SMT solder paste printing standards and common defect analysis

The global electronics manufacturing industry is entering a phase of innovation-intensive and fast-growing emerging companies. With the rapid development of component packaging, resistor-capacitance components such as PBGA, CBGA, CCGA, QFN, 0201, 01005, and 03015 have been widely used, and surface mount technology (SMT) has also developed rapidly. In its production process, the quality of welding is increasingly valued by engineers. As the basic engineering and core component of electronic components, SMT technology and electronic information technology are developing simultaneously, and playing an increasingly prominent role in the electronic information industry, and its position is becoming increasingly important.

To a certain extent, chip and miniaturization have become one of the important indicators to measure the level of development of electronic component technology.

The process flow of the whole SMT line is as follows: SMT first article tester - > printing (red glue/solder paste) - > inspection (optional 3D - SPI automatic or manual inspection) - > mounting (first paste small devices and then large parts) - > inspection (optional AOI optical/visual inspection) - > welding (soldering by hot air reflow soldering) - > inspection (AOI optical inspection appearance and functional testing and testing) - > maintenance (use tools; Soldering station and hot air desoldering station, etc.) - > PCBA splitting (manual or automatic splitting machine for board cutting)

Step 1: Add Solder Paste The purpose is to evenly apply the appropriate amount of solder paste to the pads specified by the PCB to ensure that the pads corresponding to the SMD components and the PCB can achieve a good connection and sufficient solder strength during reflow soldering. In the electronics industry, solder paste is used in SMT assembly, semiconductor packaging, and other fields, and the solder joints formed by different coating and soldering processes play the role of mechanical connection, electrical conduction, and heat conduction.

Solder paste is a paste with a certain viscosity formed by mixing a variety of metal powders, paste-like welding machines and some fluxes. At room temperature, due to the stickiness of the solder paste, the electronic components can be pasted on the corresponding pads of the PCB, and the general components will not move when the tilt angle is not large and there is no external force collision. When the solder paste is heated to a specific temperature, the flux in the solder paste volatilizes, taking away the impurities and oxides of the pad and the metal part of the component, the metal powder melts and converts into solder paste and flows, the solder end of the solder paste infiltrates the solder end of the component and the PCB pad, and the solder end and the solder pad of the component are connected together by solder after cooling to form a solder joint of electrical and mechanical connection. Rapid cooling is required to avoid oxidation of the solder paste in combination with oxygen in the air, which may affect the welding strength and electrical effect.

Solder paste is applied to the pad through special equipment, and the equipment for applying solder paste at present includes automatic solder paste printing machine, semi-automatic printing machine, manual printing station, etc.

During the solder paste printing process, the viscosity and rheology of the solder paste change due to the thrust of the squeegee. When it reaches the opening position of the stencil, its viscosity decreases, and it settles smoothly to the PCB pad through the leakage effect of the mesh, at this time, the solder paste will have a slight collapse and diffuse flow, and then the viscosity rises rapidly under the action of quenching agent, and forms a shape corresponding to the mesh, so as to obtain a good printing effect.

In this process, the chip components are accurately placed on the surface of the PCB where the solder paste or chip glue is printed using a placement machine or by hand.

The main tools for manual placement are: vacuum suction pen, tweezers, IC suction and discharge aligner, magnifying glass, etc.

Step 3: Reflow soldering Due to the characteristics of "reflow" and "self-positioning" in the reflow soldering process, the reflow soldering process has relatively loose requirements for placement accuracy, and it is relatively easy to achieve a high degree of automation and high-speed soldering. At the same time, it is precisely because of the characteristics of reflow and automatic positioning effect that the reflow soldering process has stricter requirements for the design of steel mesh holes, pad design, component standardization, component end and printed board quality, and the setting of process parameters.

Reflow soldering is a key process in SMT production, and reasonable temperature profile setting is the key to ensure the quality of reflow soldering. Improper temperature curve will cause PCB board welding defects such as incomplete soldering, virtual soldering, monument, voids, etc., which will affect product quality.

