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Thermal Profile - Eutectic v/s Lead-Free BGAs

Special care must be taken in creating profiles for both eutectic and lead-free BGAs since it involves two different processes. The reason is the lead-free profile requires a higher peak temperature compared to the eutectic profile.

A leaded or eutectic profile is about 30 to 35ºC lower in temperature for melting points than that of a lead-free profile. If the proper thermal profile is not used, definite issues will arise.

The solder paste manufacturer plays a vital role in the overall scheme for creating the correct profile. It is important that the manufacturer’s solder paste specifications be used to set up an ideal thermal profile for optimum temperature settings.

Here is a sample of a reflow processing guidelines from a solder paste manufacturer for a leaded or eutectic BGA.

  • Straight ramp of 0.5 – 1.3ºC/sec to 200ºC - 235ºC peak.
  • Soak profile of 1.5 – 2.0ºC/sec to 145ºC - 160ºC soak for a max of 90 seconds.
  • Peak temperature of 200 ºC - 235ºC, TAL (Time Above Liquidus) of 40 – 120 sec, time to peak < 4 minutes is recommended.

Figure shows a good collapse for a eutectic BGA.

The following is a sample of a reflow processing guidelines from one of the solder paste manufacturer for a lead-free BGA.

  • Ramp @ 0.5ºC -2.0ºC/sec to peak temperature 230ºC - 250ºC.
  • Soak profile of 0.5 ºC-2.5ºC/sec to 130ºC - 180ºC soak between 45 to 90 seconds.
  • Peak temperature of 230ºC - 250ºC, TAL (Time Above Liquidus) of 40 – 80 sec.

As you can see, there is a significant difference in the processing guidelines between a eutectic and a lead-free reflow profile. Various defects are certain to occur if the wrong reflow profile is used.

 

Figure shows a good collapse in process for a lead-free BGA. The main reason that companies are switching to via in pad design is due to real estate on a layout. The via in pad allows the design team more room to run circuits between the pads which the regular “dog bone” vias won’t allow.

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