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 package s
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 be tween STM icroel ectroni cs, Infi neon Tec hnolo gies, Philips Semicond uctors 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.

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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 co ntr ib uti on of s older b all s in the s old er jo in t ma ter ia l is ty pical ly o f 70 % to 80%,
which explains why fr om the thermal point of view a nd for the s older joint’s m etallurgical 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

Leaded component Tin-Lead
finish

Lead-free compon en ts Tin
finish

Supplier

Lead-free compon en ts
NiPdAu finish

Lead-free compon en ts Tin
Bismuth finish

Leaded Solder Lead-free Solder

Today and past
majority of cases

Processability: OK
Solderability: OK
Reliability: OK

Processability: OK
Solderability: OK
Reliability: OK

Processability: OK
Solderability: OK
Reliability: not recommended

Customer

Processability: OK
Solderability: OK
Reliability: OK

Processability: OK
Solderability: OK
Reliability: OK

Processability: OK
Solderability: OK
Reliability: OK

Processability: OK
Solderability: OK
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 stand ard J-STD020 is the refer ence
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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