The First Common Defects in the Dispensing System and Their Solutions

1. Wire Drawing/Tailing

(1) Wire drawing/tailing is a common defect in dispensing. Some of the most common causes are: too small an inner diameter, too high a dispensing force, too big a distance between the nozzles and the PCB or too poor quality patch glue.

(2) Solution: Replace nozzles with larger inner diameters; reduce dispensing force; adjust "stop" height. Change glue and select a glue of appropriate viscosity.

2. Nozzle Blockage

(1) The problem is the lack of glue or a low nozzle output. It is usually because the pinhole has not been cleaned completely, the patch glue is mixed with impurities and there is a hole blocking phenomenon.

(2) Solution: Use a new needle; use a good quality patch glue. The brand of the glue should be correct.

3. Printing Empty

(1) The glue dispenser is not working. The glue nozzle may be blocked or the patch glue mixed with bubbles.

(2) Solution: The glue in the injection tube should be soaked, especially if it was installed by you; replace the glue nozzle.

4. Component Displacement

(1) The component may be displaced after the glue has cured. In severe cases, pins of the chip component may not be on the pad. It is caused by uneven patch glue output, such as having one glue point more or less at the two adhesive points of the component chip; component displacement during patching or low initial adhesion; and placing the PCB too long after the dispensing of glue and it is still semi cured.

(2) Solution: Check whether the glue nozzle has been blocked; eliminate the phenomenon that glue is being dispensed unevenly; adjust the state of the patching machine; replace the glue; PCBs should not be left on the board for more than four hours after the dispensing.

5. After Wave Soldering, The Piece Will Come Off

(1) The bonding strength after curing of components is lower than specified values and the chips can fall off when they are touched. It is because the curing parameters aren't in place. The temperature isn't high enough, and the size of the component is too big.

(2) Solution: Adjust curing curve by increasing curing temperature. The peak curing temperatures of thermal curing adhesives are usually around 150 degrees. If the temperature is not reached, the chip can fall. If you are using light curing adhesive, check to see if the light curing light is old and if the lamp tube has turned black. Also, observe the amount of glue used and contaminate the PCBs and components.

6. Component Pins Float Up/Shift after Curing

(1) This fault manifests as component pins that float or move after curing. Wave soldering can cause the tin to enter the pad and, in extreme cases, open and short circuits. Uneven patch glue, too much glue or component offset are the main causes.

(2) Solution : Adjust dispensing process parameter; control dispensing quantity; and adjust the patch process parameters.

Second: Quality Analysis of Solder Paste and Patches

1. Quality Analysis of Solder Paste Printing

Poor solder paste printing can cause a variety of quality problems.

(1) Insufficient solder paste will result in an insufficient amount of solder at the solder joints after welding. This can lead to component open circuit, component deviation and component erection.

(2) Soldering paste adhesion can lead to circuit short circuits and component deviations after welding.

(3) Overall deviation in solder paste print will result in poor soldering on the entire board. This includes components with vertical parts, open circuits, insufficient tin and deviations.

(4) Solder paste pull tips are prone to short circuits, even after welding.

2. Insufficient Solder Paste Is Caused by a Number of Factors

(1) When the printer is in operation, the solder paste isn't added on time.

(2) A solder paste with a strange quality mixed with other foreign materials such as hard blocks.

(3) The solder paste that was previously unused has expired and it has been used for a second use.

(4) The quality of the board is poor. The pad has a small, inconspicuous coating such as solder resistance (green oil).

(5) The fixing clamp for the circuit board is loose.

(6) The thickness of solder paste will be uneven if it is not printed onto the stencil.

(7) The solder paste has not been printed on the stencil, or the circuit board contains contaminants (such as PCB packaging and stencil wiping papers, foreign particles floating in ambient air, etc.). ).

(8) The stencil and solder paste scraper are damaged.

(9) The equipment parameters of the solder scraper, such as pressure, angle, speed and molding speed, are not properly set.

(10) Human factors are responsible for the accidental knocking off of solder paste after it has been printed.

