The printing stage of the assembly process is vitally important. If there are errors introduced at this stage there will be “knock on” effects throughout the process. To develop the optimums solder paste printing process and achieve error free soldering. There are many considerations and advances available. Let’s discuss few…
One of the most important parts of the surface mount assembly process is the application of solder paste to the PCB. The aim of this process is to accurately deposit the correct amount onto each of the pads to be soldered. This is achieved by screen-printing the solder paste through a stencil or foil but also can be applied by jet printing. It is widely believed that this part of the process, if not controlled correctly, accounts for the majority of assembly defects. Unlike wave soldering, reflow soldering does not inherently include the solder in the soldering process. The solder needs to be introduced to the board beforehand and then melted in the reflow oven. Solder paste can be applied manually with fine instrumentation or, more realistically, modern inkjet style printers can precisely apply solder paste to the pads. However, for even small runs, using a solder stencil currently is the most common process. A PCB stencil provides a template that goes over the PCB and has openings only where solder paste needs to be applied. With a glob of solder paste and a squeegee, an entire board can have the solder paste applied within seconds. This also can be automated and done by machine.
The stencils can be of different types, as per the applications. The stainless stencils can be thicker (up to 16 mils) in order to place more solder paste on the pad. Minimum cut width is .002 mils, with .00025 inches positional tolerance and feature smooth side walls for better release of paste. A stencil size of up to 29 by 29 inches is available. Mylar and Kapton stencils are lower cost than stainless steel stencils and used mostly for small runs and prototypes. They have a lower cycle life and are cut with UV laser. They are typically .003 to .005 thick and supported by a stiff cast aluminum frame. “Globally electronics & software are eating the share for almost all the products we come across in our professional or personal life at any stage of our existence. To meet the global demand the whole electronics manufacturing sectors seeing a surge of innovative processes, complex yet effective mechanisms are in place to make sure the end product lasts longer. An important process among other stages of electronics hardware manufacturing is “Stencil Printing” stage which is the foremost stage and because it is first stage it requires utmost attention and consideration. Normally 85% of the PCB defects accrue due to misjudgment of variables caused during stencil printing process and this pose a challenge for a production department to look out for the variables which affect the stencil printing drastically. A stencil printer is more of a chemical process oriented chamber than a piece of hardware and this means giving enough weight age to sheer mass of the machine as well because a good workmanship is also important.” said Mr. Amit Madan, Country Manager, Trans-Technology India in an exclusive talk. He further added to enable stencil printing set up equipment should be made to handle “Flexible PCBs, etc. Likewise the process gets even when the necessary features like stencil vacuum assist remaining solder paste detector options are present.
Challenges
Stencil printing process has to be accurate to get the PCB assembled error-free. A significant challenge in newer, smaller electronics assembly is the huge difference in size among components. Therefore, trying to apply the right amount of solder paste for each component with one stencil is difficult. The biggest problem is how to produce quick-turn prototypes without disrupting series production that is already running in the line. Product changeover requires time-consuming tweaks to the stencil printing process, while unnecessarily shutting down an expensive assembly line to change the product. The inability of the stencil’s technology to vary solder paste volume by part, on the run, remains the biggest impact on the soldering quality. The latest generations of ultra low-profile and highly miniaturized components, such as QFN ICs, LGA power devices, and 03015 (0.3mm x 0.15mm) chip resistors challenge surface mount manufacturers to control screen printing within a narrower window, particularly in respect of positional alignment and solder-paste volume. At the same time, however, manufacturers are also under pressure to minimize setup and changeover times, to maintain productivity. In high-volume scenarios, there is also pressure to sustain fast cycle times. Solder paste parameter optimizing after setup can take considerable time away from production. When changeovers occur often, they create a bottleneck in assembly speed. Stencil manufacturing and lead-time, as well as occasional replacement, add to the delays. Each PCB design and design change require a new stencil. Thus, stencil costs and two- to three-day time delays can happen with multiple revision changes.
Achieving a reliable process
- Accept the fact that 1 size does not fit all – 5 mil stencil rarely yields 5 mils of paste
- Determine what % of defects are print related – Most likely very high percentage – corrective actions
- Understanding “First-Pass” Yield – Be honest – Improved print process = reduced rework
- Implement proactive 3D Solder Paste Inspection – Fast ROI – Pay dirt all the way!
