LPC2921 datasheet(75/81 Pages) PHILIPS

75 / 81 page

LPC2921_2923_2925_0

Preliminary data sheet

Rev. 00.01 — 24 October 2008

75 of 81

NXP Semiconductors

LPC2921/2923/2925

ARM9 microcontroller with CAN and LIN

12.4 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 32) than a SnPb process, thus

reducing the process window

• Solder paste printing issues including 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). In addition, the peak temperature must be 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 38 and 39

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 32.

Table 38.

SnPb eutectic process (from J-STD-020C)

Package thickness (mm)

Package reflow temperature (

°C)

Volume (mm3)

< 350

≥ 350

< 2.5

235

220

≥ 2.5

220

Table 39.

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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