Issue link: https://iconnect007.uberflip.com/i/686649
June 2016 • SMT Magazine 97 Apolinario: As devices become smaller, the probability of defects from a solder paste print- ing standpoint increases. The key is we need to do a printing process that is consistent and with high accuracy to serve those requirements. We have to consider the appropriate stencil design, the machine's accuracy, and the solder paste. Also, devices such as ICs and BGAs, which are the critical components of the PCBA and are sensitive to defects. For example, voiding in BGAs cannot be seen without using x-ray in- spection. So, if the printing process is not good, it's a waste of time. The challenge is to obtain an ac- curate result in the first pass. Las Marias: What can you say about the advan- tages and disadvantages between solder jet printer and screen printing? Apolinario: Both printers can be used in boards with tighter tolerances and smaller pitches. The difference, however, is that jet printing is quite slow—it's cycle time is about six min- utes—while screen printing can be used in high-volume production with a printing cycle time average to less than 30 seconds. The ad- vantage of jet printing is that you don't need a stencil. You can use jet printing in qualifi- cations for low-volume, high-mix, or in NPI. Screen printing, meanwhile, is used in me- dium- to high-volume production. Accuracy- wise, they are comparable. The only difference is the speed—solder paste printing is much faster compared to jet printing. Bantigue: Aside from speed, jet printing has limitations when it comes to smaller compo- nents because there is a minimum dispense dot size for jet print. For example, if you are dis- pensing 0.40 mm diameter of solder paste on jet print, you cannot use it for the 0.4 mm CSP mounting. Smaller components usually use sol- der paste printing instead of jet. Although jet printing will not require a stencil, you actually use a more expensive solder paste—usually type 5 or finer solder—unlike for the solder paste printing, you can use type 4 or larger solder pastes. Las Marias: Does it make sense to have both equipment in a factory? Apolinario: Yes. If you are running a low-vol- ume project, it is better to use jet printing. If you have an R&D or NPI project, running one board, two boards, then it makes sense to use a jet printer. But for your medium- to high- volume operation, then you should be using a screen printer. In terms of performance, accu- racy, it's just the same for both. Las Marias: What are the best practices to con- sider when it comes to solder paste printing? Apolinario: First, the product should undergo the DFM to come up with the optimum aper- ture design and finish of both stencil and PCB. For complex boards that combine small and big components, you have to consider the multi- step or step-design stencil, or what we call over- printing. Next thing to consider is the solder paste se- lection—what type and kind of solder to use, what's best for certain products or projects. Lastly, you need to consider what equip- ment to use. Las Marias: What do you think is the future for solder paste printing? Apolinario: We are seeing smaller pitches, such as 0.3 mm (300 microns) and below, as a chal- lenge going forward. This is the same challenge for the equipment manufacturers—they have to be able to support this trend. Bantigue: Maybe, combining both solder paste printing and flux (sticky flux) dip for CSP. In- stead of using solder paste on the pads of CSPs, you will dip the solder spheres (of CSP) on flux before mounting on the printed circuit board. Las Marias: Thank you very much for you time, and it's great speaking with you guys. Apolinario: Thank you, Stephen. SMT SOLDER PASTE PRINTING: A USER'S PERSPECTIVE