SMT soldering quality specific gravity analysis chart There are many factors that affect the tin property and solder joint strength of solder paste resoldering, which are summarized here into five main aspects. According to years of field experience, solder paste, printing and reflow profile account for more than 70% of the soldering quality.

In recent years, with the miniaturization and high integration of electronic terminal products, especially portable products such as smartphones and smart watches, as well as the increase in the cost of raw materials, SMD ceramic capacitors have been developing in the direction of small size.

In the smartphone market, the mainstream MLCC sizes have transitioned to 0201 (0.6×0.3 mm), 01005 (0.4×0.2 mm), and even smaller 008004 (0.2×0.1 mm) are being evaluated in-house by a few vendors. In its production process, the influence and role of solder paste printing on the entire production process are increasingly valued by engineers.

In the SMT industry, enterprises generally agree that in order to obtain good welding results and long-term reliable quality products, the first thing to pay attention to is the printing of solderpaste.

In production, it is necessary not only to master and apply SMT solder paste printing technology, but also to be able to analyze the causes of problems and apply improvement measures to production practice.

SMT solder paste printing standards and common defects include: less tin, even tin, sharpening, shifting, missing prints, more tin, collapse, PCB board dirt, etc., the solder paste printing thickness is - 0.02mm ~ +0.04mm of the thickness of the stencil.

SMT solder paste printing standard parameters

<!--[if !supportLists]-->1.     <!--[endif]-->CHIP component printing standard

<!--[if !supportLists]-->·       <!--[endif]-->No offset of solder paste;

<!--[if !supportLists]-->·       <!--[endif]-->The amount and thickness of solder paste meet the requirements;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, and there is no collapse and fracture;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste covers more than 90% of the pads.

<!--[if !supportLists]-->2.     <!--[endif]-->CHIP component printing is allowed

<!--[if !supportLists]-->·       <!--[endif]-->There are shrinkage porosity in the opening of the stencil, but 85% of the solder paste still covers the pads;

<!--[if !supportLists]-->·       <!--[endif]-->The amount of solder paste is uniform;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste is within the required specifications;

<!--[if !supportLists]-->·       <!--[endif]-->Print offset is less than 15%.

<!--[if !supportLists]-->3.     <!--[endif]-->CHIP component printing rejection

<!--[if !supportLists]-->·       <!--[endif]-->insufficient amount of solder paste;

<!--[if !supportLists]-->·       <!--[endif]-->The amount of solder paste at two points is uneven;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste printing offset by more than 15% pads.

<!--[if !supportLists]-->4.     <!--[endif]-->SOT component solder paste printing standard

<!--[if !supportLists]-->·       <!--[endif]-->No offset of solder paste;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste completely covers the pads;

<!--[if !supportLists]-->·       <!--[endif]-->Three-point solder paste is uniform;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste meets the test requirements.

<!--[if !supportLists]-->5.     <!--[endif]-->SOT component solder paste printing is allowed

<!--[if !supportLists]-->·       <!--[endif]-->The amount of solder paste is uniform and the forming is good;

<!--[if !supportLists]-->·       <!--[endif]-->More than 85% of the solder paste covers the pads;

<!--[if !supportLists]-->·       <!--[endif]-->Print offset less than 15%;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste meets the specifications.

<!--[if !supportLists]-->6.     <!--[endif]-->SOT component solder paste printing rejection

<!--[if !supportLists]-->·       <!--[endif]-->More than 85% of the solder paste is not covered with pads;

<!--[if !supportLists]-->·       <!--[endif]-->There is a serious shortage of tin.

<!--[if !supportLists]-->7.     <!--[endif]-->Diode, capacitor solder paste printing standards

<!--[if !supportLists]-->·       <!--[endif]-->Good solder paste printing;

<!--[if !supportLists]-->·       <!--[endif]-->No offset in solder paste printing;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste thickness test meets the requirements.

