Hi YS
I think there is a casual relationship between Tg and MOT and I am suggesting two things to Patricia:
1. Upgrade FR-4 to Tg 150C which is consistent with practice for industrial equipment and lead-free assembly.
2. Check her PCB supplier to verify they can supply PCBs UL Listed at 130C MOT, which is generally employed for industrial equipment (verses 105C or 120C for some consumer products).
This ensures that, regardless of Tg :
(a) the raw CCL materials have adequate retained properties after LTTA to obtain and FR-4 rating (at least 120C MOT).
(b) that the fabricated PCBs have passed Bond Strength and Delamination Tests after accelerated aging with the MOT rated temperature as an index multiplied by the acceleration factors for the 10 and 56 day aging conditions. These tests have been well correlated to actual long-term performance and this is why OEMs or industrial, medical or automote equipment generally require full recognition verses just Flammability Testing (which proves nothing about long term reliability).
I agree that Tg is not an absolute indicator of anything and some high Tg materials may fail to gain FR-4 rating due to poor retained properties. But low Tg FR-4 epoxies tend to have poor thermal resistance to lead-free soldering and difficulty passing accelerated aging tests to obtain a 130C MOT rating, so I would ask, can you suggest a 135C Tg material that has good capability for 130C MOT? My experience is materials less than 140C are a risk.
If we look at the materials commonly used in many industrial or automotive applications requiring high service reliability, they are typically 150C nominal class FR-4 since these materials have high enough thermal resistance to pass lead-free assembly and ATC tests, but are less brittle than many high Tg materials (eg, 170C or above) so the lower Tg materials have better mechanical toughness to survive shock and vibration for industrial applications.
In general I will consider Tg, Td, Z-CTE and Elastic Modulus (Young's Modulus) as basic indicators of material properties to match them to application. In very high temperature applications materials with high temperature strength retention would be required but it seems not to be the case here. What I assumed:
Ambient 30C max
Temp Rise 55C max
Joule Heating 20C max (the general IPC recommended limit for continuous resistive heating of the PCB (something the designer apparently is ignoring!)
That sums up to 105C. Perhaps a 120C (minimum FR-4) rated PCB would be OK, but most of the OEM now specify 130C as a minimum for any product containing hot components for a safety factor.
Our company's practice is to default to 130C MOT and 145C (TMA)/150C (DSC), and go up from there, even for consumer products.
What is your further advice/comment? Always interested in others experience.
Regards,
C.B. Katzko ¹Å¥K°ª
Research + Development
Meadville Group
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"Yuet Shan Chan" <[log in to unmask]">[log in to unmask]> 2009/11/13 12:23
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Re: [TGAsia] PCBA operating temperature enquiry
Dear Chris,
I think there is no direct relationship between Tg and MOT. A laminate
with high Tg or low Tg can also have 130oC MOT. In UL's standard 746E, it
stated FR4 can have 130 MOT. UL use other term called RTI (Ralative Thermal
Index). FR4 can reach 130oC RTI in electrical properties and 140oC RTI in
mechanical properties.
Best Regards,
>>> [log in to unmask]">[log in to unmask] 13/11/2009 11:56 >>>
Hi Patrica
Most FR-4 materials are rated at 120C or 130C MOT (Maximum Operatiog
Temperature). Which may be adequate assuming your customer spec ....
30C (maximum room temp)
55C (specified maximum temp rise)
=85C
However they should take care to review how UL defines the MOT criteria and
apply it to their end product use.
However, the UL MOT rating is tested at the bare board level so you should
check the PCB suppoer MOT rating for current products.
I doiubt an FR-4 with Tg 135C will meet 130C MOT and suggest you assume an
upgrade to 150C nominal, many such materials can qualify 130C MOT.
Regards
C.B. Katzko
Research + Development
Meadville Group
by Blackberry
----- Original Message -----
From: Patricia Lui [[log in to unmask]">[log in to unmask]]
Sent: 11/13/2009 10:42 AM ZE8
To: [log in to unmask]">[log in to unmask]
Subject: [TGAsia] PCBA operating temperature enquiry
Dear Friends,
I have a customer who used my PCB boards on laser machines and below are
the advise operating peak temperature and temperature cycle during
operations, they have also attached an emissivity balance thermal image, can
someone advise me will a standard FR4 material with Tg 135deg C sustain in
running at such an operating temperature? or customer need to change their
PCB requirement to a high Tg material? Can you identify any possible issue
if the boards are operating at such temperature environment? And assuming
that the boards are stored for one year before using, so it have absorbed
moisture after the components population and long storage.
1) Peak Temperature the Assembled Board may reach during operation.
– See point 3
2) Normal running temperature of the Assembled Board during operation.
– See attached emissivity balanced thermal image
3) Does the Assembled Board run at a sustained temperature or does the
temperature cycle? If so, what is the high and low cycle temperatures?
Assembled PCB in product would operate at a room temperature, but we specify
an operating ambient of 0degrees Celsius to +45degrees Celsius. Additional
heat input from the laser could increase the base temperature by a further
10 degrees Celsius. There is a temperature sensor in the device (adjacent to
the hottest component) which shut-down the laser above 55 degrees Celsius.
The operating environment for this PCB is considered benign and would not be
considered a problem. The thermal image indicates the hot-spots as being
power components mounted off-board from the PCB or components that are
designed to dissipate a controlled amount of power such as fuses, but not
PCB traces. Once the laser product is powered it normally remains in this
state, and reaches steady-state thermal equilibrium, which
inevitably deviates by a small amount as the laser operating conditions
are changed.
Hope to have your favourable advise.
Thanks & Best Regards
Patricia Lui
Xuan Technologies.
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