Glass Transition and Decompsition Temps for Lead Free

What is the correct glass transition temperature (Tg) and decomposition temperature (Td) for the lead free process?

R. M.

Expert Panel Responses

The glass transition temperature should be determined using the Thermo-Mechanical Analysis (TMA) method as per IPC-TM-650, 2.4.24C[3]. The TMA method is preferred over the other two methods sometimes used to determine the glass transition temperature, DSC[4] and DMA[5], because the thermal expansion of the PCB is a critical parameter, which is given by TMA as a function of temperature. For PCBs subject to soldering processes using the more elevated soldering temperatures required for lead-free solders, specifying a glass transition temperature, Tg, of 140C will not be adequate for PCBs thicker than about 50 mils. Furthermore, for thicker PCBs, it is recommended that a minimum decomposition temperature, Td, determined as per IPC-TM-650, 2.4.24.6[6] as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z), determined as per IPC-TM-650, 2.4.41[7] be specified. CTE(z) values should be given separately for temperatures below Tg and above Tg; however, frequently the thermal expansion, TE in %, is lumped together from 50 to 260C or even 50 to 288C. Typically, the decomposition temperature is given as Td(5%) to a 5% weight loss; the decomposition temperature, Td(2%), to a 2% weight loss, has been found a very good indicator, but is not as yet widely available. Frequently, the time to delamination, either T-288 or T-260, are specified either in addition to Td or instead of it. The T-288 delamination time provides a more appropriate level of performance given the process temperature required for LF-soldering. The delamination time is sometimes combined with the requirement that that temperature needs to be survived for 4 to 5 excursions. The easiest way to specify the three properties critical for the survival of the PCB and the PTH/via interconnect structure - Tg, Td, thermal expansion (TE) - is by specifying a minimum Soldering Temperature Impact Index, STII, which is defined as STII = Tg/2 + Td/2 - (TE%(50 to 260C) x 10). For PCBs with thicknesses of 0.06 inches (1.5 mm) or more, an STII-value of 215 or larger is recommended. However, the STII-concept is not widely used as yet. Thanks to Werner Engelmaierfor all the above information. I had this data since fall of 2006.

Terry Jeglum
President/CEO
Electronic Technology Corporation

Mr. Jeglum has 35+ years experience and is the founder of Electronic Technology Corporation. He is responsible for 22 years of program management for the Company.

Requirements are primarily driven by peak temperature, which is driven by component and board size / thickness / complexity. With small components and a small board, Tg between 130 and 150C can be sufficient. Larger components and a larger board tend to need Tg closer to 170C to 180C.

Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions

Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

Tg and Td are important material properties, as they influence the resin system in organic substrates. In lead-free processing,

Peak assembly temperatures can reach a point where resin decomposition begins.
Higher temperatures result in increased thermal expansion and moisture absorption and stress on plated holes
Vapor pressure of the absorbed moisture at lead-free assembly temperatures can lead to blistering/delamination

Glass Transition Temperature (Tg): The temperature at which the resin system changes from a rigid or hard material to a soft or rubber like material. Issue: Several properties change as the Tg is exceeded, including the rate at which a material expands with temperature. Temperatures above Tg results in,

Higher moisture permeability
Higher CTE
Reduction in modulus

Decomposition Temperature (Td): The temperature at which material weight changes by 5%. This parameter determines the thermal survivability of the resin material. Issue: Resin decomposition can result in adhesion loss and delamination. The chemical bonds break upon exceeding Td resulting in permanent degradation and damage to the material. Preferred attributes for Pb-free Assembly: Decomposition Temperature (Td) >340C For standard thickness printed wiring boards (1.6mm) a material with a glass transition temperature of greater than 170C is recommended as a minimum requirement

Bjorn Dahle
President
inspīre solutions LLC

Bjorn Dahle is the President of inspīre solutions LLC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

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