Solder paste typography for PCB's with BGA and leadless devices. How to avoid solder paste defects on SMT boards.

Here we look the many issues surrounding production faults of PCB assemblies during the SMT assembly process, and how best to avoid them.

If you are involved in the procurement or quality aspect of PCBA manufacturing, you'll of already seen a wide variety of solder defects and problems over the years. In fact, when you goggle 'solder paste defects' you'll be greeted with a wide range of forums, blogs, statistics and solutions for controlling the paste application process. 

Even with a top of the range stencil printer, along with all if its related cleaning and handling equipment comes a complicated set of rules to ensure that paste application is correct. At Gemini we use a completely different technology called solder jet printing. We are among a number of global based companies who are now adopting this process, to remove the many constraints of traditional stencils. In fact companies such as GE, Thales and BAE have chosen to adopt solder jet printing.

If you've previously had a problem with floating  QFN devices, or poor connections on BGA devices, its typically due to the volume of paste during the SMT process. Traditional stencils have a physical limit, for the distance between devices and the Z height.

Here we can see the the reasons why stencils have reached the limit of capability for high mix technology PCB assemblies. Stepped stencils that can cater for demanding PCB boards designs require a new set of design rules to be used. This is not the case with solder jet printing, due to the ability to land solder paste on any device, and pad at any size.

Here we have a picture of a typical QFN deposit, made with a solder jet printer.
 

As you can see the paste typography is unlike a stencil deposit, and is created with a unique construction for each pad type. With QFNs, the issue of floating after reflow is thus removed. The native paste deposit caters for the spread of paste. And even with stitching vias inside the pad, paste can be applied to fill the via holes, and avoid solder drain. Standard resistors and capacitors have a small deposit inside the heel to avoid tomb stoning during reflow. The dot is around 0.3 mm wide - a size that cant be pasted with repeatability with stencils, as the aperture hole gets blocked up)

When a PCB is reflowed the parts on the board required less remedial work to ensure they comply with IPC standards. Reworking complex parts can often lead to joint integrity problems or damaged devices, so being able to reflow first time is of course the optimum position for any CEM involved with SMT Assembly.

Fist pass rates at AOI (automated optical inspection) are much higher when the solder fillets and joints are correct. False calls are eliminated, when every joint on each board follows a uniform and consistent typography. Less re-work leads to a lower hidden costs for manufactures, that ultimately can be removed from the cost of manufacture.

 

Solder jet printing improves yields and reliability and drives out the out the hidden cost of PCB assembly. For complex PCB and SMT assembly nothing can rival the capability of jet printing.

Gemini Tec use 2 world-class jet printers for all its SMT assembly. Ensuring that we can avoids the common issues found with solder paste application using traditional stencils. Both jet printers provide a full AOI to paste deposits before placement to ensure do deviation on such a critical stage of the SMT assembly process.

In this video we can see within the MY600 jet printer. Filmed in slow motion, the advanced DSP electronics moves the head, with over 3g force, shooting solder paste deposits onto the PCB, through its unique ejector head. Small red lights are just visible showing the AOI process.
 

Here is a example of the latest XILINX Kintex-7 devices, being mounted to a production board built at Gemini during November 2014. Over 1000 solder connections on the FPGA, with the added complication of a mixture of devices that need differing paste volumes.

Complex PCB assembly with XILINX FPGA devices at Gemini Tec Ltd

If you would like to see more about jet printing and how its used to build complex SMT products, contact us at Gemini Tec for more help.

Resources:
A link to our supplier of jet printers and SMT placement equipment.
http://www.mycronic.com/

More details on our services. 
http://www.geminitec.co.uk/smt-production-service

Gemini Tec Ltd

1 The Brook Trading Estate

Deadbrook Lane

Aldershot

Hampshire

GU12 4XU

United Kingdom

+44 (0) 1252 333 444

www.geminitec.co.uk