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AN10365_3

© NXP B.V. 2008. All rights reserved.

Application note

Rev. 03 — 22 April 2008

20 of 24

NXP Semiconductors

AN10365

Surface mount reflow soldering description

Another visual aspect in Pb-free soldering is that Pb-free solder joints tend to be less
shiny than SnPb solder joints, and they may have striation marks. This is due to the
different microstructure that is formed during solidification. Although SnPb solder joints
should be rejected if they look this way, this is normal for Pb-free, and no reason to reject
Pb-free solder joints.

5.2 Repair

Sometimes, a package lead that has not been soldered properly may be repaired simply
by heating this single lead with the tip of a soldering iron. In that case, it is sufficient to
heat the lead until the solder melts completely, and a new device should not be necessary.

In other situations, however, there may be a need to replace an IC package on the board.
In that case, the rework process should consist of the following steps:

1. Device removal

2. Site preparation

3. Application of solder paste to the site

4. Device placement

5. Device attachment

It is advised to dry bake the board for 4 hours at 125

°

C, prior to steps 1 to 5.

a. SnPb solder joints

b. Pb-free solder joints

Fig 18. Difference in wetting between SnPb and Pb-free solder joints

a. SnPb solder joints

b. Pb-free solder joints

Fig 19. Difference in appearance between SnPb and Pb-free solder joints

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Содержание AN10365

Страница 1: ...low soldering description Rev 03 22 April 2008 Application note Document information Info Content Keywords surface mount reflow soldering Abstract This application note provides guidelines for the board mounting of IC packages ...

Страница 2: ...nd an email to salesaddresses nxp com NXP Semiconductors AN10365 Surface mount reflow soldering description Revision history Rev Date Description 03 20080422 various text amendments to Section 1 2 1 2 2 2 3 3 4 1 4 2 4 3 and 6 02 20060726 updates in Table 1 Table 8 and on page 20 the minimum peak reflow temperature when using SnPb solder is changed from 210 C to 215 C 01 20050524 initial version ...

Страница 3: ... as mechanical carriers for electronic components they also provide the electronic interconnection between these components and also between these components and the outside world These electronic components may be ICs or other types such as capacitors and resistors Through component selection and the use of Cu interconnections between the components an electronic system such as a mobile phone can...

Страница 4: ...ons of the solder lands on the PCB to make reliable solder joints between the IC package and the PCB The PCB footprints of NXP Semiconductors packages can be found by clicking Package Information on the Product Information page of the NXP Semiconductors web site at the URL given in Contact information at the bottom of page 2 The unique identifier for the PCB footprint is the NXP package outline co...

Страница 5: ...for the SOT266 1 package SSOP20 DIMENSIONS in mm Ay By D1 D2 Gy Hy P1 7 200 4 500 1 350 0 400 C 0 600 6 900 5 300 Gx 7 450 Hx 7 300 0 650 SOT266 1 solder land occupied area Footprint information for reflow soldering of SSOP20 package Ay By Gy C Hy Hx Gx P1 Generic footprint pattern Refer to the package outline drawing for actual layout P2 0 125 0 125 D1 D2 4x P2 0 750 001aaf255 ...

Страница 6: ...cludes accuracies in the process and IC package board and stencil tolerances The footprint design is directly related to these aspects of the soldering process the calculation of these dimensions is based on process parameters that are compliant with modern machines and a state of the art process A solder resist layer also known as a solder mask layer is usually applied to the board to isolate the...

Страница 7: ...ay from the solder land to prevent bleeding of the solder resist onto the Cu pad Typical values for these distances are 50 µm to 75 µm The footprints referred to in this document indicate the areas that can be soldered The footprint shown in Figure 2 is redefined for both a solder resist and a Cu layout in Figure 5 Note that the overlap gap between the solder resist and the Cu is 0 05 mm in this p...

Страница 8: ...ends on the package family SnPb finish is still used but only for selected applications Leaded packages are reflow solderable standard gull wing packages are wave solderable only if the lead pitch is equal to or larger than 0 65 mm and no exposed heatsink is present wave soldering smaller pitches will lead to a higher defect level Leadless packages with solder balls e g BGA and TFBGA These IC pack...

Страница 9: ...dless packages with solder lands are reflow solderable only they cannot be wave soldered IC packages may have heat sinks at the top or the bottom of the package A few extra remarks are made on IC packages with heat sinks at the bottom such as HVQFNs Even if the exposed pad does not have to be soldered to the board for electrical or thermal purposes the package reliability may improve if it is sold...

Страница 10: ...eflow For more information on the solder paste please contact your solder paste supplier 3 Moisture sensitivity level and storage If there is moisture trapped inside a package and the package is exposed to a reflow temperature profile the moisture may turn into steam which expands rapidly This may cause damage to the inside of the package delamination and it may even result in a cracked IC package...

