With the development of economy and scientific technology, people begin to hold increasingly higher requirement to electronic products that have to meet the requirements of multiple functions, miniature, high density and high performance and feature high quality. Therefore, for SMT industry, high soldering quality is the life insurance of electronic products.
However, in the practical manufacturing, soldering defects usually take place, especially in the stage of reflow. As a matter of fact, the soldering issues seen in this stage aren't completely caused by reflow technology because SMT soldering quality is closely correlated with manufacturability of PCB pad, stencil design, components and PCB pad solderability, manufacturing equipment state, quality of solder paste and technical parameters of each link and operation skills of each worker. Manufacturing procedure of SMT is displayed in Figure 1 below.
Problems may take place in each link of this procedure, influencing soldering quality of SMT. In this article, elements that will possibly influence soldering quality of SMT will be discussed and analyzed in order to avoid similar problems in the practical manufacturing.
As one of the most important composites in SMT, the quality and performance of BOM is directly correlated the quality of reflow soldering. Specifically, the following aspects have to be taken into consideration:
a. Component packaging has to meet the automatic mounting requirement of mounter.
b. Component figure has to meet the requirement of automatic SMT in that it has to have standard shape with high dimension accuracy.
c. Solderable ends of components and PCB pad soldering quality should meet the requirement of reflow soldering and solderable ends of components and pad are not polluted or oxidized. If solderable ends of components and PCB pad suffer from oxidation, pollution or damp, some soldering defects will possibly take place such as bad wetting, pseudo soldering, solder beads or cavity. This is especially true for humidity sensor and PCB administration. Humidity sensors have to be stored in drying cabinet after vacuum packaging and it's necessary to roast them prior to manufacturing next time.
Manufacturability Design of PCB Pad
Level of SMT resides in PCB design quality and is the first element influencing surface mounting quality. Based on statistics from HP, 70% to 80% manufacturing defects derive from PCB design problems in terms of substrate material selection, component layout, pad and thermal pad design, solder mask design, component package types, assembly method, transmitting boundary, positioning via, optical positioning points, EMC (Electromagnetic Compatibility), etc.
For PCBs that have correct pad design, even if a little skew takes place in the process of surface mounting, it can be corrected under the effect of surface tension of melting soldering tin, which is called auto-positioning or self-correcting effect. However, if PCB pad is incorrectly designed, even if mounting position is quite accurate, soldering defects still come across such as component position shift and tombstoning. Therefore, the following aspects have to be delicately considered in terms of SMT pad design.
• Symmetry of pad. In order to avoid problems of position shift and tombstoning after reflow soldering, for chip components 0805 or below, pads at both ends should maintain symmetrical in terms of pad size and heat absorbing and dissipation capacity in order to keep the balance of surface tension of melting soldering tin. If one end is on the large copper foil, it's suggested that single-line connection is applied to connect pad on large copper foil.
• Space between pads. In order to ensure suitable lap joint size between component ends or pins and pad, soldering defects tend to be caused when space between pads is too large or too small.
• Residual size of pad has to ensure the meniscus shape of soldering points after lap joints between component ends or pins and pad.
• Width of pad should be basically compatible with that of component ends or pins.
• Through-hole vias mustn't be placed on pad. Otherwise in the process of reflow soldering, melting tin will possibly flow away along through-hole vias, producing pseudo soldering and tin insufficiency. It may flow to the other side of board to cause short circuit.
Solder Paste Printing
Solder Paste Printing technology primarily aims to solve problems concerning incompatibility of solder paste printing amount (filling amount and transferring amount of solder paste). Based on professional statistics, with PCBs correctly designed, 60% of PCBs for rework are caused as a result of bad solder paste printing. In solder paste printing, three important "S"s must be remembered: solder paste, stencil and scraper. If correct selections are made, excellent printing effect can be obtained.
• Quality of solder paste
As a necessary material for reflow soldering, solder paste is a kind of paste solder evenly mixed by alloy powder and paste flux (rosin, diluents, stabilizer etc.) among which alloy powder is the key element to composition of solder joints. Flux is the critical material to eliminate surface oxidation, increase wettability and ensure quality of solder paste. In terms of quality, generally speaking, 80% to 90% of solder paste belongs to metal alloy while it accounts for 50% in terms of volume. Solder paste quality insurance comes primarily in two aspects: storage and application. Solder paste is usually stored between 0 and 10°C or stored according to requirement of manufacturers. For its application, temperature of SMT workshop has to be 25°C±3°C and humidity has to be 50%±10%. Moreover, its time to recovery has to be 4 hours or more and full stirring has to be implemented prior to its application so that its viscosity features excellent printability and demoulding deformation. Solder paste cover has to be correctly placed after application and board with solder paste on has to implement reflow soldering in two hours.
