Surface Mount Assembly Defects

Surface mount is a mature technology but still we seem to suffer from common process defects during assembly. With the expertise now available in industry we should be able to overcome most problems but often they return in a couple of months to haunt us all over again. This illustrates that we often do not find the real cause of the problem that's why drawing on the experience of others can beneficial. 

As with the Printed Circuit Board Defects covered in a previous column a CD-ROM is available which illustrates the most common surface mount defects and why they occur. The interactive disks also feature video clips showing some of the defects occurring. CD ROM is a technology which lends itself to storage and retrieve of process information and pass on solutions to process engineering colleges.

The following are just some of the common process defects found during assembly today.

Crystallised Joints
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This type of joint formation is often seen on LCCC (Leadless Ceramic Chip Carriers) and is a surface effect on the solder joint. It may be caused by the joint remaining in the liquid phase for too long with no flux on the surface. This then causes the rough surface to appear. It does not effect the joint quality and is considered a cosmetic defect.

Cracked Joint
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Obvious joint cracks, like the example, has been caused by flexure of the board assembly. It may also be caused by quality staff probing the joints during inspection after reflow or, in the case of subcontract, at goods receipt at the customer's site.

It is surprising just how much force can be developed with an inspection probe. It is well in excess of the 800-1000 grams seen during joint pull off measurement.

Poor Pad Wetting
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Poor solderability on printed boards may be seen more often with nickel gold and copper boards. They are a relatively new coatings to the assembly industry and some PCB manufacturers and care needs to be taken during their introduction and use. A SMART Group video was produced last year showing the benefits on alternative solderable finishes and how to evaluate there use.

In the example it is clear that the copper pad is not solderable as all the solder paste has reflowed to form a solder ball on the pad surface. Poor solderability on copper boards may be caused by ageing, poor handling, washing the board, excessive reflow temperature or it may have been poor when supplied.

Component Lift
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Component movement and lifting are related to the soldering process and the design of the board. If one termination reflows and wets before the other, surface movement is generated either rotational or vertical. The different solderability of each termination has also be shown to cause lifting.

The use of vapour phase reflow is more likely to cause movement than convection due to the fluid movement on the board surface. The example shown lifted because of the thick solder mask, during reflow it acted like a fulcrum lifting the part just above the pad surface. The incidence of component lift is increasing due to the increased reduction in size of  passive parts.

Non Wetting
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Poor solderability of the leads has caused the poor wetting in the example. The solder paste during reflow has wetted the pads but has been unable to flow up the leads. In the case of the example it is a component problem which needs to be discussed with the supplier.

As a minimum requirement there should be 5 microns of tin/lead coating on the pins to provide a long solderable life.

Joint Failure
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A microsection taken though a joint area shows the complete separation of the joint from the pad. In this example the nickel plating on the copper pad has failed. The soldering operation was completely successful. The surface of the pad was not correctly cleaned prior to the plating operation.

The void in the microsection did not contribute to the solder joint failure.

Joint Voiding
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Voids in surface mount joints occur due to either volatile gas not escaping or non metallic material not being displaced before the solder solidifies. Basic profiles are provided by the paste suppliers but it is still necessary to make adjustments to these profiles to improve voiding, the visual appearance of the joint surface and the amount of flux left after reflow is complete.

Voids will often be seen under leads, the heel of gull wing leads and in Ball Grid Array terminations. Excessive voiding will reduce joint pull strength it does not necessarily reduce the reliability of the joint.
  
Poor Solderability
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Poor solderability of the lead as supplied from the manufacturer. In the example the solder paste has reflowed and wet the pad but failed to solder the J lead. The lead shows very little evidence of any solderable coating.

Solder Beading
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Solder balls forming at the side of components and not at the joint surface are referred to as solder beads. This is simply to avoid the confusion between the many other solder ball phenomena. The subject of solder beading was covered in AEE some months back so contact the office for any back issues of this column.

During the assembly operation solder paste gets under the body of the component. As the board passes through the reflow oven and into the reflow zone the paste turns into a liquid. All the solder balls coalesce together to form a solder joint. The same thing occurs to paste under the part in this case the increase in size of the solder lifts the part to allow the liquid to escape. The component lowers back on to the board leaving the solder bead.

To determine when the paste enters under the parts check paste printing quality. Remove parts prior to reflow and check for paste. Pass a fully loaded board through reflow changing the final zone temperature to prevent paste reflow. When the board has exited check for paste under parts.
By finding out when the paste gets under the parts it is possible to eliminate the problem.

Solder Beading
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With Leadless Ceramic Chip Carriers (LCCC) it is more likely that any solder beading is due to placement. When the component is placed on to the solder paste it is forced under the body of the device. The LCCC termination and ceramic body are all relatively flat hence some paste displacement must occur. 

Remember to also look at paste slumping as a cause.

Solder Wicking
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Solder wicking occurs when either surface has poor solderability causing the solder to flow to one surface in preference to the other. It may also occur if there is a significant difference in the reflow temperature of the pad or pin. Correct temperature profiling will eliminate this possibility.

The J lead termination shown is a classic example of the wicking defect and was supplied by Texas Instruments as part of the SMT training course originally run in the UK by the author.

Bob Willis is a process engineering consultant based in England dealing with all aspects of electronic manufacture. For further information  or to obtain a copy of some of the material discussed in this column contact him via his Internet Home Page http://www.bobwillis.co.uk