<!--[if !supportLists]-->8.     <!--[endif]-->Diode, capacitor solder paste printing allowed

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste is sufficient;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste covers more than 85% of the pads;

<!--[if !supportLists]-->·       <!--[endif]-->Good solder paste forming;

<!--[if !supportLists]-->·       <!--[endif]-->Print offset is less than 15%.

<!--[if !supportLists]-->9.     <!--[endif]-->Diode and capacitor solder paste printing refusal

<!--[if !supportLists]-->·       <!--[endif]-->More than 15% of the solder paste of the pad is not completely covered;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is offset by more than 15% of the pads.

<!--[if !supportLists]-->10.  <!--[endif]-->PAD PITCH = 1.25 - 0.7MM SOLDER PASTE PRINTING STANDARD

<!--[if !supportLists]-->·       <!--[endif]-->Each solder paste covers 100% of each pad;

<!--[if !supportLists]-->·       <!--[endif]-->The amount of solder paste is uniform, and the thickness is within the test range;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, and there is no lack of tin and collapse;

<!--[if !supportLists]-->·       <!--[endif]-->No offset.

<!--[if !supportLists]-->11.  <!--[endif]-->PAD PITCH = 1.25 - 0.7MM SOLDER PASTE PRINTING ALLOWED

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, the component solder foot tin is full, and there is no collapse and no bridging;

<!--[if !supportLists]-->·       <!--[endif]-->There is offset, but not more than 15% of the pads;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste thickness test meets the requirements;

<!--[if !supportLists]-->·       <!--[endif]-->There are no defects in post-furnace welding.

<!--[if !supportLists]-->12.  <!--[endif]-->PAD PITCH = 1.25 - 0.7MM SOLDERPASTE PRINT REJECTION

<!--[if !supportLists]-->·       <!--[endif]-->More than 15% of the solder paste does not cover the pad;

<!--[if !supportLists]-->·       <!--[endif]-->Offset by more than 15%;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste almost covers the two pads, which is easy to cause short circuit after the furnace;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste printing forms a bridge.

<!--[if !supportLists]-->13.  <!--[endif]-->PAD PITCH = 0.65MM SOLDER PASTE PRINTING STANDARD

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste printing on each pad is 100% covered on the pad;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, no collapse, no offset, no bridging;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste meets the requirements.

<!--[if !supportLists]-->14.  <!--[endif]-->PAD SPACING = 0.65MM SOLDER PASTE PRINTING ACCEPTANCE

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, no bridging, no collapse;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste is tested within the specification;

<!--[if !supportLists]-->·       <!--[endif]-->The offset of the solder paste at each point is less than 10% of the pads;

<!--[if !supportLists]-->·       <!--[endif]-->There are no defects in post-furnace welding.

<!--[if !supportLists]-->15.  <!--[endif]-->PAD PITCH = 0.65MM SOLDER PASTE PRINTING REJECTION

<!--[if !supportLists]-->·       <!--[endif]-->More than 10% of the solder paste does not cover the pads;

<!--[if !supportLists]-->·       <!--[endif]-->Offset by more than 10%;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste almost covers the two pads, which is easy to cause short circuits after the furnace.

<!--[if !supportLists]-->16.  <!--[endif]-->PAD SPACING ≤ 0.5MM SOLDER PASTE PRINTING STANDARD

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste printing on each pad is 100% covered on the pad;

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is well formed, and there is no collapse;

<!--[if !supportLists]-->·       <!--[endif]-->The thickness of the solder paste meets the requirements.

<!--[if !supportLists]-->17.  <!--[endif]-->PAD SPACING: ≤ 0.5MM SOLDER PASTE PRINTING ALLOWED

<!--[if !supportLists]-->·       <!--[endif]-->Although the solder paste forming is slightly poor, the solder paste thickness test is within the specification;

<!--[if !supportLists]-->·       <!--[endif]-->There is no offset, no bridging, and no collapse of the solder paste at each point;

<!--[if !supportLists]-->·       <!--[endif]-->There is no less tin and false soldering after the furnace.