3. Solder Paste Adhesion Is Caused by Several Factors

(1) The pad spacing on the circuit board is too small.

(2) The hole position of the stencil is incorrect.

(3) The stencil is not wiped clean.

(4) The stencil problem is the cause of poor solder paste.

(5) Poor solder paste performance.

(6) The circuit board in the printer is not firmly attached.

(7) The solder paste scraper is not set properly in terms of pressure, angle and speed.

(8) Due to human factors, the solder paste must be squeezed into place and then adhered.

4. Solder Paste Printing: The Main Causes of Overall Deviation

(1) The positioning reference point of the circuit board does not appear to be clear.

(2) The stencil reference point is not aligned to the positioning reference point on circuit board.

(3) The circuit board is not firmly attached to the printer. The pin for the positioning ejector is not in position.

(4) The optical positioning system fails.

(5) The stencil opening is not marked with solder paste and the design file for the circuit board does not match.

5. What Are the Main Reasons Why the Solder Paste Pulls the Tip?

(1) Solder The performance parameters, such as the paste viscosity, are problematic.

(2) There are issues with the molding parameters when separating the board from the printing plate.

(3) There are burrs in the hole of the screen printing plate.

SMT Quality Analysis

Some of the most common problems with SMT patches are: damaged or missing parts; flipped or side parts; and parts that are not aligned properly.

1. What Are the Main Causes of Missing Patches?

(1) The component feeder (feeder), does not place the material.

(2) The component's suction is not working properly, either because the air flow is blocked or damaged.

(3) The vacuum air path is blocked and has failed.

(4) The circuit board has been badly purchased and is deformed.

(5) There is either no solder or not enough solder on the pad.

(6) Component quality issues, thickness is inconsistent.

(7) Either there are errors in the programming of the patch machine or the parameters for component thickness have been incorrectly selected during the programming.

(8) Human Factors are Careless and Bumped Off.

2. SMC Resistors Can Flip or Fall During Patching Due to Several Factors

(1) The component feeder (feeder), feeds abnormally.

(2) The nozzle height on the placement head should be corrected.

(3) The height of the head that grabs the material is wrong.

(4) The component is flipped due to vibration when the loading hole of the component tapes are too large.

(5) The direction of the bulk materials is reversed when they are placed into the tape.

3. Component Patch Deviation Is Caused by a Number of Factors

(1) The X-Y coordinates are wrong when programming the SMT.

(2) The SMT nozzle can cause unstable suction.

4. Damage to Components Is Caused by a Number of Factors During SMT

(1) The position pin is too high. This causes the circuit board to be too high. Components are squeezed when SMT.

(2) The Z coordinates are wrong when programming the SMT.

(3) The nozzle spring is stuck.

Third, Influence of Reflow Soldering Quality Factors

1. Factors Affecting Solder Paste

Re-flow soldering quality is affected by a number of factors. The most important are the temperature curve and composition parameters of solder paste. These high performance re-flow furnaces can now be adjusted and controlled more accurately. The trend towards high density and miniature has led to a shift in solder paste printing, which is now the key for re-flow soldering.

The shape of the alloy powder particles in the soldering paste is directly related to the quality of welding of narrow pitch devices. In addition, the composition and viscosity of the paste must be chosen appropriately. The solder paste should also be stored in a refrigerator. It can only be opened after the solder paste has returned to room temperature. Due to temperature variations, it is important to pay attention to prevent water vapor from mixing with the solder paste. When necessary, use a mixer for stirring the solder paste.

2. The Influence of Welding Equipment

Welding equipment can have a significant impact on the environment.

Vibrations of the conveyor belt in the re-flow soldering equipment can also affect the quality of welding.

3. Influence of the Re-Flow Soldering Procedure

The re-flow will cause quality problems after excluding the paste.

(1) Cold welding is often caused by a low re-flow temp or a lack of time in the re-flow zone.

(2) The temperature rise rate of the preheating zone of tin beads is too high (it is generally required that the temperature rise slope is less than 3 degrees/second).

(3) The tinned board or component is damp. Excessive moisture can cause a tin blast and tinning.

