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March 28, 2011

Chip Component Cracking During Placement

Occasionally we notice chip resistor cracking during placement. Most often the cracks are in the center of 0201 and 0402 size chips.

Does your experience indicate this is typically a placement equipment adjustment problem (excessive force), or could it be a component design issue also?



D. C.

Experts Comments

There are two main causes of mechanical cracking. The first is impact cracking, which occurs during the placement of the component on the printed circuit board (PCB). The second is flex cracking due to bending or board flexure.

Impact cracks are typically caused by improper setup of the pick-and-place machine, while bending cracks are typically caused by excessive board flexure after soldering the component to the PCB.

Impact cracking will appear on the surface of the component, usually as a discolored circular or half moon shaped crack at or near the center of the Capacitor. These small cracks can evolve into larger cracks as the additional stresses of subsequent processing are applied to the component, including those stresses caused by bending of the PCB.

Impact cracking can be avoided by selecting the right placement head technology into the pick & place machine: a low mass, spring loaded, nozzle tip attached to a servo-controlled vertical movement of the placement head. An intelligent impact sensing algorithm (patent by Assembleon) takes care of impact point detection and smooth force relaxation after placement.

Only this type of technology will eliminate component cracking and will give extremely low DPM figures While it is possible capacitors could be supplied already cracked in its incoming packaging, it is not likely. Most component manufacturers take great care to ensure final inspection and shipment is done correctly. Cracking modes other than impact and handling can include thermal, in-circuit testers and H absorption.

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Sjef van Gastel
Manager Advanced Development
Assembleon
Sjef van Gastel is manager of advanced development at Assembleon Netherlands B.V. He is responsible for technology roadmapping, technology investigations leading towards new machine concepts and for competitive dynamics. He is principal author of the book 'Fundamentals of SMD assembly.'

Your question raises so many more questions.

First, can you trace the cracked components to a specific product, area of the board, reference designator, reel of parts or supplier? It can be simply too much placement force, but it can also be underside supports are set in correctly causing the board to warp upward creating excess force at placement, or it can be dirty nozzles making a spring compliant nozzle too resistive.

The component height might alsobe set incorrectly in the program causing the nozzle to place with greater force than expected.

The "0" down stroke point of either the pick-up or the placement can be set in correctly or you could beusing a nozzle that is smaller than recommended for the part your placing, this can put added stress on the center of the part during placement.

More unlikely, but, there is also the chance that the parts are already cracked before you take them out of the package.

My recommendation would be to eliminate possible culprits one-by-one until you narrow it down to the root cause. I'm sorry there's no clear answer, good hunting!

Scott Wischoffer
Marketing Manager
Fuji America Corporation
Scott Wischoffer has been in the SMT industry since 1986 with experience in service, training, sales & applications, and marketing.
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