ST AN2034 APPLICATION NOTE

AN2034

APPLICATION NOTE

Soldering Compatibility (Backward and Forward)

COMPATIBILITY OF LEAD-FREE COMPONENTS

The semiconductor industry is moving towards the elimination of lead from packages, in accordance with new international regulations.

This concerns both solder paste, for the board mounting process, as well as semi-conductor packages themselves.

A major impact is on the solder reflow process step, as most of the lead-free alloys under investigation for solder paste melt at temperatures that are 30 to 40°C higher than eutectic SnPb (Tin, Lead) solder.

For a transitional period, leaded and lead-free technologies already coexist, either by having leaded packages mounted with lead-free paste or by having eutectic Sn/Pb paste used with lead-free components.

The tight collaboration between STMicroelectronics, Infineon Technologies, Philips Semiconductors and Freescale (E4 group) lead to the proposal of consistent and compatible solutions to customers and made it possible to promote the necessary standardization.

ST Lead-free Finishing Solutions

For lead-frame based packages

■Post-plated of matte Tin for insertion components (DIP), Power packages and some Surface Mount Technology Components (PLCC).

Note: Post-plated SnBi finishing in some products is used only as an exception, based on customer request.

■Hot dipped tin coating

■Pre-plated of 3 layers of NiPdAu coating for Surface Mount Technology signal components (SO and QFP families mainly)

For Ball Grid Array packages:

■SnAgCu (SAC) solder balls.

November 2004

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AN2034 - APPLICATION NOTE

PACKAGE FINISHING AND SOLDERING PROCESSES COMPATIBILITY

For component coating materials and board solder materials to be compatible, the three criteria below have to be met:

■processability

■solderability

■solder joint reliability

For Lead-frame packages, the contribution of connection coating in solder joint material is typically of 10% to 20%. The solder paste is the major contributor.

For BGA packages, the contribution of solder balls in the solder joint material is typically of 70% to 80%, which explains why from the thermal point of view and for the solder joint’s metallurgical uniformity, the SnPb soldering profile may not be suitable for Lead-free BGA assembly

For compatibility assessment purposes, Lead-frame and BGA packages therefore have to be considered separately.

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AN2034 - APPLICATION NOTE

LEAD-FRAME PACKAGE COMPATIBILITY

The present situation, with the on-going conversion of leaded electronic components and board mounting processes to Lead-free finishing and mounting processes leads us to consider three new cases (as summarized in Table 1.):

■Customers who use leaded solder on lead-free components (backward compatibility)

■Customers who use lead-free solder (and a 30 to 40°C higher soldering temperature) on leaded components (forward compatibility)

■Customers who use lead-free solder on lead-free components

Table 1. Forward and Backward Compatibility: Lead-frame Based Components

	Lead-frame Based Packages		Customer
			Leaded Solder	Lead-free Solder
		Leaded component Tin-Lead	Today and past	Processability: OK
				Solderability: OK
		finish	majority of cases
				Reliability: OK
		Lead-free components Tin	Processability: OK	Processability: OK
			Solderability: OK	Solderability: OK
		finish
			Reliability: OK	Reliability: OK
	Supplier
		Lead-free components	Processability: OK	Processability: OK
			Solderability: OK	Solderability: OK
		NiPdAu finish
			Reliability: OK	Reliability: OK
		Lead-free components Tin	Processability: OK	Processability: OK
			Solderability: OK	Solderability: OK
		Bismuth finish
			Reliability: not recommended	Reliability: OK

Processability

Components are warranted to withstand the soldering temperatures required for soldering them with standard alloys (SnPb) and with Lead-free alloys (SnAgCu). Jedec/IPC standard J-STD020 is the reference standard used to ensure the soldering resistance of components.

■For reflow soldering: paste application and component placement do not require specific actions

■For wave soldering: glue application, component placement and glue cure do not require specific actions

■The main differences in the soldering process are time and temperature (both of which are higher when using lead-free solder pastes)

International standards for the soldering resistance of components, such as IPC/ JEDEC J-STD020, should not be confused with the required profiles for the soldering of components on board.

The aim of the soldering resistance profile is to describe the maximum heat that components have to withstand during soldering. The soldering process is possible at lower temperatures: see Table 2.

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