<!--[if !supportLists]-->18.  <!--[endif]-->PAD SPACING ≤ 0.5MM SOLDER PASTE PRINTING REJECTION

<!--[if !supportLists]-->·       <!--[endif]-->The solder paste is poorly formed and broken;

<!--[if !supportLists]-->·       <!--[endif]-->solder paste collapse, bridging;

<!--[if !supportLists]-->·       <!--[endif]-->Solder paste coverage is significantly insufficient.

Solder paste printing defect improvement measures The solder paste printing station mainly has the following defects: insufficient solder paste, too much solder paste, solder paste bridging, solder paste adhesive scraper.

Insufficient solder paste Machine/Process Possible Causes Improvement Measures

<!--[if !supportLists]-->1.<!--[endif]-->The stencil is too thin Increase the thickness of the stencil Basic guidance for the application of stencil thickness Templates Applicable components 8 ~ 20 mil Chip components 8 mil Component Pin Spacing > 31mil 6 mil Component pin spacing 20 - 25 mil

6 mil component pitch < = 20 mil Possible Causes Improvements

<!--[if !supportLists]-->1.<!--[endif]-->The template opening is too small, and the template opening is increased

<!--[if !supportLists]-->2.<!--[endif]-->Poor template quality Check the template quality

<!--[if !supportLists]-->3.<!--[endif]-->Scraper deformation Check scraper Material/process Possible cause Improvement measures

<!--[if !supportLists]-->4.<!--[endif]-->The viscosity is too high Check the viscosity of the solder paste and ask the supplier

<!--[if !supportLists]-->5.<!--[endif]-->The solder balls inside the solder paste are too large Inspect the solder balls inside the solder paste and ask the supplier

<!--[if !supportLists]-->6.<!--[endif]-->The metal content in the solder paste is too low Check the metal content in the solder paste and ask the supplier

<!--[if !supportLists]-->7.<!--[endif]-->There are bubbles inside the solder paste Ask the supplier

<!--[if !supportLists]-->8.<!--[endif]-->Solder paste is not warm enough Solder paste is warm

<!--[if !supportLists]-->9.<!--[endif]-->The amount of solder paste on the template is not enough, add solder paste

Too much solder paste Machine/Process Possible Causes Improvement Measures

<!--[if !supportLists]-->1.<!--[endif]-->Template Damage Inspect the template

<!--[if !supportLists]-->2.<!--[endif]-->The quality of the template is not good, and the quality of the template is checked

<!--[if !supportLists]-->3.<!--[endif]-->Loose formwork Heavy stretch formwork

<!--[if !supportLists]-->4.<!--[endif]-->Scraper speed is too fast Reduce scraper speed

<!--[if !supportLists]-->5.<!--[endif]-->Scraper pressure is too small Increase scraper pressure Material/process Possible causes Improvement measures

<!--[if !supportLists]-->6.<!--[endif]-->The viscosity of the solder paste is too high: Check the viscosity of the solder paste

<!--[if !supportLists]-->7.<!--[endif]-->Board bending screens PCB boards

<!--[if !supportLists]-->8.<!--[endif]-->The height of the PCB board between the pads screens the PCB board

<!--[if !supportLists]-->9.<!--[endif]-->No solder mask between the pads of the PCB board and the pads Modify the design

Solder paste scraper Machine/Process Possible Cause Improvement N/A N/A Material/Process Possible Cause Improvement

<!--[if !supportLists]-->1.<!--[endif]-->The amount of tin on the template is not enough, add solder paste

<!--[if !supportLists]-->2.<!--[endif]-->Solder paste viscosity is too high Check the solder paste viscosity and question the supplier

The key skills to pay attention to when printing solder paste are as follows:

<!--[if !supportLists]-->1.<!--[endif]-->Before printing, it is necessary to check the scraper, steel mesh and other utensils to ensure that they are clean, free of dust and sundries, so as to avoid contamination of solder paste and affect the solder fall; The squeegee edge should be straight and without notches. The steel mesh should be straight without obvious deformation. There must be no residual solder slurry lumps or other debris on the edge of the open groove.

