Home > Industry news

How to Prevent the Tombstone and Open Defects during the SMT Reflow Process


The tombstone phenomenon is a defect that occurs when passive SMT components are partially or completely lifted from the PCB pad. What usually occurs is that one end of chip is soldered on the PCB pad while opposite end of the chip stand ups vertically, looking similar to a tombstone in a cemetery.

Analysis of Root Causes

The root cause of the tombstone phenomenon is that the wetting properties (e.g., different wetting speeds) cause an imbalanced torque at the two ends of the chip terminals when the solder paste starts to melt. The tombstone phenomenon has other causes, including:
  1. The solder pads are improperly designed:
    • The components’ terminals are not covered by more than 50% of PCB pads due to the space between the two pads is too wide.
    • The two pads are designed with different sizes.
  2. The solder paste printing is uneven.
  3. The component placement is inaccurate.
  4. The reflow oven temperature is uneven.
  5. The thermal conductivity of the PCB material has different heating capacities.
  6. The presence of nitrogen has a tendency to increase the occurrence of the tombstone defect.
  7. Placing the chip parallel with the reflow oven’s conveyor.
Corrective Actions
  1. Design the pad sizes according to the recommendation specified on the datasheet.
  2. Reflow profile -- for lead-free solder paste, use a gradual soak ramp rate before reaching the melting point.
  3. Improve the accuracy of the solder paste printing.
  4. Impove the accuracy of chip placement by decreasing the placement speed.
  5. Esnure the pick and place nozzles have been adjusted to the correct pressure.
Another phenomenon is the OPEN circuit defect. This occurs when there is an electrical continuity OPEN circuit between at least one terminal of the chip and the solder pad. This phenomenon is very critical because even AOI cannot detect this failure most of the time.
open circuit defect
Analysis of Root Causes

The root cause of the OPEN circuit phenomenon is similar to the tombstone defect. However additional causes must be considered:
  1. Component oxidization has occurred.
  2. The pad sizes are not correct. The component’s terminals are not covered by more than 50% of PCB pads due to the space between the two pads is too wide.
  3. There is poor flux activity.
Corrective Action
  1. Design the pad sizes according to the recommendation specified on the datasheet.
  2. Select new components.
  3. Reflow profile -- for lead-free solder paste, use a gradual soak ramp rate before reaching the melting point.
  4. Adjust the pressure for pick and place nozzles.

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