Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


seeking low cost fix to tce mismatch

Jim

#8043

seeking low cost fix to tce mismatch | 27 December, 1999

Is there a way to avoid the reliability problems arising from the tce mismatch between ceramic chip smc's and FR4 laminates, without spending an arm and a leg on expensive substrate materials? Are there inexpensive substrates with reasonably compatable tce's, or are there inexpensive R's and C's either with leads or with compatable tce's?

reply »

Justin Medernach

#8044

Re: seeking low cost fix to tce mismatch | 28 December, 1999

Jim, I wouldn't sweat a TCE mismatch between Ceramic chip components and FR4. No need to worry. 99.5% of consumer product out there utilizes this exact technology with no threat of failure due to CTE mismatch. The terminices of the components occupy roughly 20 to 30% of the overall surface area of the contact side on the component. This allows for extremely robust solder joints which will take up the majority of the CTE mismatch.

Regards, Justin

reply »

Jim

#8045

Re: seeking low cost fix to tce mismatch | 28 December, 1999

Justin,

I have a customer who is considering changing from TH to SMT, but his preliminary reliability testing yielded solder joint failures, because his product must endure severe temperature swings. He can't change to an expensive substrate, or he'll suffer competitively. There's the rub.

Jim

reply »

Mike Naddra

#8046

Re: seeking low cost fix to tce mismatch | 29 December, 1999

Justin,

I would be currious as to your customers application, and if the temperature delta and rate are great enough to cause solder joint failures as a function of mismatched Tce then you may want to consider that even if you are able to identify and procure a substrate with a similar Cte,that the majority of passive components are rated at 4 deg C. / sec, and active at 2-3 deg C / sec. Assuming the customers application submits the unit to a ramp of not more than 4 deg C / sec. this should not cause a solder joint failure. (Slopes of 4 deg C. sec are not uncommon in the reflow process). however, if the customers application submits the unit to slopes greater than 4 deg C / sec. You have the opportunity of seeing component failure in the field. If the customers application does not exceed this slope (4 deg C. /sec) and you are sill expierencing solder joint failure. I would take a look at the reflow profile, and the resultant metelurgical grain growth associated with it. Or mechanical stress on the joint that is exasserbated by thermal cycling.

Hope this helps, Mike

reply »

Jim

#8047

Re: seeking low cost fix to tce mismatch | 30 December, 1999

Mike,

Thanks for the reply. It's not really a matter of slope. It is the long term thermomechanical fatigue that the solder joint undergoes as a result of repeated severe temperature swings, while the product is in use. Non-leaded parts are susceptable to this failue mode, because they don't have leads to absorb the strain caused by the TCE. For them, the solder joints feel the stress. It is a cyclical compressive and tensile force that causes stress cracks to form in the joint, and eventually leads to failure.

What I am trying to find out is if there are any relatively inexpensive substrates out there that have TCE's similar to ceramic chips, or (the other way around) if there are any chip caps made out of another type of material that have TCE's similar to FR4, or that have leads.

Know of anything?

Jim Gustin

reply »

Mike Naddra

#8048

Re: seeking low cost fix to tce mismatch | 6 January, 2000

Jim,

The use a a teflon substrate, would mitigate the TCE mismatch you are expierencing.

Mike

reply »

PCB X-Ray Inspection

PCB equipment