Wafer Bumping Using Solder Paste
Now day, Wafer bumping using solder paste with very fine solder powder has become into focus as more cost effective than conventional sputtered or plated methods. This additive method resolves around a stencil printing process similar to conventional SMT with the exception of extremely small pitch and desired deposit size. An optimal print process for a high aperture density design has been established. Print process variables such as squeegee type, print gap, and separation speed are tested and quantified as to their effect of deposit variability.
Stencil-Free Jet Printing Technology
Stencil-free jet printing technology has been developed to meet the demand for greater flexibility in modern electronics production, significantly improving the throughput speed. The CAD data (or Gerber data) for a particular PCB, compiled offline, is sent to the inkjet printer for instant printing. High-speed solder paste depositing by jet printing is made possible by a unique ejection method. The technique ejects tiny droplets of solder paste from a cartridge through an ejector mechanism onto the PCB at the positions required by the Gerber file. The ejector system operates at speeds up to 500 droplets per second, which enables solder paste printing on the fly. The different types of solder paste used for jet printing are delivered in standard cartridges. The cartridge is swiftly snapped into the machine in seconds, so one can switch from tin/lead to lead-free solder paste in a matter of minutes. A barcode label on the solder paste cartridge and identification chip in the cassette ensure that the wrong type of solder paste or solder paste that is past its due date is never loaded by mistake. The electronic identification barcode label and cassette memory make machine settings automatic. Once the paste code type is entered, the printing can start. Touch-less jet printing technology applies no force to the PCB, hence no support pins are needed, further improving start-to-finish speed. In addition, the print program automatically aligns and adjusts to the PCB board stretch based on the fiducially marks. Accurate temperature control ensures that proper viscosity of the solder paste can be maintained at all times, which in turn leads to higher levels of application accuracy. The process is completely controlled by software. The solder paste volumes can be altered on-demand. The inkjet printing process allows control of solder paste deposits with precision, in three dimensions. It is possible to fine-tune the volume, area coverage, height, and layers of solder paste that need to be applied for every individual pad, component, and package. Inkjet can print pads for components with pitches as small as 0.4 mm (16mil). With this level of control, small deposits can be printed next to large ones, something stencils had a hard time doing, which resulted in time-consuming hand-solder operations. To maintain a high degree of flexibility, printing programs can easily be adjusted on the fly if revisions are needed. The absence of stencils means other time-reduction benefits such as no need for special paper for underside wiping, no need for stencil cleaning machines, no need for cleanup and storage of stencils, and no risk of damaging the stencils during handling.
YCP10 Automatic Solder paste printer from Yamaha, Trans-Tec
Yamaha is in the business of manufacturing a solder paste printer way back in early 90s, currently YCP10 is latest model. Few highlights of YCP10 (Automatic Solder paste printer) are,
- Monocoque frame, printer is built on YS12 series mounter.
- Unique 3S (Swing single squeegee) head design
- Repeatability 10microns @ 6sigma.
- Standard vacuum assist stencil feature
- PCB size 510 mm x 460 mm
- Dedicated 2D camera for post print solders inspection (Optional)
- Auto cleaning (Dry/ Wet / Vacuum) _ Frequency can be programmed
- Printing attach angle (45 deg – 60deg) can be controlled dynamically
Advent – SP1200 Series from SANSON
SANSON has been setting the industry standard for printers by combining highly skilled technical expertise with ingenious product design and adding the most important element of all – customer input. The Advent- SP1200 series Solder paste printer offers customer based technological features and combines them with unprecedented ease of operation. User friendly stencil adjustment system simplifies every aspect of set-up and printer’s full capabilities in just a very short time. The Advent- SP1200 series is engineered to achieve the highest levels of precision and consistency in paste deposition. The Advent – SP1200 “Wide Board” Manual & Semi-Automatic solder paste printer offers advanced technological features and combines them with unprecedented ease of operation. An Omron based embedded system allows either first time or experienced operators to use the printer’s full capabilities in just a short time. Resistive touch-screen technology is also employed to reduce liabilities associated with keyboards and mouse operations.
