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When it comes to technology and quality, an SMT assembler's capabilities will be reflected when you go through the key elements in the SMT process. This article provides key points to consider in evaluating a PCB assembler, which allows you to test whether your circuit boards assembled in that factory conform to your required standard.
Among the key elements in evaluating SMT quality are solder paste printing quality, reflow quality, how to avoid misplacement of components, how to avoid manual placement, capabilities on stencil thickness calculation and modification, size of aperture, up-to-date equipment or instrument. Of these elements, the quality control of solder paste printing is the most critical. If solder paste is badly printed, your circuit boards are incontrovertibly in low quality, whether component mounting is accurately completed and reflow temperature is suitably modified. After all, non-standard amount of printed solder paste is closely associated with quality of soldering.
In terms of the other elements, the accuracy of the surface mounter has already achieved the required level and reflow temperature curves have been determined in advance, therefore few modifications are needed.
To evaluate solder paste printing capability, there are two aspects to consider: solder paste quality administration capability and solder paste printing capability.
Solder Paste Quality Administration
High-quality solder paste relies on its brand and freshness degree. The freshness degree of solder paste has to be tracked starting from the time of warming up, to jar opening and stirring. Different manufacturers conform to different regulations stipulating that solder paste has to be used up within a certain period of time, otherwise it will oxidize and lead to insufficient soldering in the reflow process. Furthermore, significant administration has to be implemented on solder pastes applied in stencil.
Solder paste is recommended to be stored in low-temperature storage to maintain its activity, and warming up (generally more than four hours) is required prior to application to prevent its temperature from being incompatible with room temperature. When the temperature varies dramatically, water drop will be generated on the surface of the solder paste, leading to splashing during high-temperature reflow.
In addition, you should also consider issues such as how the solder paste applied in stencil will be processed, how the solder paste is timed, and how the solder paste that has been applied in original stencil will be administered and controlled when the stencil is modified.
Another item that needs to be carefully studied is the time when the first batch of solder is warmed up, especially for PCB assemblers that don’t run 24 hours a day. As the SMT line starts working only when the solder paste is completely warmed up, some manufacturing houses will run warm-up about four hours earlier, or even one day earlier, to save time and improve the efficiency of the SMT lines. It’s necessary to know that the solder paste quality will be greatly decreased if the warming-up procedure took place one day before application. In practice, professional PCB assemblers will surely scrap the solder paste if the warm-up happened over 12 hours before application.
Solder Paste Printing Capability
In terms of solder paste printing capability inspection, PCBs containing fine pitch (0.4 mm or 0.5 mm) BGAs should be selected for inspection. Repeating solder paste printing should be implemented on the same piece of PCB for five to ten times. Each printing result should be inspected under a microscope to see whether issues like bridging or displacement occur.
An SMT manufacturer with a solder paste inspection (SPI) equipment can also measure the solder paste amount or volume.
Stencil cleaning is also another element affecting the quality of solder paste printing. As solder paste leakage tends to be caused by long-term printing, leading to bridging, the stencil should be cleaned with non-dust cloth or with an ultrasonic wave to avoid hole blocking issue.
Solder paste quality administration and solder paste printing capability are the main focus of SMT process inspection. Of course, genuine solder paste printing technique contains far more items that’ll be summarized into the following aspects:
1. Solder paste: Solder paste is mainly composed of tin powder (metal alloy powder including Sn, Ag, Cu, Bi) and flux, whose volume ratios account for 50%, respectively. It’s necessary to select a type of suitable solder paste compatible with the requirements of your products. Furthermore, tin powder can be rated with different numbers. The larger the number is, the smaller the particle is. Generally, No. 3 tin powder is used for SMT while No. 4 tin powder is applied for fine pitch or small soldering pad mounting.
2. Stencil: Steel is usually used as a stencil material. Apertures of steel are generated generally based on three leading different methods: etching, laser cutting and electrotyping. In terms of products with fine pitch ICs, laser cut stencil is suggested since aperture wall through laser cutting is more accurate and neat. In spite of excellent performance of electrotyping stencils, they have limited effect and the price is relatively high.
Stencil thickness and size of aperture greatly influence both the solder paste printing and reflow quality. According to principles, the key administration point lies in tin volume, in that the amount of solder paste has to be compatible with the final required soldering amount. In theory, the smaller the SMD component is, the thicker the stencil has to be. However, keep in mind that the thinner the solder paste is, the more difficult it is to control the tin amount. Basically, the thickness of an ordinary stencil is within the range from 0.12 mm to 0.15 mm. When it comes to fine pitch components (0201 or 01005), a stencil with thickness of 0.1 mm below is needed.
Silkscreen Printing Parameter Setting and Modification
a. Scraping blade pressure: A slight modification of scraping blade pressure leads to tremendous influence on solder paste printing. If blade pressure is too low, the solder paste will fail to fall at the bottom of stencil aperture and to be effectively transferred to the pad. If blade pressure is too high, the solder paste will be too thin or the stencils will even be damaged. The optimal condition is that the solder paste is scrapped from the surface of stencil totally.
b. Printing thickness: Printing thickness largely relies on the thickness of the stencil. A slight modification of solder paste printing thickness can be obtained through blade speed modification and blade pressure. Suitable reduction of printing speed of blade also leads to increasing of solder paste amount on PCB.
c. Stencil cleaning: In solder paste printing, the stencil should be cleaned after every 10 units of PCBs have been successfully printed in order to eliminate deposits at the bottom of stencils and pervasive solder paste. Generally, alcohol without water is applied as a cleaning agent.
To obtain genuinely high SMT quality, investigation and analysis have to be carried out on each manufacturing link and key elements so that effective control methods can be captured. In the SMT assembly process, solder paste printing is the most critical. As long as reasonable parameters are set and corresponding laws between them are mastered, high-quality solder paste printing can be achieved.
This article was originally published in the March 2017 issue of SMT Magazine.