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Inspection Standard for Solder Joints
All text Copyright Bob Willis, EPS
1.
INTRODUCTION
In view of the high reliability requirements of products supplied by the
Electronics Industry, the establishment of a quality standard for
soldered joints is necessary. The characteristics determined herein
represent the minimum requirements for the types of soldered joints that
are typical for printed board assemblies, containing conventional
components, surface mount components and wire terminations.
Successful soldering depends, to a large degree, on a number of
individual contributing factors, e.g. solder alloy, paste type, flux, wave
form, soldering iron tip, reflow temperature, design of soldering lands,
the shape and material of component leads, materials and
finishes.
The method used to produce soldered joints, without affecting the
reliability, may affect the visual appearance. In this respect hand
soldered joints may vary from machine soldered joints, as will the
appearance when using other production processes. This specification
takes these into consideration.
2.
SCOPE
This specification is designed to cater for the type of printed board
assembly manufactured within the Electronics Industry. Other
specifications (typically customer specifications) should be considered
as they apply. Where required Inspection Departments should be
instructed and training to the appropriate standards.
For the purposes of visual evaluation pictorial examples of all
types of soldered joints should be provided and described. These apply
to:Non-plated through hole boards and plated through hole boards. For the
purposes of this specification multi-layer printed board assemblies are
considered as being plated through hole boards.
Solder joints for wire terminations, the preparation and pre-tinning are
also considered.
The
specification
relates
to
joints
formed
on lightweight
components, i.e. under 20 grams, where the soldered joint acts as a
mechanical and electrical connection. On heavier components
consideration needs to be given for additional mechanical fixing, where
this specification should still apply.
The specification is applicable (unless countermanded by product design
documentation) to all printed boards assembled by the company or by
sub-contractors.
3.
CRITERIA FOR THE ASSESSMENT OF SOLDERED
JOINTS
3.1
Solderability
Solderability can be defined as the ability of a surface or surfaces
to be wetted with molten solder resulting in the formation of
a smooth, continuous solder film or fillet.
Reference should be made to the Specification for Solderability
contained within company quality procedures.
Good solderability of all surfaces to be joined is a pre-requisite
for the production of good quality soldered joints.
Poor solderability results when the surfaces to be joined are not in
the correct combination. This
results
in
non-wetting,
de-wetting or a combination of both and gives rise to poor quality
soldered joints.
Inspection of soldered joints should be conducted under
conditions of adequate lighting with use of approximately x5
magnification. If required, a maximum of x10 magnification
may be used in cases where the joint standard cannot be judged, at
x5.
An acceptable joint can be judged by the wetting produced
between the component and the board. Limited penetration of
solder on boards containing plated through holes, provided it is
not widespread, is acceptable but the process, components and
board should be investigated to assess the cause.
3.2
Non-Wetting
Non-Wetting is the incomplete wetting of the base metal or
metals and surfaces to be joined which results in the formation
of discontinuous solder fillets.
In non-wetted areas, the colour of the surfaces to be joined is
visible. Hence, where the base metal is copper, non-wetting is
clearly distinguished. However, on plated through hole printed
wiring boards with a tin/lead coating on the conductor and lands,
non-wetting may not be clearly visible.
3.3
De-Wetting
De-Wetting of a solder coating presents an appearance similar to
that of water lying on a grease-contaminated surface. It arises
when the molten solder, after initially wetting the surface,
retracts into discrete globules and ridges which exhibit high
contact angles. The areas between the globules and ridges retain a
solder colour, even when the base metal is copper, but the
solder coating in these areas is extremely thin.
4.
CHARACTERISTICS OF SOLDERED JOINTS
Good and bad characteristics of soldered joints appear in many
combinations and to various degrees. The assessment of soldered joints
is subjective and evidence of some poor characteristics does not
necessarily indicate a faulty joint.
4.1
Good Characteristics

Good solder wetting of the component lead or termination
and PWB land.

Formation of a solder-cone with a concave fillet, the solder
cone tapering off evenly towards the tip of the component
lead.

Evidence of capillary action in plated through holes on
mixed technology boards (viewed from the components
side of the PWB).

Sufficient quantity of solder on the soldered join with the
profile of the component lead or wire visible.

Smooth and shiny appearance, generally. High temperature
solders may have a dull appearance.
4.2
Poor Characteristics

Incomplete wetting of component lead or termination or
PWB land.

Insufficient solder leaving the component connection
exposed.

Insufficient penetration of solder in plated through holes.

Excess solder, where it appears widespread.

Voids and surface contamination both metallic and non
metallic.

