PCA88021 datasheet(29/34 Pages) NXP

29 / 34 page

PCA8802_1

Product data sheet

Rev. 01 — 19 February 2009

29 of 34

NXP Semiconductors

PCA8802

Smartcard RTC

16. Soldering of WLCSP packages

16.1 Introduction to soldering WLCSP packages

This text provides a very brief insight into a complex technology. A more in-depth account

of soldering WLCSP (Wafer Level Chip-Size Packages) can be found in application note

AN10439 “Wafer Level Chip Scale Package” and in application note AN10365 “Surface

mount reﬂow soldering description”.

Wave soldering is not suitable for this package.

All NXP WLCSP packages are lead-free.

16.2 Board mounting

Board mounting of a WLCSP requires several steps:

1. Solder paste printing on the PCB

2. Component placement with a pick and place machine

3. The reﬂow soldering itself

16.3 Reﬂow soldering

Key characteristics in reﬂow soldering are:

• Lead-free versus SnPb soldering; note that a lead-free reﬂow process usually leads to

higher minimum peak temperatures (see Figure 36) than a PbSn process, thus

reducing the process window

• Solder paste printing issues, such as smearing, release, and adjusting the process

window for a mix of large and small components on one board

• Reﬂow temperature proﬁle; this proﬁle includes preheat, reﬂow (in which the board is

heated to the peak temperature), and cooling down. It is imperative that the peak

temperature is high enough for the solder to make reliable solder joints (a solder paste

characteristic) while being low enough that the packages and/or boards are not

damaged. The peak temperature of the package depends on package thickness and

volume and is classiﬁed in accordance with Table 16.

Moisture sensitivity precautions, as indicated on the packing, must be respected at all

times.

Studies have shown that small packages reach higher temperatures during reﬂow

soldering, see Figure 36.

Table 16.

Lead-free process (from J-STD-020C)

Package thickness (mm)

Package reﬂow temperature (

°C)

Volume (mm3)

< 350

350 to 2000

> 2000

< 1.6

260

1.6 to 2.5

260

250

245

> 2.5

250

245

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