Summary of SMT quality problems (2)

- May 09, 2020-


3Factors affecting the quality of reflow soldering

 

1. Influencing factors of solder paste

 

The quality of reflow soldering is affected by many factors. The most important factor is the temperature curve of reflow soldering furnace and the composition parameters of solder paste. Now the commonly used high-performance reflow soldering furnace can easily and accurately control and adjust the temperature curve. In contrast, in the trend of high-density and small-size, solder paste printing has become the key to the quality of reflow soldering

 

The particle shape of solder paste alloy powder is related to the welding quality of narrow spacing devices, and the viscosity and composition of solder paste must also be selected appropriately. In addition, solder paste is generally stored in cold storage, and the cover can only be opened after the temperature is restored to the room temperature when it is used. Special attention shall be paid to avoid the solder paste mixed with water due to the temperature difference, and the solder paste shall be evenly stirred with a mixer when necessary

 

2. Impact of welding equipment

 

Sometimes, the vibration of the conveyor belt of reflow welding equipment is one of the factors that affect the welding quality

 

3. Influence of reflow welding process

 

After eliminating the quality abnormality of solder paste printing process and chip mounting process, reflow soldering process itself will also cause the following quality abnormality:

 

. Cold welding usually means low reflow temperature or short reflow time

 

the temperature climbing speed in the preheating area of solder bead is too fast (generally, the slope of temperature rising is less than 3 degrees per second)

 

(3) the circuit board or components with tin are damped, and too much moisture may cause tin explosion

 

. The crack is generally the temperature drop in the cooling zone is too fast (generally, the temperature drop slope of lead welding is less than 4 degrees per second)

 

IV. SMT welding quality defects reflow welding quality defects and Solutions

 

1. In the reflow welding of the phenomenon of erecting the stele, the phenomenon of erecting the chip components often occurs. The root cause of the phenomenon is that the wetting force on both sides of the component is unbalanced, so the torque on both sides of the component is unbalanced, which leads to the phenomenon of erecting the stele

 

The following conditions will cause imbalance of wetting force on both sides of the component during reflow welding:

 

1.1. The pad design and layout are unreasonable. If the pad design and layout have the following defects, the wetting force on both sides of the component will be unbalanced

 

1.1.1 one of the pads on both sides of the element is connected with the ground wire or the area of the pad on one side is too large, and the heat capacity at both ends of the pad is uneven;

 

1.1.2. The temperature difference across the PCB surface is too large to cause uneven heat absorption on both sides of the component pad;

 

1.1.3 uneven temperature may occur at both ends of the pad of large-scale devices QFP, BGA and small chip components around the radiator

 

Solution: change pad design and layout

 

1.2 there are problems in solder paste and solder paste printing. Solder paste has low activity or poor solderability of components. After solder paste melts, the surface tension is not the same, which will cause the imbalance of pad wetting force. The solder paste printing quantity of two pads is not uniform, and many sides will be unbalanced due to the increase of solder paste heat absorption and the lag of melting time

 

Solution: select solder paste with high activity, improve solder paste printing parameters, especially the window size of template

 

1.3. Uneven stress in z-axis direction of chip displacement will lead to uneven depth of component immersed in solder paste, and uneven wetting force on both sides will be caused due to time difference during melting. If the component chip displacement will directly lead to monument

 

Solution: adjust the process parameters of mounter

 

1.4. If the furnace temperature curve is not correct, if the reflow furnace body is too short and the temperature zone is too small, the working curve of PCB heating will be incorrect, so that the humidity difference on the board surface is too large, resulting in the imbalance of wetting force

 

Solution: adjust proper temperature curve according to different products

 

1.5. In nitrogen reflow welding, the oxygen concentration will increase the wetting force of solder by adopting nitrogen protection reflow welding. However, more and more examples show that the phenomenon of erecting a monument increases when the oxygen content is too low. Generally, it is considered that the oxygen content is controlled at (100 ~ 500) × 10 to the sixth power, which is the most appropriate

 

2. Xizhu

 

Tin bead is one of the common defects in reflow soldering, which not only affects the appearance but also causes bridging. Tin bead can be divided into two categories: one appears on one side of chip components, often a large independent ball; the other appears around IC pin, in the form of scattered beads. There are many reasons for tin bead generation, which are analyzed as follows:

 

2.1. The incorrect reflow soldering curve can be divided into four sections, namely preheating, heat preservation, reflow and cooling. The purpose of preheating and heat preservation is to make the surface temperature of PCB rise to 150 within 60-90s, and heat preservation is about 90s, which can not only reduce the thermal shock of PCB and components, but also ensure that the solvent energy of solder paste is partially volatile, Avoid spatter caused by too much solvent during reflow soldering, which will cause solder paste to rush out of the pad and form solder bead

 

Solution: pay attention to the heating rate, and take a moderate preheating, so that it has a good platform to make most of the solvent volatile

 

2.2 quality of solder paste

 