(4) Cracks are usually caused by a rapid temperature drop (generally the temperature slope for lead welding is below 4. degrees per second).

Re-Flow Defects: Fourth SMT Welding Defects Solutions to the Defects

1. Tombstone Phenomena

Tombstone phenomena In re-flow soldering, chip components are often standing up. The cause: The tombstone effect is caused by an imbalance in the wetting forces on both sides of a component. This causes the torques at the ends to be unbalanced.

In the following cases, The wetting forces on both sides of a component will be unbalanced when re-flow soldering is performed:

(1) The pad layout and design are unreasonable. The pad design and layout will have these defects. This will result in an unbalanced wetting force for both sides of the component.

1). The pad area of one side of the component may be too large or the heat capacity of both ends of the pad may be uneven.

2). The temperature differences between different parts of the surface of the PCB are too great, leading to uneven heat absorption both on the front and back of the component pad.

3). The temperature will be uneven at the ends of the small chip components and around large devices QFP BGA.

Solution: Change pad layout and design.

(2) There are problems with the solder paste or solder paste printing. Solder paste activity is low or component solderability is poor. The surface tension will be different after the solder paste has melted. This will result in an unbalanced pad wetting force. Due to the uneven amount of solder on the pads, the side with the most solder will melt. Unbalanced wetting forces due to a time lag.

Solution: Use solder pastes with higher activity and optimize the parameters for solder paste printing, including the size of the template window.

(3) Uneven force along the Z-axis of the patch shift will result in an uneven depth of immersion of the component in solder paste. The wetting force will also be unbalanced on both sides due to the difference in time when melting. The tombstone will be directly affected if the component patch shifts.

Solution: Modify the parameters of the patching machine.

(4) The PCB surface will have an excessive difference in wetness if the re-flow oven is too small and the temperature zones are too few.

Solution: Adjust temperature curves according to the product.

(5) Oxygen concentration during nitrogen re-flow. Nitrogen protection re-flow increases the wetting force, but as more examples are shown tombstone can occur when the oxygen level is too low. It is generally believed that the best way to control the phenomenon of increased oxygen content is to keep it between (100-500x10)x10 to negative sixth power.

2. Tin Beads

Tin beads is a common defect in re-flow soldering. Bridging is not only a problem for appearance, but can also cause it. Tin beads can be divided in two categories. The first category is often seen on the chip component side, as a large sphere. The second category can be found around the IC pins as small beads. The generation of tin bead is due to many factors, as explained below.

(1) Incorrect Temperature Curve

The re-flow curve can be broken down into four sections: preheating and heat preservation, re-flowing, and cooling. Preheating and heat conservation is used to raise the PCB surface to 150 degrees within 60-90 minutes and to keep it warm until 90. It can reduce thermal shock to the PCB, components and also allow the solvent to be partially volatilized. This will prevent splashing caused by too much solder paste during re-flow.

Solution: Preheat the solution at a moderate rate to ensure that the solvent is vaporized.

(2) Quality Solder Paste

1). Metal content is (90+-0.5 c/o) in most solder pastes. A low metal content can lead to excess flux components. Due to the difficulty in volatilizing during the preheating phase, excess flux can cause flying beads.

2). Flying beads can also be caused by increased water vapor or oxygen in the solder paste. Water vapor can enter the soldering paste bottle if it isn't allowed to recover properly after being taken out of the fridge. Water vapor can also enter if it is not sealed in time.

The remaining solder paste should be treated separately after the template has been completed. It will cause the solder in the original bottle to deteriorate, resulting in tin bead production.

Solution: Solder paste storage and usage requirements: Select high-quality paste.

(3) Printing and Patching

1). The template and pad will be offset in the solder paste printing process. The solder paste can flow out the pad if the offset is large. Tin beads will also appear when the solder paste is heated. A poor working environment can also cause tin bead formation. Ideal printing conditions are 25+3 and relative humidity between 50c/o-65c/o.

Solution: Adjust the clamping of your template carefully to avoid loosening. Work in a better environment for printing.