<!--[if !supportLists]-->2.<!--[endif]-->There should be a fixture or vacuum equipment to fix the base plate to prevent the PCB from shifting during the printing process and to improve the separation effect of the stencil after printing.

<!--[if !supportLists]-->3.<!--[endif]-->Adjust the position between the stencil and the PCB to match as closely as possible (large gaps will cause solder leakage, and horizontal misalignment will cause solder paste to be printed outside the pads).

<!--[if !supportLists]-->4.<!--[endif]-->At the beginning of printing, the amount of solder paste added to the stencil should be appropriate, generally about 200g for A5 standard stencil, about 300g for B5 standard stencil, and about 400g for A4 standard stencil.

<!--[if !supportLists]-->5.<!--[endif]-->As the printing operation progresses, the amount of solder paste on the stencil will gradually decrease, and the appropriate amount of fresh solder paste should be added at the appropriate time.

Solder paste is an extremely important part of the SMT process, which will directly affect the ability of the entire SMT process. It is reported that defects caused by solder paste account for 60% - 70% of defects in SMT. At present, the use and management of solder paste products on the market are quite complex.

When storing, the ambient temperature is required to be about 2 - 10 °C, if the temperature is too high, the flux and the alloy solder powder will have a chemical reaction, which will increase the viscosity and affect its printability; If the temperature is too low (below zero), the rosin in the flux will crystallize, deteriorating the shape of the solder paste, and jeopardizing the rheological properties of the solder paste when thawing. Before use, it is necessary to go through the "reheating" process, if the reheating time is insufficient, the temperature difference between the solder paste and the room temperature will form condensation, resulting in uneven solder beads and solder and other defects; In addition, too low a temperature will affect the fluidity of the solder paste, resulting in poor printing. Improper storage and reheating can greatly increase the risk in the SMT production process. Therefore, how to solve the above problems is particularly important to improve product yield and reduce production costs.

In recent years, electronics factories have begun to introduce constant temperature and humidity equipment to control the internal temperature and humidity of the solder paste printing machine, the purpose of which is to provide operating conditions for the SMT solder paste printing machine and comprehensively improve the printing quality in the SMT manufacturing process.

In the process of solder paste printing, why use a constant temperature and humidity machine?

<!--[if !supportLists]-->19.  <!--[endif]-->Effect of temperature on solder paste. Experiments have shown that the viscosity of the solder paste decreases as the temperature increases.

<!--[if !supportLists]-->20.  <!--[endif]-->Lower viscosity results in the following defects:

<!--[if !supportLists]-->·       <!--[endif]-->The decrease in viscosity will cause the backlog to diffuse, and the solder paste will spread to the side and bridge during printing;

<!--[if !supportLists]-->·       <!--[endif]-->Low viscosity will cause collapse, and the printed solder paste block will collapse and bridge due to insufficient retention, which generally occurs after the printing process (debris flow);

<!--[if !supportLists]-->·       <!--[endif]-->Lower viscosity can lead to defects, poor mold release, hole plugging, and poor molding.

It can be seen that temperature has a great influence on the viscosity of wet paste, which directly affects the core quality of the product, so it is very important to control the internal working temperature of the printing machine during the printing process.

Solder paste printing ability evaluation refers to the situation of printing multiple times in a row with a fine pitch (such as a QFP pad) or a small diameter round pad, etc., in the hope that there will be no change in viscosity value or sag resistance, and even if it is left for 10 hours, it will not collapse. This characteristic is important for continuous construction, and it is not difficult for the site to be inspected, and it is not included in the American code.