2.2.1. The metal content in solder paste is usually (90 ± 0.5)%. Too low metal content will lead to too much flux composition, so too much flux will not volatilize easily in preheating stage and cause flying beads

 

2.2.2. The increase of water vapor and oxygen content in solder paste will also cause flying beads. As solder paste is usually refrigerated, when it is taken out of the refrigerator, if the recovery time is not ensured, it will cause water vapor to enter; in addition, the cover of solder paste bottle shall be tightly closed after each use, and if it is not tightly closed in time, it will also cause water vapor to enter

 

After the completion of the solder paste printed on the template, the remaining part shall be treated separately. If it is put back into the original bottle, the solder paste in the bottle will deteriorate and the solder bead will be producedSolution: readjust the height of Z axis

 

2.3 printing and placement

   2.3.1 in the printing process of solder paste, due to the deviation between the template and the pad, if the deviation is too large, the solder paste will be immersed outside the pad, and the solder bead will easily appear after heating. In addition, the poor printing working environment will also lead to the formation of solder bead. The ideal printing environment temperature is 25 ± 3 ℃, and the relative humidity is 50% - 65%

   Solution: carefully adjust the clamping of the template to prevent loosening and improve the printing working environment


  2.3.3 thickness and opening dimension of formwork. Too large thickness and opening dimension of formwork will increase the amount of solder paste and cause the solder paste to flow out of the welding pad, especially the template made by chemical corrosion method

 

  Solution: select the appropriate thickness of the template and the design of the opening size. Generally, the opening area of the template is 90% of the pad size

 

3. Core suction 

  Core suction phenomenon, also known as core pulling phenomenon, is one of the common welding defects, which is often seen in gas-phase reflow welding. Core suction phenomenon causes solder to separate from the pad and move up the pin to between the pin and the chip body, usually resulting in a serious lack of solder. The reason is that as long as the thermal conductivity of the component pin is large, the temperature rises rapidly, so that the solder first wets the pin, The wetting force between solder and pin is much greater than that between solder and pad. In addition, the upwarping of pin will aggravate the phenomenon of core suction

 

Solution:

 3.1 for gas-phase reflow welding, SMA shall be fully preheated before being put into gas-phase furnace;

 3.2. The solderability of PCB pad shall be checked carefully. PCB with poor solderability cannot be used in production; 

 3.3. Pay full attention to the coplanarity of components, and do not use the devices with poor coplanarity in production

 

  In infrared reflow soldering, the organic flux in PCB substrate and solder is a good absorbing medium of infrared, while the pin can partially reflect infrared. Therefore, the solder preferentially melts, and the wetting force between solder and pad will be greater than that between solder and pin, so the solder will not rise along the pin, so the probability of core suction is much smaller

 

4. Bridging is one of the common defects in SMT production. It will cause short circuit between components and must be repaired in case of bridging. There are many main reasons for bridging:

 

4.1 quality problems of solder paste

 

 4.1.1. The metal content in solder paste is too high, especially if the printing time is too long, it is easy to increase the metal content, resulting in IC pin bridging;

 

 4.1.2 the solder paste has low viscosity, and flows to the outside of the welding pad after preheating;

 

 4.1.3. The solder paste tower has poor falling degree, and flows to the outside of the welding pad after preheating;

 

Solution: adjust solder paste ratio or use solder paste with good quality


4.2 printing system

 

4.2.1 the printing machine has poor repetition accuracy and alignment (poor steel plate alignment and PCB alignment), which causes solder paste to be printed outside the solder pad, especially the QFP pad with fine spacing;

 

4.2.2. The incorrect design of the size and thickness of the template window and the uneven Sn Pb alloy coating in PCB pad design lead to more solder paste

 

Solution: adjust printing machine, improve PCB pad coating;

 

4.3. Too large sticking pressure and full flow of solder paste after being pressed are common reasons in production. In addition, insufficient sticking precision will cause displacement of components and deformation of IC pin

 

4.4. The reflow soldering furnace heats up too fast, and the solvent in the solder paste is too late to volatilize

 

Solution: adjust the height of Z axis and the heating speed of reflow soldering furnace

 

5. Wave soldering quality defects and Solutions

 

5.1 pulling point refers to excessive needle shaped solder at the end of solder joint, which is a unique defect in wave soldering process

 

Causes: improper PCB transmission speed, low preheating temperature, low tin pot temperature, small PCB transmission angle, bad wave crest, solder failure, poor solderability of component lead

 

Solution: adjust the transmission speed until it is appropriate, adjust the preheating temperature and tin pot temperature, adjust the PCB transmission angle, optimize the nozzle, adjust the wave peak shape, change the new flux and solve the solderability problem of the lead wire

 

5.2. Causes of faulty welding: poor solderability of component lead, low preheating temperature, solder problem, low flux activity, too large pad hole, oxidation of lead-in plate, pollution of plate surface, too fast transmission speed, low tin pot temperature

 

