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Lead Bending and Soldering Considerations for International Rectifier's Power Semiconductor Packages

Posted: 27 Jun 2003     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2003JUN/A/2003JUN27_POW_ICP_AN1.PDF

Application Note

AN-1031

Lead Bending and Soldering Considerations for

International Rectifier's

Power Semiconductor Packages

By Doug Butchers and Mark Steers, International Rectifier

Introduction

This application note is intended to

address the two most frequently asked

package-related questions of IR's

power semiconductors.

These are:

7 How can the legs of through-

hole packaged devices be bent

safely without endangering part

reliability?

7 How can through-hole and

SMD parts be soldered

ensuring no damage to the parts

in the process?

Lead bending

Through-hole packaged parts are

mostly supplied to customers with the

leads projecting straight out of the

plastic body. Many practical power

circuits however use bulky heatsinks in

contact with the device tabs to enhance

thermal performance; but this may

preclude the straight-leaded orientation

arrangement of the standard part.

Consequently it is quite usual to

change the lead direction to make a

more convenient electrical connection

on an adjacent printed circuit board.

Where the customer carries out this

function, there are certain important

guidelines that should be observed.

(Please note that IR can offer a lead

bending option for many of the more

common variations- for further details

please contact your local Sales office-

details available via "contact us" on

the IR Web site www.irf.com )

Clamping

In order to limit any stress that the

bending action imposes on the

leads, it is essential to firmly clamp

leads at the body-refer to Figure 1.

The minimum distance that a bend

should start away from the plastic

body will vary from part to part -

Table 1 provides these values for

the most common power packages.

In general, the larger the clamp

area the more reliable the quality of

the lead form.

Under no circumstances must the

plastic body be held or restricted

while lead forming as this has the

potential to mechanically damage

the package-particularly at the

metal-to plastic interface.

Bend radius

IR recommends that the bend

radius should never be less than the

thickness of the lead material. The

general rule of thumb is to create a

radius that is one to two times the

material thickness- "T" in figure 1.

However, in certain design

constrained situations, where lead

length is critical, the radii may be

made equal to the leadmaterial

thickness.

With the majority of lead forms,

there will be an element of micro

cracking on the solder plating on

Application Note

AN-1031

the outer radii which will expose

the copper. This is not at all

uncommon and will not affect the

strength of the leads. However, if

the leadform radii are too small

compared with the lead thickness,

then deep cracks will appear,

leading to reliability issues at

some later stage of the part's

operating life. Refer to Table 1 for

recommended dimensions.

Figure 1. Material Thickness

Table 1. Recommended Leadform Dimensions

Package Outline X min.

(Clamping Dim)

"T" nominal

Super-247 2.75mm 0.6mm (dam bar area only)

Super-220 1.75mm 0.8mm

TO-247 2.75mm 0.6mm

TO-220 1.75mm 0.46mm

TO-220 Full-pak 1.75mm 0.46mm

Hand bending

Where lead bending is required on

relatively low quantities of parts, i.e.

for development, pre-production and

even limited pilot production, bending

may carried out by hand. It is still very

important to abide by the rules laid

down above. Clamp the leads with

pliers ensuring that static

precautions are fully observed.

For accuracy of bend it is better to use

un-tapered snipe-nose pliers.

Other considerations

There may be other compelling reasons

for having a lead bend apart from the

requirement to re-direct the leads

purely for connection purposes.

Stress relief

For some mechanical arrangements the

heatsink and the lead termination point

may be subjected to relative movement

(Figure 2). In these cases where these

forces are unavoidable, it is desirable

to introduce a stress-relievinglead

bend in order to re-position this stress.

With a simple bend of this sort, the

stresses which would normally have

degraded either the lead-to-body joint

or the lead-to-pcb joint, will be

absorbed along the lead length.

Application Note

AN-1031

Board clearance

Where leads are terminated in a

PCB or similar substrate, the

original in-line alignment of the

leads (Gate, Drain, Source for a

power MOSFET) may not

provide an adequate electrical

clearance/ creepage distance

between track pads. By

offsetting the centre leg as

shown in Figure 3 this problem

may be avoided.

Figure 2. Heatsink and Lead Termination

Plan view of hole arrangement

Figure 3. Center Leg Offset

Soldering

Through-hole parts

Most through-hole parts will be

soldered into circuit using Wave or

Dip soldering. The leads of IR's

through-hole parts are prepared with a

eutectic solder finish, which melts at

about185 0C.

Application Note

AN-1031

As a general rule for the soldering

process, restrict the temperature of the

leads, at 1.6mm from the body, to 300

0C for a maximum of 10 seconds. This

will avoid damage to the remainder of

the package.

The same temperature restrictions

should apply when hand soldering

parts.

SMD parts

SMD parts will be soldered to

substrates using a number of re-flow

techniques. The most common of

these are:

Convection reflow

Vapour phase reflow

Infra-red (IR) reflow *

*Generally not recommended for

packages larger than "D"-pak as the

"black body" absorption tends to

encourage too high temperatures.

In all cases the desired temperature

profile will consist of three main

stages:

Preheating

Soldering / reflow

Cooling

As a recommendation for a suitable

temperature profile, IR suggests the

guidance of the JEDEC specification

JESD22-A112-B. Refer to Table 2 for

these details.

As with many JEDEC standards this

specification provides recommended

ranges for parameters. Within those

limits there will be a large number of

individual temperature profiles used,

which provide variations on particular

profile features.

The preheating stage brings the parts to

the soldering temperature at a rate that

limits excessive thermal stresses

within the body of the device. This

period may be typically up to 120

seconds.

The soldering stage defines the time

necessary, at maximum temperature, to

reflow the solder paste and creating an

even solder layer between the part

heatsink and the substrate track.

Typical Reflow cycle

0 1 0 1 1 0 120 130 1 4 0 150 160

50

0

2 5 0

2 0 0

1 5 0

1 0 0

3 0 0

Time -seconds

Temperature-degC

Pre-heat

Reflow

Cooling

Application Note

AN-1031

Table 2. JEDEC Reflow recommendations

Convection or IR

/Convection

VPR

Average ramp-up rate (183 00 C to Peak) 3 00C / second max 10 00C / second max

Preheat temperature 125 (1 2501 250 C) 120 seconds

Temperature maintained above 183 00 C 60-150 seconds

Time within 5 00 C of actual peak temperature 10-20 seconds 60 seconds

Peak temperature range (220+5/-0) 00C or

(235+5/-0) 00C

215-219 00 C or (235+5/-0) 00 C

Ramp-down rate 6 00C / second 10 00C / second

Time 25 00 C to peak temperature 6 minutes max

SMD parts are finished with a standard 90 (Sn)/ 10 (Pb) solder plating.





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