2). The pressure on the Z-axis during the process of patching is another important factor that causes tin bead formation, but this does not always attract the attention of people. Some patch machines have a Z-axis that is adjusted according to the component thickness. Incorrectly adjusting the Z-axis will cause the pad to be squeezed when the component is attached. This portion of the paste will produce tin bead during welding. In this case, the size of the tin beads generated is slightly larger.

Solution: Adjust the Z-axis of the placement machine.

3). The thickness and size of the opening in the template. The amount of solder will increase if the template thickness and size are too large. Solder paste may also flow out of the pad if the template is made using chemical etching.

Solution: Select a template that has the right thickness and size of opening. The template opening is usually 90c/o the pad size.

3. Wicked Phenomenon

Known as core pulling, the wicking phenomena is a common defect in welding. It is most common when vapor phase re-flow is used. The wicking phenomena causes the solder detachment from the pad, and to move up the pin between the pin body and the chip. This leads to severe cold soldering. This is because the component pin has a high thermal conductivity, which causes the temperature to rise rapidly. The solder then wets the chip body first. The upward warping will also aggravate the wicking effect.

Solution:

(1) For vapor phase re-flow soldering the SMA must be preheated fully before it is placed in the vapor phase furnace.

(2) The solderability of PCB pads should be checked carefully. PCBs that are difficult to solder cannot be used in production.

(3) Attention must be paid to the coplanarity between the components. Devices that have poor coplanarity should not be used in production.

The organic flux on the PCB substrate, as well as the solder, is an excellent infrared absorber, while the pins are able to partially reflect the infrared light. The solder melts before the pads, because the wetting forces between the pad and solder are greater. The solder won't rise up along the pins and the likelihood of wicking will be much lower.

4. Bridging

Bridging is a common defect in SMT. Short circuits can occur between the components. It is important to repair bridging. Bridging can be caused by many things, but the main ones are:

(1) Quality Issues with Solder Paste

1). The metal content of the solder paste can be too high. This is especially true when the print time is too short.

2). After preheating, the solder paste is low viscosity and flows outside of the pad.

3). Poor solder paste tower drops, flowing outside the pads after preheating

Solution: Use a higher quality solder or adjust the ratio of solder paste.

(2) Printing System

1). Poor alignment and poor repeat accuracy of the printer (poor PCB alignment, poor steel plate alignment). Solder paste is printed outside of the pad.

2). Incorrect template design with incorrect thickness and size, and an uneven Sn-pb plating on the PCB pads results in excessive solder paste.

Solution : Improve the PCB pad and adjust the printer.

(3) In production, excessive placement pressure and full flow after pressure are common causes. A lack of placement accuracy can also lead to component displacement and IC pin deformation.

(4) The re-flow oven heats up to fast and the solvent in solder paste doesn't have time to evaporate.

Solution: Increase the heating speed of the re-flow soldering oven and adjust the Z-axis height.

Five Wave Soldering Quality Defects and Solutions

1. Pull tip: The excess solder on the tip of the needle is called a pull tip. This defect occurs only in wave soldering.

Causes: Incorrect PCB transmission speed; low preheating temperatures; low tinpot temperatures; small PCB transmitting angle.

Solution: Adjust the transmission speed, the PCB transmission angles, the nozzles, the wave crest shapes, the flux replacement, and the adjustment of the preheating temperatures.

2. Cause of cold soldering: low preheating temperatures, poor component lead solderability (lead board oxidation), too large pad holes, board surface contamination, board surface contamination, and too fast transmission speeds.

Solution: Solve the lead solderability. Adjust the preheating temperatures, test for tin content and impurities in the solder. Adjust the flux density. Reduce the pad hole when designing. Remove PCB oxides. Clean the board surface.

3. Couse of thin tin: Poor solderability, large pad sizes (except those that require large pads), large pad holes, large welding angles, fast transmission speeds and high tin pot temperature.

Solution: Solve the solderability problem by reducing pads and padholes during design. Reduce welding angles. Adjust transmission speeds. Adjust tinpot temperature. Check pre-flux device. Test solder content.

