Improvement Measures and Factors Impacting Quality of SMT Soldering
With the advancements made in technology and having a hype and strength in economy consumers have expected to have higher expectations from the electrical and electronic products to meet their multi-purpose requirements. With having multi-purpose capabilities, consumers also expected electronic products to have higher performance, high density, high quality, and to be miniature. Surface mount technology (SMT) is the way to keep such expectations from electronic products.
In the manufacturing of electronic products, soldering defects in printed circuit boards often takes place which are difficult to tackle. Soldering problems are caused by numerous reasons such as reflow technology, because the quality of SMT soldering is relevant to manufacturing of PCB Pads, Installation of components, stencil design, and PCB pad soldering capability along with soldering paste quality and other technical parameters such as technique of operation of worker. The flow diagram of manufacturing process of SMT is given as follows.
Each step mentioned in figure above may have an error which would influence the SMT quality. This article has all the elements which may possibly have an influence on quality of SMT. The details of each influence is discussed in detail and analyzed to as to avoid these problems while manufacturing of PCBs is in progress.
The Preparation of BOM
BOM abbreviated as bill of materials is one of the key composites of SMT. Therefore performance quality of bill of materials is directly reflected and correlated to the flow of soldering. The following are some major aspects which needs to be considered for better quality SMT.
- The packaging of component must be meeting the requirements of autonomous mounting of mounter.
- The figure of component must be meeting the requirements of autonomous SMT so that it may have a standard shape having higher dimension accuracy.
- The quality of PCB pad soldering as well as solderable ends of constituent elements must be meeting requirements of reflow technology so that it must not be oxidized or polluted. If the PCB pads and solderable ends of constituent elements are oxidized or polluted, there is a possibility of soldering defects e.g. Pseudo soldering, bad wetting, cavity, or solder beads etc. The case is true for PCB administration and humidity sensor.
The PCB Pad’s Manufacturability Design
The quality of PCB design has SMT level and it is also first key element which has an influence on the SMT quality. Grounded on the statistics acquired from HP almost 70-80 percent of the manufacturing faults are derived from the problems of PCB design when factors such as component layout, selection of substrate substance, design of thermal pad, types of component packages, transmitting boundary, design of solder mask, method of assembly, electromagnetic compatibility (EMC), points of optical positioning, and via positions etc. are considered.
The PCBs which are having accurate pad design in cases where there is little skew taking place while surface mounting procedure is in progress, and is easily corrected through melting of soldering’s impact of surface tension tin known as auto-positioning effect. In case of the incorrect design of PCB pad even when position of mounting is almost correct, the soldering defects will come across strongly e.g. tomb-stoning and shift in component position etc. The following are some major aspects which must be considered delicately when designing Pad of SMT.
- Pad’s Symmetry:For avoiding the problems of tomb-stoning and position shift, after process of reflow soldering is completed for components of chip of 0805 and below the pads must maintain a symmetry at either ends when heat dissipation and absorption, size of pad, and keeping a balance in surface tension of melting the soldering tin are considered.
- Spacing among Pads:For ensuring the considerable size of lap joint among ends of components or pads and pins, soldering defects are caused in case if spacing among pads is either too small or large.
- Pad’s Residual Size:For ensuring meniscus outline for soldering points afterward the joints among ends of components, pad, and pins.
- Pad’s Width:It must be compatible with the pins or ends or the components.
- Vias of Through-hole shouldn’t be positioned on Pad:If placed on pad the procedure of the reflow soldering and melting tin would possibly have a flow away which produce tin insufficiency and pseudo soldering.
The Solder Paste Printing
The aim of soldering paste printing technology is solving the problems which are concerned with incompatibility of printing of paste of solder. On the basis of statistics acquired by professionals when corrected designed PCB is considered 60 percent of PCBs are requiring rework because of poor solder paste printing. 3 things should be evoked i.e. scraper, solder paste, and stencil while catering solder paste printing.
The Solder Paste Quality:
Reflow soldering’s integral ingredient is solder paste. Solder paste which is a mixture of paste flux and alloy powder. Powder of alloy is major ingredient for composition of joints of soldering. Flux is considered as the material for removal of surface oxidation and increasing wettability for ensuring solder paste quality. When quality is considered, 80-90 percent of paste of solder is having alloy but in volume it has only 50 percent quantity. Application and storage of soldering paste are key factors for having an idea of its quality. Solder paste must be stored in places having 0 to 10°C
Design of pcb Stencil:
The major purpose of stencil is lying in coating of even solder paste over pad of PCB. Stencil is key element in PCB printing and its eminence is having a major influence on eminence of solder paste printing. There are a total of 3 techniques through which stencils are designed such as galvanoformung, laser cutting, as well as chemical corrosion. The following characteristics must be suitably dealt in order to have a better stencil design.
1.Steel Sheet Thickness:
For guaranteeing soldering quality and solder paste quantity, the stencil’s surface must be even and steel thickness must also be determined through component having smallest of all spacing among pins. The relationship among thickness of sheet of steel and least pitch is briefed in table below.
