resulting in poor tinning property and uneven force distribution at both ends.

4. The different widths of the pads at both ends result in different affabilities.

5. If the solder paste is left for too long after printing, the FLUX will evaporate excessively and its activity will decline.

6. Insufficient or uneven preheating of REFLOW leads to higher temperatures in areas with fewer components and lower temperatures in areas with more components. The areas with higher temperatures melt first, and the tensile force formed by the solder is greater than the adhesive force of the solder paste on the components. Uneven force application causes monument erection.

Iii. Short Circuit
1. The STENCIL is too thick, severely deformed, or the holes of the STENCIL are deviated and do not match the position of the PCB pads.

2. The steel plates were not cleaned in time.

3. Improper setting of the scraper pressure or deformation of the scraper.

4. Excessive printing pressure causes the printed graphics to become blurred.

5. The reflux time at 183 degrees is too long (the standard is 40-90 seconds), or the peak temperature is too high.

6. Poor incoming materials, such as poor coplanarity of IC pins.

7. The solder paste is too thin, including low metal or solid content within the solder paste, low shake solubility, and the solder paste is prone to cracking when pressed.

8. The solder paste particles are too large and the surface tension of the flux is too small.

Iv. Offset:
1). Offset before REFLOW:

1. The placement accuracy is not precise.

2. The solder paste has insufficient adhesion.

3. The PCB vibrates at the furnace inlet.

2) Offset during the.REFLOW process:

1. Whether the PROFILE temperature rise curve and preheating time are appropriate.

2. Whether there is any vibration of the PCB in the furnace.

3. Excessive preheating time causes the activity to lose its effect.

4. If the solder paste is not active enough, choose solder paste with strong activity.

5. The design of the PCB PAD is unreasonable

V. Low tin/Open Circuit:
The temperature of the board surface is uneven, with the upper part being higher and the lower part lower. The solder paste at the bottom melts first, causing the solder to spread out. The temperature at the bottom can be appropriately reduced.

2. There are test holes around the PAD, and solder paste flows into the test holes during reflow.

3. Uneven heating causes the component pins to be too hot, resulting in the solder paste being led onto the pins, while the PAD has insufficient solder.

4. There is not enough solder paste.

5. Poor coplanarity of components.

6. The pins are soldered or there are connection holes nearby.

7. Insufficient tin moisture.

8. The solder paste is too thin, causing tin loss.

The phenomenon of "Open" actually mainly has four types:

1. low solder is usually called low tin

2. When the terminals of a part do not come into contact with tin, it is usually called empty soldering

3. When the terminal of a part comes into contact with tin but the tin does not climb up, it is usually called false soldering. However, I think it is better to accept the refusal to solder

4. The solder paste has not completely melted. It is usually called cold welding

Soldering beads/solder balls

1. Though rarely, solder balling is generally acceptable in no-rinse formulations; But solderbeading doesn't work. Solder beads are usually large enough to be seen with the naked eye. Due to their size, they are more likely to fall off the flux residue, causing a short circuit somewhere in the assembly.

2. Solder beads differ from solder balls in several aspects: Solder beads (usually with a diameter greater than 5-mil) are larger than solder balls. Tin beads are concentrated on the edges of larger chip components very far from the bottom of the board, such as chip capacitors and chip resistors 1, while tin balls are anywhere within the flux residue. A solder bead is a large tin ball that comes out from the edge of a sheet component when the solder paste is pressed under the body of the component and during reflow instead of forming a solder joint. The formation of tin balls mainly results from the oxidation of tin powder before or during reflux, usually just one or two particles.

3. Misaligned or overprinted solder may increase the number of solder beads and solder balls.

Vi. Core Suction Phenomenon
Core-suction phenomenon: Also known as core-pulling phenomenon, it is one of the common soldering defects, mostly seen in gas phase reflow soldering. It is a severe false soldering phenomenon formed when the solder separates from the pad and ascends along the pins to the area between the pins and the chip body.

The reason is that the thermal conductivity of the pins is too high, causing rapid temperature rise and resulting in the solder wetting the pins first. The wetting force between the solder and the pins is much greater than that between the solder and the pads. The upward curling of the pins will further intensify the occurrence of core suction.

Carefully inspect and ensure the solderability of the PCB pads.

2. The coplanarity of components cannot be ignored.

3. SMA can be fully preheated before welding.