Solution: solve the solderability of lead wire, adjust preheating temperature, test tin and impurity content of solder, adjust flux density, reduce pad hole in design, remove PCB oxide, clean board surface, adjust transmission speed, adjust tin pot temperature

 

5.3. Causes of tin thin: poor solderability of component lead, too large pad (except for the need for large pad), too large pad hole, too large welding angle, too fast transmission speed, high tin pot temperature, uneven solder coating, insufficient solder content

 

Solution: solve the solderability of the lead wire, reduce the pad and pad hole, reduce the welding angle, adjust the transmission speed, adjust the tin pot temperature, check the pre coated flux device, and test the solder content

 

5.4. Causes of missing welding: poor solderability of lead wire, unstable wave peak of solder, failure or uneven spraying of flux, poor solderability of PCB, shaking of conveyor chain, immiscibility of pre coated flux and flux, unreasonable process flow

 

Solution: solve the solderability of lead wire, check the wave crest device, change the flux, check the pre coated flux device, solve the solderability of PCB (cleaning or returning), check and adjust the transmission device, use the flux uniformly, and adjust the process flow

 

5.5 blistering of solder mask of printed board after welding

 

After SMA welding, there will be light green blisters around individual welding points, and even blisters of nail plate size in serious cases, which will not only affect the appearance quality, but also affect the performance in serious cases. This kind of defect is also a frequent problem in reflow welding process, but most of it occurs in wave soldering

 

Cause: the fundamental reason for the blistering of solder mask is the presence of gas or water vapor between solder mask and PCB substrate. These trace gases or water vapor will be entrained in different processes. When high welding temperature is encountered, the gas expands, resulting in the delamination of solder mask and PCB substrate. When welding, the temperature of solder pad is relatively high, so the bubble first appears around the solder pad

5.5.1 in the process of PCB processing, it is often necessary to clean and dry the PCB before the next process. If the drying temperature is not enough at this time, water vapor will be entrained into the next process, and bubbles will appear in case of high temperature during welding

 

5.5.2 before PCB processing, the storage environment is not good, the humidity is too high, and the PCB is not dried in time during welding

 

5.5.3 in the wave soldering process, the flux with water content is often used now. If the preheating temperature of PCB is not enough, the water vapor in the flux will enter into the PCB substrate along the hole wall of the through hole, and the water vapor will enter around the pad first, and the bubble will be generated after encountering the high welding temperature

terms of settlement:

  

5.5.4 strictly control each production link. The purchased PCB shall be put into storage after inspection. Generally, the PCB shall not appear blistering within 10s at 260

 

5.5.5 PCB shall be stored in a ventilated and dry environment for a period of no more than 6 months;

 

5.5.6 before welding, PCB shall be placed in an oven and pre baked for 4 hours at (120 ± 5)

 

5.5.7 the preheating temperature in wave soldering shall be strictly controlled. Before entering wave soldering, it shall reach 100-140 . If water containing flux is used, the preheating temperature shall reach 110-145 , so as to ensure the evaporation of water vapor

 

6. Blistering on PCB substrate after SMA welding

After SMA welding, blisters of nail size appear. The main reason is that there is moisture in the PCB substrate, especially in the processing of multi-layer boards. Because multi-layer boards are formed by pre forming and hot pressing of multi-layer epoxy resin semi curing sheets. If the storage period of epoxy resin semi curing sheets is too short, the resin content is not enough, and the pre drying can not remove the water vapor cleanly, After hot pressing, it is easy to carry water vapor. Air bubbles will also be left due to the insufficient glue content of the semi-solid sheet and the insufficient bonding force between layers. In addition, after the purchase of PCB, due to the long storage period, the humid storage environment, the pre baking before the production of the chip is not timely, and the blistering phenomenon will easily occur after the damp PCB chip

 

Solution: PCB should be checked and accepted before entering the warehouse; PCB should be pre baked at (120 ± 5) for 4 hours before mounting

  

7. Open circuit or faulty welding after IC pin welding

  

Causes:

7.1. Poor coplanarity, especially fqfp devices, lead to pin deformation due to improper storage. If the placement machine does not check coplanarity, it is not easy to be found sometimes

  

7.2. Poor solderability of pins, long storage time of IC, yellow pins and poor solderability are the main causes of faulty welding

 

7.3. Solder paste with poor quality, low metal content and poor weldability is generally used for welding of fqfp devices, and the metal content shall not be less than 90%

  

If the preheating temperature is too high, it is easy to cause IC pin oxidation and poor solderability

  

7.5 the window size of the printing template is small, so that the solder paste quantity is not enough 

terms of settlement:

  

7.6. Pay attention to the storage of devices. Do not take components or open the package

  

7.7 during production, the weldability of components shall be checked, especially the IC storage period shall not be too long (within one year from the date of manufacture), and the IC shall not be subject to high temperature and humidity during storage

 

7.8 carefully check the size of the template window, which should not be too large or too small, and pay attention to matching with the PCB pad size


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