Home > Industry news

An Analysis of SMT Solder Paste Printing Defects


In SMT PCB production, solder paste printing is a critical step. Since the solder paste is used to directly form the soldering joint, the quality of solder paste printing affects the performance and reliability of the surface mount assembly. Quality solder paste printing guarantees a quality solder joint and final product. Statistics demonstrate that 60% to 90% of soldering defects are related to solder paste printing defects. So it is very important to understand what causes defects in solder paste printing.

The following table lists an analysis of solder paste printing defects:

Item Factors Analysis
1 Solder Paste Powder formation The irregular shape of solder powder will easily clog stencil apertures. This will cause a big slump after printing. It can also cause solder ball and short bridge defects after reflow.
A spherical shape is best, especially for fine-pitch QFP printing.
Particle Size If the particle size is too small, the results will be poor paste adhesion. It will have a high oxygen content and cause a solder ball after reflow.
The particle size should be controlled to about 25 ~ 45 ?m in order to meet the requirements for fine-pitch QFP soldering, If the partical size desired is 25 to 30 ?m, it should applied with less than 20 ?m solder paste for an ultra fine-pitch IC.
Flux Flux contains a thixotropic agent, which allows the solder paste to have pseudoplastic flow characteristics. Since the viscosity decreases when the paste passes through the stencil apertures, the paste can be applied to the PCB pads rapidly. When the external force stops, the viscosity will recover to ensure no deformation occurs.
The flux in the solder paste should be controlled to between 8 and 15 percent. A lower flux content will result in excess amount of solder paste applied. Conversely, a high flux content will result in an insufficient amount of solder applied.
2 Stencil Thickness A stencil that is too thick will cause a solder bridge short.
A stencil that is too thin will cause an insufficient solder to be applied.
Aperture size When the stencil aperature size is too big, a solder bridge short can occur.
When the stencil aperature size is too small, and insufficient solder paste will be applied.
Aperture shape It is best to use a circular-shaped stencil aperture design. Its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow.
3 Printing parameters Blade Angle Speed & Pressure The blade angle affects the vertical force applied on the solder paste. If the angle is too small, the solder paste will not be squeezed into the stencil apertures. The best blade angle should be set around 45 to 60 degrees.
A higher the printing speed means that less time will be spent in applying the solder paste through the stencil aperture surface. A higher printing speed will cause insufficient solder to be applied.
The speed should be controlled to around 20 ~ 40 mm/s.
When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the stencil.
When the blade pressure is too high, it will result in more paste leakage. The blade pressure is typically set at about 5N ~ 15N / 25mm.
4 Printing process control PCB moisture If the PCB moisture is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and creating solder balls.
Dry the PCB if it was fabricated over 6 months ago. The recommended drying temperature is 125 degrees for 4 hours.
Paste storage If the solder paste is applied without a temperature recovery period, the water vapour in the surrounding environment will condense and penetrate the solder paste; this will cause the solder to splash.
Solder paste should be stored in a refrigerator at 0 to 5 degrees.
Two to fours hours before use, place the paste in a normal temperature environment.

Flason Electronic Co.,ltd provide a full smt assembly line solutions, including SMT reflow ovenwave soldering machinepick and place machinesolder paste printerPCB loaderPCB unloaderchip mounterSMT AOI machineSMT SPI machineSMT X-Ray machineSMT assembly line equipmentPCB production Equipment smt spare parts etc any kind SMT machines you may need, please contact us for more information: wechat whatsapp:008613691605420, Skype: flasonsmt, Email: sales@flason-smt.com


FAQ

1) This is the first time I use this kind of machine, is it easy to operate?

There is English manual or guide video that show you how to use machine.

If you still have any question, please contact us by e-mail / skype/ phone /trademanager online service.

2) If machine have any problem after I receive it, how can I do ?

Free parts send to you in machine warranty period.

If the part is less than 0.5KG, we pay the postage.

If it exceeds 0.5KG, you need to pay the postage.

3) MOQ ?

1 set machine, mixed order is also welcomed.

4) How can I buy this machine from you? ( Very easy and flexible !)

A. Consult us about this product on line or by e-mail.

B. Negotiate and confirm the final price , shipping , payment methods and other terms.

C. Send you the proforma invoice and confirm your order.

D. Make the payment according to the method put on proforma invoice.

E. We prepare for your order in terms of the proforma invoice after confirming your full payment.

And 100% quality check before shipping.

F.Send your order by air or by sea.

5)Why choose us ?

A. Gold supplier on Alibaba !

B. Trade assurance to US$54,000 !

C. Best price & Best shipping & Best service !


Contact information



CONTACT US

Contact: Andrey

Phone: 13691605420

Tel: 0755-85225569

Email: sales@flason-smt.com

Add: Flason Industry park Shajing Town Baoan district shenzhen China 518104