ST AN2639 APPLICATION NOTE

AN2639

Application note

Soldering recommendations and package information for Lead-free ECOPACK® microcontrollers

Introduction

STMicroelectronics microcontrollers support various types of Lead-free ECOPACK® package to meet customer requirements.

The mounting technologies are Surface mount technology (SMT), and Through hole technology (THT). Beside the available mounting technology, the choice is often driven by technical and economical concerns. This application note describes the various package types used for microcontrollers, introduces the different mounting technologies, and gives soldering recommendations.

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www.st.com

Contents	AN2639

Contents

1	Lead-free packages at STMicroelectronics . . . . . . . . . . . . . . . . . . . . . .		. 3
2	Microcontroller packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
3	SMD presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	3.1	Handling SMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
4	Soldering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	4.1	Soldering methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
		Bending leads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
		Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
	4.2	Soldering recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	4.3	Compatibility with leaded soldering process . . . . . . . . . . . . . . . . . . . . . . .	7
	4.4	Reflow soldering conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
5	SMD gluability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
6	Dry packing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
	6.1	Pop-corn effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	6.2	Dry pack opening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14

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AN2639	Lead-free packages at STMicroelectronics
1	Lead-free packages at STMicroelectronics
	STMicroelectronics is fully committed to environment protection and sustainable
	development and started in 1997 a voluntary program for removing polluting and hazardous
	substances from all devices. In 2000, a strategic program, named ECOPACK has been
	officially launched to develop and implement solutions leading to environment friendly
	packaging and ban progressively Pb and other heavy metals from our manufacturing lines.
	ECOPACK is a registered trademark of STMicroelectronics.
	STMicroelectronics ECOPACK products are RoHS compliant according to EU 2002/95/EC
	directive.
	For more detailed information please go to http://www.st.com.
Note:	RoHS stands for ‘Restriction of the use of certain Hazardous Substances’. The directive
	2002/95/EC of the European Parliament and of the Council of 27th January 2003 on the
	RoHS in electrical and electronic equipment entered into force on 13th of February 2003.
	The aim of this directive is to ban heavy metals (Lead, Mercury, Cadmium, Hexavalent
	Chromium) and two brominated compounds (PBB and PBDE). The directive will be
	applicable throughout EU by 1st July 2006.

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Microcontroller packages	AN2639

2 Microcontroller packages

The different packages available at STMicroelectronics for microcontrollers are described

Table 1.

Table 1.	Microcontroller package types(1)
	Surface mount technology		Through hole technology
Package		Pin count	Package	Pin count
SON		8,16	PDIP .3	8, 16, 20
SOW		16, 20, 24, 28, 34 Shrink	PDIP .4	32S
PLCC		44, 520	PDIP .6	28, 42S, 56S
PQFP		44, 52, 80, 100, 144, 208
LQFP		32, 44, 48, 52, 64, 80, 100, 128
LFBGA,TFBGA		Up to 144
PBGA		Up to 480
TSSOP		20
VFQFPN		8, 20, 24, 36, 40, 48, 64

1. Larger package portfolio can be proposed upon request.

Through hole technology (THT) and Surface mount technology (SMT) imply different soldering technologies leading to different constraints.

In THT, the package body is exposed to relatively low temperatures (< 150 °C) because the lead extremities are only dipped in the soldering alloy, whereas in SMT the whole package body is exposed to a very high temperature (> 240 ° C) during reflow soldering process.

In addition, molding compounds used for integrated circuit encapsulation absorb moisture from the ambient medium. During rapid heating in solder reflow process (see Section 4: Soldering for more details), this absorbed moisture can vaporize, generating pressure at lead frame pad / silicon to plastic interfaces in the package, with a risk of package cracking and potential degradation of device reliability.

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AN2639	SMD presentation

3 SMD presentation

Unlike through hole technology where leads are inserted into the printed circuit board, SMD (surface mount device) package is attached directly onto mounting pads of the substrate. SMT is extensively used in electronic applications because it has the following advantages:

●Packages are smaller and support higher pin counts

●Packages are light and compact, thus reducing system sizes

●Mounting can be done on either side of the PCB

●No cost for drilling holes into the PCB

Surface mount technology also comes along with a few disadvantages:

●Increased sensitivity to soldering heat because of their thinner dimension

●Soldering conditions harder to determine (use of finer structures and higher pin count)

3.1Handling SMDs

Though the intrinsic reliability of SMD packages is now excellent, the use of inappropriate techniques or unsuitable tools during mechanical handling can affect the long term reliability of the device, or even destroy it.

When handling a SMD package, it is strongly recommended to use adapted tools such as vacuum pipes to avoid touching the pins as much as possible. Manual handling could affect lead coplanarity and cause lead contamination or scratches that could generate solderability problems. It is also not allowed to widen the interval between two consecutive pins.

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