4. Cause of leaking solder: faulty solderability, instabile solder wavecrests, uneven or ineffective flux spraying; poor local solderability on PCBs; shaking of the transmission chains, incompatibility between pre-flux, flux and process flow.

Solution: Solve PCB solderability problems (cleaning, return, etc. ), replace flux, test preflux devices and check flux levels.

5. After soldering Blistering on printed circuit board: Small bubbles of light green color will appear after soldering around the individual SMA solder joints. In extreme cases, fingernail sized bubbles will appear. This not only affects appearance but also performance. The same defect can also occur during the reflowing process, but is more prevalent when wave soldering.

Cause: The primary cause of blistering on the solder mask is the presence of water or gas between the soldermask and PCB substrate. The trace amounts of water or gas will be entrained into different processes. Gas expands when exposed to high welding temperatures and can cause the PCB substrate and solder mask to delaminate. The pad temperature is high during welding. This causes bubbles to appear first around the pad.

There Are Many Reasons Why PCB Can Carry Water Vapor

(1) During PCB processing, it is necessary to dry and clean the board before proceeding with the next step. After etching it is important to dry the PCB before applying the soldermask. In the event that the temperature of the drying process is insufficient, water vapour will enter the next step. When welding at high temperatures, bubbles will appear.

(2) The storage environment is poor before PCB processing, the humidity level is high and the drying process during welding is not done in a timely manner.

In wave soldering, flux containing water is commonly used. The water vapor from the flux can enter the PCB substrate if the preheating temperature of the PCB is too low. Water vapor will enter first the area surrounding the pad and then bubbles will form when the high temperature welding is encountered.

Solution:

1). Control each link of production. Before storing the purchased PCB, it should be checked. PCBs should not bubble after 10 seconds at 260 degrees.

2). PCBs should be stored in an air conditioned, dry environment for a maximum of six months.

3). Before welding, the PCB should be baked at 120+-5 for four hours.

4). The preheating temperature should be controlled strictly in wave soldering. Before wave soldering, the temperature should be between 100 and 140 degrees Fahrenheit. Preheating temperatures for water containing flux should be between 110 and 145 degrees Fahrenheit to allow the water vapor to evaporate.      

Sixth, Bubbles Appearing on the PCB Substrate Following SMA Welding

After SMA weld, bubbles the size of a fingernail appear. This is due to the PCB substrate being entrained with moisture vapor. The multi-layer board is composed of multiple-layer epoxy semi-cured pre-formed sheets that are then hot pressed. If the storage time of the semi-cured epoxy sheets is too short or the resin is insufficient and the pre-drying is not done properly, it's easy to entrain water vapor. The semi-cured sheets will leave bubbles if they contain too little glue, and the bonding between the layers is not strong enough. Due to the humid and long-term storage of the PCB, the patch may not be pre-baked before production. The damp PCB patch will also bubble after patching.

Solution: Before the PCB can be patched it must be pre-baked for four hours at 120+-5 degrees Celsius.

Seventh, Cold Solder or Open Circuit Joint after IC Pin Weld

Causes:

1. Poor Coplanarity,especially with FQFP devices the pins deform due to improper storage. It can be difficult to detect if the placement machine doesn't have a function for checking coplanarity.

2. The pins are not solderable. The IC has been stored for a very long time. The pins have turned yellow. This is the primary reason for false soldering.

3. The paste solder is not of good quality and has a low metal content. It also has a poor solderability. Metal content in the solder paste used to weld FQFP devices should not be lower than 90%.

4. The temperature of preheating is too high. This can cause IC pin oxidation, and result in a worsening of solderability.

5. The printing template is too small resulting in an insufficient amount of solder paste.

Solution:

(1) Do not open or remove the packaging without first checking the location of the device.

(2) The solderability should be tested during production. This is especially important since the storage of ICs should not exceed one year (from the date of manufacture) and should not experience high temperatures or high humidity.

(3) Check the size of the template window carefully to ensure it is not too big or too small. Also, make sure that the size matches the size of the PCB pads.