PCF8564AUSLASH5GESLASH1 datasheet(39/44 Pages) NXP

39 / 44 page

PCF8564A_1

Product data sheet

Rev. 1 — 8 October 2009

39 of 44

NXP Semiconductors

PCF8564A

Real time clock and calendar

For further information on temperature proﬁles, refer to application note

AN10365

“Surface mount reﬂow soldering description”.

19.3.1 Stand off

The stand off between the substrate and the chip is determined by:

• The amount of printed solder on the substrate

• The size of the solder land on the substrate

• The bump height on the chip

The higher the stand off, the better the stresses are released due to TEC (Thermal

Expansion Coefﬁcient) differences between substrate and chip.

19.3.2 Quality of solder joint

A ﬂip-chip joint is considered to be a good joint when the entire solder land has been

wetted by the solder from the bump. The surface of the joint should be smooth and the

shape symmetrical. The soldered joints on a chip should be uniform. Voids in the bumps

after reﬂow can occur during the reﬂow process in bumps with high ratio of bump diameter

to bump height, i.e. low bumps with large diameter. No failures have been found to be

related to these voids. Solder joint inspection after reﬂow can be done with X-ray to

monitor defects such as bridging, open circuits and voids.

19.3.3 Rework

In general, rework is not recommended. By rework we mean the process of removing the

chip from the substrate and replacing it with a new chip. If a chip is removed from the

substrate, most solder balls of the chip will be damaged. In that case it is recommended

not to re-use the chip again.

MSL: Moisture Sensitivity Level

Fig 34. Temperature proﬁles for large and small components

001aac844

temperature

time

minimum peak temperature

= minimum soldering temperature

maximum peak temperature

= MSL limit, damage level

peak

temperature

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