Solder spikes and bridges on the component lead or PWB
land.
When examining solder joint wetting angles, consideration should be
given to the board design and land areas as this may affect the shape of
the joint. Depending on the process used for assembly this will also
affect the appearance. In cases where too much solder has been applied
this may give the appearance of a bad wetting angle. Where this is
evident, the process of assembly shall be examined.
4.3
Classification of Soldered Joints
During the assessment of soldered joints by Inspectors and
operators the following classifications, which are pictorially
illustrated should be considered.
a)
Satisfactory
This is a satisfactory condition which should be achieved
and used as the standard for manufacture.
b)
Acceptable
This condition represents the maximum acceptable
departure from the 'satisfactory' condition. Joints within
this limit of deviation will not require reworking.
Individual clarification accompanies each illustration.
c)
Unacceptable
This applies to an unacceptable joint condition which
should not be reworked without the causes of the fault
being established. Rework may be possible after the
assessment of the fault.
d)
Investigation of Soldering Process
The Production Department should examine the soldering
process and the condition of the component termination
points when the joint quality falls and remains within the
standards of Acceptability. It should not, however, be
cause for rejection.
e)
Criteria for Reworking Joints
As referred to above, reworking is normally only
considered on joints judged to be Unacceptable. The reason
why the joint is Unacceptable should be established prior to
commencing the rework since this may well have a bearing
on whether successful reworking is achievable.
5.
FEATURES OF SOLDER JOINTS ON SURFACE MOUNT
BOARDS
5.1
General
It should be noted that with surface mounted technology the
solder joint may be the only means of attachment between the
component and the board. Consideration must be given to this
point during design to ensure the ability to manufacture, test and
inspect the board.
The visual standard provided via illustrations is an aid when
determining the acceptability of the joints on the connected
component. The figures provide an illustration of Satisfactory,
Acceptable and Unacceptable soldered joints and are marked
accordingly.
The visual appearance of solder joints on surface mounted
components may vary due to the type of process used to assemble
and solder the components. Surface mounted components must all
meet the standard for solderability contained within company
specifications.
Inspection of solder joints should be conducted under conditions
of adequate lighting with use of approximately x 5 magnification,
if required. A maximum of x 10 magnification may be used in
cases where the standard cannot be judged, at x 5.
There should be no evidence of solder balls or slivers on the
board. If present, the process must be investigated to determine
the cause.
Components which are soldered in place using wavesoldering will
be glued to the board. Minimal adhesive on components
terminal points is acceptable, provided that it does not reduce the
minimal solder fillet. Evidence of terminal contamination by
adhesive must be examined and corrective action taken before
further production.
Any evidence of poor solderability or leaching must be
investigated and tests for solderability on remaining stocks
should be conducted. Repair work on boards containing surface
mounted components need special consideration. Reference
should be made to relevant section repair manuals.
5.2
Resistor, Capacitors with Metallised Terminations
Solder joints shall exhibit a visible solder fillet between the pad
and component termination. The fillet should rise to a minimum
of 25% of the terminal, with evidence of good wetting. A
continuous fillet shall be visible around 75% of the metallised
area and termination.
Evidence of leaching on metallised components is unacceptable.
This is an indication that a problem exists with the components or
the process. Both should be examined for possible correction.
If leaching has occurred, it will generally be visible on the corners
or edges of the metallisation.
Due to their construction
capacitors may delaminate or crack and evidence of this will
require removal and investigation of components and process.
No more than 25% of the component termination should overhang
the pad, provided
that the minimum clearance between
conductors is maintained. Components should ideally be flat to
the surface of the pad.
5.3
Melf Resistors and Round Leads
Solder joints on round contacts shall exhibit a visible fillet
between the pad and component termination. The fillet should
rise to a minimum of 25% of the terminal, with evidence of good
wetting. The outline of the termination should ideally be visible
in the solder fillet. A continuous solder fillet shall be visible
around 75% of the termination.
5.4
SO and SOT Packages
Leaded chip carriers solder joints on flat leads shall exhibit a
visible fillet between the pad and component. The fillet should
rise to the top of the lead with the outline of the component lead
ideally visible in the joint. In the case of flow soldered joints the
lead form is not generally visible. A continuous solder fillet shall
be visible around 75% of the lead.
Minimal marks left by probes used to hold components in
position during reflow are acceptable. The heel fillet shall be
continuous between the heel of the lead and the pad, wetting shall
extend to a midpoint between upper and lower bend as a
maximum. A solder fillet shall be visible to a minimum of half the
lead thickness.
Any sign of non-wetting or de-wetting should require
examination of boards and components for satisfactory
solderability.
Leads may be raised off the pad surface provided this does not
exceed two lead thicknesses, however a good solder fillet must
still be visible and meet the requirements of this specification.
Leads may have a side overhang provided that it does not exceed
25% of the lead width and the minimum clearance between
conductors is maintained. Any machine misplacement found or
damaged leads must be investigated for correction on future
production.
Gull wing chip carriers shall exhibit the same soldering
requirements as other leaded devices. Leads, not having wettable
areas by design are not required to have fillets, however, the joints
shall permit easy inspection of all wettable surfaces.
Inspection consideration should be given to the condition of all
lead frames and their correct plane height. Any excessive
variation should be investigated for causes of damage whether it
be assembly or poor packing.
5.5.
Leadless Chip Carriers
Solder joints shall exhibit a visible solder fillet between the pad
and termination. A joint is not required where no pad is present.
The solder fillet should rise to a minimum of 25% of the
metalised termination with evidence of wetting between
casterlation terminal and pad.
Overhang of the terminal is allowable to a maximum of 25%
provided the minimal joint is present.
All termination points should ideally have a tin/lead finish prior to
processing. Components should be obtained in this condition
from the supplier.
For further information on EPS services Email Bob Willis or visit his Internet
Home Page.
Email: [email protected]
Home Page: http://www.bobwillis.co.uk