Страница 11: ...spected at all times Naturally this also means that IC packages with a critical MSL may not remain on the placement machine between assembly runs Nor should partly assembled boards between two reflow steps be stored longer than indicated by the MSL level The IC package floor life as a function of the MSL can be found in Table 2 Fig 10 Example of MSL information on sticker note the two MSLs corresp...

Страница 12: ...ongst others on the loading and speed of the squeegee the board separation speed the printing direction and the aperture orientation In essence all of these parameters must be adjusted so that all solder paste deposits on one board from the smallest to the largest are printed properly Consequences of insufficient solder paste printing are usually open contacts or bad joints These may arise because...

Страница 13: ...n stencil aperture dimensions that are 50 µm smaller than the corresponding solder land dimensions see Figure 12 This rule does not apply for BGAs for BGAs the solder paste deposit is shown explicitly in the PCB footprint specification Although BGA balls and their solder pads are circular square stencil apertures are sometimes preferred for BGAs Another exception lies with the very large solder la...

Страница 14: ...tern the paste plus the area between the deposits has a length of 3 02 mm The pattern area 3 02 mm 3 02 mm divided by the land area yields a paste pattern coverage of approximately 35 Depending on the solder paste used the solder paste deposits printed on a large land may not always coalesce completely In some cases individual solder joints can still be recognized between the exposed die pad and s...

Страница 15: ...component mounters or chip shooters can be used These machines can process up to 100 000 components per hour The placement force may also be an important parameter for some packages In theory an IC package is always pressed down into the solder paste until it rests on a single layer of solder paste powder particles the rest of the solder paste is pressed aside A consequence that is immediately app...

Страница 16: ...below the temperature at which the components and boards are damaged 3 Cooling down the board is cooled down rapidly so that soldered joints freeze before the board exits the oven The peak temperature during reflow has an upper and a lower limit 1 Lower limit of peak temperature measured at the solder joint the minimum peak temperature must be at least high enough for the solder to make reliable s...

Страница 17: ...r small solder paste deposits a fast temperature profile is preferred The cold spots on the board will warm up far more slowly The oven settings should be planned so that the cold as well as the hot spots will have reached roughly the same temperature by the end of the preheat phase The second phase in the reflow profile is the reflow zone in which the solder melts and forms soldered joints The mi...

Страница 18: ...ualification conditions At the bottom left some regions on the board are exposed to temperatures that are too low resulting in unreliable joints At the right all of the regions on the board have peak temperatures that fall within the upper and lower limits Reflow may be done either in air or in nitrogen In general nitrogen should not be necessary in that case air is preferred because of the lower ...

Страница 19: ... need 215 C so they will also melt properly OK Pb free solder balls are combined with a SnPb paste in this case only the Pb free balls require a higher reflow temperature whereas the rest of the process does not therefore this combination is not advised Pb free solder balls are combined with a Pb free paste now the paste requires the same reflow temperature and so do the solder balls OK The text a...

Страница 20: ...epaired simply by heating this single lead with the tip of a soldering iron In that case it is sufficient to heat the lead until the solder melts completely and a new device should not be necessary In other situations however there may be a need to replace an IC package on the board In that case the rework process should consist of the following steps 1 Device removal 2 Site preparation 3 Applicat...

Страница 21: ...hat package removal should not be initiated until the solder has melted entirely Re use of removed IC packages is not recommended 5 2 2 Site preparation After the device has been removed the board area must be prepared for the new device Prepare the site by removing any excess solder and or flux remains from the board Ideally this can be done on an appropriate de soldering station using solder wic...

Страница 22: ...ope or split beam system possibly in a special repair station If this is not possible try to align the device with board markers 5 2 5 Soldering To reflow the solder apply a temperature profile that is as close as possible to the original reflow profile used for assembling the board Take care that the board and or IC package are not moved or tilted until the solder has solidified completely Note t...

Страница 23: ...published in this document including without limitation specifications and product descriptions at any time and without notice This document supersedes and replaces all information supplied prior to the publication hereof Suitability for use NXP Semiconductors products are not designed authorized or warranted to be suitable for use in medical military aircraft space or life support equipment nor i...

Страница 24: ...s 1 Introduction 3 2 Materials 3 2 1 Printed circuit boards and footprints 3 2 2 IC packages 7 2 3 Solder paste 10 3 Moisture sensitivity level and storage 10 4 Surface mounting process 12 4 1 Solder paste printing 12 4 2 IC package placement 15 4 3 Reflow soldering 16 4 4 Solder and terminal finish or solder ball compatibility 18 5 Inspection and repair 19 5 1 Inspection 19 5 2 Repair 20 5 2 1 De...

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