• Stencil design
The key function of stencil lies in uniform solder paste coating on PCB pad. Stencil is a must-be in printing technology and its quality directly influences quality of solder paste printing. Up to now, there are three methods to manufacture stencils: chemical corrosion, laser cutting and galvanoformung. Stencil design won't be ensured until the following aspects are fully considered and suitably dealt with.
a. Thickness of steel sheet. To guarantee solder paste amount and soldering quality, surface of stencil has to be smooth and even and selection of thickness of steel sheet should be determined by the component with the smallest space between pins. Relationship between thickness of steel sheet and minimum pitch, value of components can be summarized in Table 1 below.
b. Apertures design. Apertures're trapezoid section apertures with their opening as flared mouth. Their walls are smooth with no burr. Width-to-thickness ratio=width of apertures/thickness of stencil (for Fine-Pitch QFP, ICs); area ratio=base area of apertures/aperture wall area (for 0201, BGA, CSP parts).
c. Anti-solder ball processing. Anti-solder ball processing implemented on stencil aperture of 0603 or above CHIP components can effectively avoid generation of solder balls after reflow. For components with too large pad, it is suggested that mesh partition be applied to stop too much tin producing.
d. MARK. At least 3 MARK points should be generated on Side B of stencil and stencil should be compatible with MARK on PCB. There should be a pair of MARK points with the longest diagonal distance in order to increase printing accuracy.
e. Printing direction. Printing direction is also a critical control point as well. In the process of printing direction determination, components with fine space between each other shouldn't be too near track. Otherwise, bridge connection will possibly be caused as a result of too much tin.
Scrapers, to some extent, make a difference to printing quality based on their different hardness material and shapes. Generally, steel scrapers with nickel plating are used and scrapers with 60° are ordinarily applied. If there are through-hole components, scrapers with 45° are advised so that tin amount on through-hole components can be increased.
• Printing parameters
Printing parameters mainly encompass scraper speed, scraper pressure, stencil down-release speed, stencil cleaning mode and frequency. Restrictive relationship does occurs between angle of scraper and stencil and viscosity of solder paste, so printing quality of solder paste won't be ensured until these parameters are correctly controlled. Generally speaking, a low speed of scraper leads to relatively high printing quality with possibility of vague shape of solder paste. Furthermore, an extremely low speed even decreases manufacturing efficiency. On the contrary, s high speed of scraper will possibly lead to insufficient solder paste filling in net hole. Too much scraper pressure may lead to insufficient tin and increasing abrasion between scraper and stencil while extremely low pressure leads to incomplete solder paste printing. Therefore, speed should be increased as much as possible with solder paste normally scrolled. Moreover, scraper pressure should be adjusted in order to obtain a high printing quality. An extremely high down-release speed will possibly cause icicles of solder paste or bad formation while a low speed will influence manufacturing efficiency. Unsuitable stencil cleaning mode and frequency will lead to incomplete stencil cleaning and continuous tin electrodeposit or insufficient tin in stencil holes tend to be caused to products with narrow space.
• Equipment accuracy
During printing products with high density and small space, printing accuracy and repeated printing accuracy will affect stability of solder paste printing.
• PCB support
PCB support is an important adjustment content for solder paste printing. If PCB lacks effective support or features an unsuitable support, high thickness of solder paste or uneven solder paste. PCB support should be arranged flat and uniform in order to guarantee closeness between stencil and PCB.
Quality of components mounting depends on three elements: correct selection of components, accurate placement and suitable mounting pressure. Correct selection of components refers to the fact that components has to be compatible with requirement of BOM. Accurate placement means that mounting coordinates have to be correct and accuracy of mounter has to ensure mounting stability and correct components mounting on pad. Meanwhile, attention has to be paid to mounting angle in order to ensure the direction correctness of components. Suitable mounting pressure refers to pressing thickness of components and it should never be too small nor too big. Mounting pressure can be determined by setting PCB thickness, component package thickness, mounter pressure of nozzle and adjustment of Z axis of mounter.
Soldering quality of welding spots lies in correct setting of reflow soldering temperature curves. A good reflow soldering curve requires that all mounting components on PCB have to receive excellent soldering and welding spots should feature both excellent appearance and high quality. If temperature rises too fast, on the one hand, components and PCB will suffer from heat so fast that components tend to be damaged and PCB be deformed. On the other hand, solvent in solder paste is volatillized too fast, metal composites will be splashed out as tin plating balls. Peak temperature is usually set to be higher than melting point of solder paste by 30°C to 40°C. If temperature is too high and reflow time is too long, heat-resistive components or component plastic will be damaged. Contrarily, reliable welding spots will be formed as a result of incomplete melting of solder paste. In order to strengthen soldering quality and stop components from oxidization, nitrogen reflwo soldering can be applied. Reflow curves are usually set according to the following aspects:
a. It can be set according to the temperature curve recommended by solder paste. Ingredient of solder paste determines its activation temperature and melting point.
b. Based on thermal performance parameters of heat-resistive components and valuable components, the highest soldering temperature has to be taken into consideration concerning some special components.
c. It should be set according to PCB substrate material, size, thickness and weight.
d. It should be set according to reflow oven structure and temperature zone length and different reflow ovens should receive different settings.
There are a lot of elements influencing SMT soldering quality, including component solderability, PCB quality, PCB pad design, quality of solder paste, manufacturing quality of PCB, SMT manufacturing equipment situation, technical parameters of each link of SMT and operational skills of each worker. Among these elements, quality of components, PCB and solder paste and PCB design is fundamental in reflow soldering quality insurance because soldering defects led by these elements are difficult or not possible to be solved through technical solutions. As a result, the privillege of improving excellent soldering quality lies in good control of material quality and excellent PCB pad design. Furthermore, technical parameters of each link in the process of solder paste printing, mounting and reflow soldering have to be administered and optimized with perfect manufacturing technical procedure implemented.
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