2. Design of Aperture:
The aperture has an opening like flared mouth. The walls of aperture are smooth having no burr.
3. The Processing of Anti-Solder Ball:
The processing of anti-solder ball is implemented on the aperture of stencils of 0603 and above components. The components which are having larger pads are suggested to have mesh partition to be applied for stopping tin production.
There must be 3 mark points on stencil’s side B and it must also be well-matched with the PCB’s mark. There must be couple of Mark’s points having extended diagonal distance for having an increased printing accuracy.
5. Printing Direction:
The direction of printing as well is serious control point. In the progression of determination of direction of printing, those components having fine spacing among each other should not be near to each other on track else bridge connection among them will most probably have abundant tin.
The printing quality is somehow enhanced by scrappers based on the fact that these have different shapes and hardness material. Most used scrappers are made of steel having nickel plating. In case if PCB is having through hole components then the scrappers with 45 degreesare used for increasing the extent of through-hole components.
Parameters for Printing:
There are numerous parameters of printing e.g. pressure of scraper, speed of scraper, speed of down release of stencil, frequency, and cleaning mode of stencil. There is a restrictive relationship among angle of stencil and scraper along with viscosity of the soldering paste so as to ensure the quality of printing of soldering paste to be controlled in a correct manner. High printing eminence is achieved through use of lower speed scraper and there is also opportunity of unclear shape of the soldering paste. However, an extreme lower speed scrapper may decrease efficiency of manufacturing. On the other hand, a scrapper having higher speed will be leading inadequate filling of solder paste in the net hole. In case if there is much scrapper pressure would lead to insufficient abrasion among stencil and scraper and lower pressure would leading to imperfect soldering paste printing. Hence, it has to be noted that speed must be increased in a controlled manner for having enough solder paste scrolled. The scrapper pressure must be adjusted for obtaining good quality printing. Higher speed of down release will be causing icicles of the soldering paste while lower speed will be influencing the efficiency of manufacturing.
The Accuracy of Equipment:
While the printing of products is in progress having higher density and smaller space, the printing precision and repetitive printing accuracy is affecting the stability of soldering paste printing.
The PCB support considered as a key regulation content for the printing of soldering paste. In case if PCB is lacking efficient support, the higher thickness of soldering paste or non-uniform soldering paste. Therefore, the support of PCB must be arranged so as uniform and flat so as to have a guarantee of closeness among PCB and stencil.
The quality of mounting of components is largely dependent on 3 elements i.e. selection of right components, appropriate mounting pressure, and accurate placement of components. The correct selection of components is referring to the components which are compatible with BOM. The accurate placement of the components is referring to mounting the components on PCB on correct dimensions along with an exactness of mounter for ensuring stability of mounting and correct mounting of components on pad. There must be enough attention paid to the mounting angle so as to ensure the correct direction of components. The suitable or appropriate mounting pressure is referring to pressing the component’s thickness so as it must not be either too small or big. The pressure of mounting is evaluated by setting the thickness of PCB, thickness of component package, nozzle’s mounter pressure, along with the adjustment of mounter’s z-axis.
The Reflow Soldering
The quality of soldering of spots to be welded is dependent in the right setting of temperature curves of reflow soldering. The good curve of reflow soldering is requiring the mounting components on the PCB to have perfect welding and soldering spots featuring excellent in quality and appearance. In case if there is a rise in temperature abruptly then PCBA and its components would suffer from the heat at a fast speed and components will damage as well PCB will tend to deform. The solvent in the soldering paste is volatile and evaporates at fast pace, metal composites are splashed out in the form of plating balls of tin.
The highest temperature is set more than that of melting temperature of soldering paste. The peak temperature is in range of 30-0°C. In case when temperature is much higher and time of reflow is also long then the heat-resistive components are damaged. However, the consistent welding spots are fashioned as a consequence of imperfect melting of the soldering paste. Nitrogen reflow soldering is applied so as to remove oxidation and given strength to soldering quality. The following criteria is set for the reflow soldering.
- 1) The reflow soldering may be set in accordance to temperature curve which is recommended by the soldering paste. The ingredients of soldering paste is determining melting point and activation temperature.
- 2) The reflow soldering must be set in accordance to material of PCB substrate, weight, size, and thickness.
- 3) It must also be set in accordance to structure of reflow oven along with considering length of temperature zone. However, dissimilar reflow ovens must obtain diverse settings.
There are numerous elements which have an influence on soldering quality of SMT comprising of solderabiliyt of components, Pad design of PCB, quality of PCB, quality of manufacturing of PCB, solder paste quality, technical constraints of SMT link, situation of manufacturing equipment of SMT, and some operational abilities of workers. Among all these elements having influence on SMT quality, eminence of components, soldering paste, PCB, and design of PCB are essential in reflow soldering eminence insurance as the soldering defects are leading to problems which are too difficult to be resolved.
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