12 Flexible Printed Circuit Boards
13.14 Quality Control of Solder Joints
Solder joints need to be inspected and checked before being accepted as good and reliable. In machine soldering, it is extremely important to continuously monitor the soldering process in order to provide the feedback necessary for maintaining the solder line under best operating conditions.
The present day trend is to employ computer assisted inspection techniques.
The very look of a solder joint would normally provides a good indication of a proper job done.
If a surface is not adequately wetted by the solder, a good joint cannot be achieved. A proper solder joint can be achieved by:
a Using the right temperature, approximately 30 °C to 35 °C above the melting point of the solder as well as the metal being joined.
a Clean and deoxidized metal surface.
a Using the proper and non-contaminated solder
a Use of proper flux in order to remove oxides and prevent new oxidation while soldering.
a Keep the contact time between the base metal and the molten solder as short as possible.
In addition to the optical inspection as well as monitoring the process parameters during the soldering process, the complete evaluation of a solder joint involves many other tests and procedures.
Some of the commonly used tests are:
Mechanical Tests:
a Pull Test a Vibration Test
a Micro-section: measuring the intermolecular bond thickness which should be between 0.5 mm and 1 mm.
The structure of the intermolecular bond is made by crystals which tend to grow under the influence of higher temperature and as a function of time. Absence of crystals does not provide sufficient physical strength, where as larger crystals reduce the bond strength. Therefore, it is necessary that proper intermolecular bond is established.
Electrical Functions:
a Resistance Testing: It is often not very helpful because even a bad joint may sometimes show a very low resistance. The difference sometimes will not be discernible.
a Joule Test: It is carried out by providing a constant current to solder joints in series. They heat up to a different extent according to their different resistances.
a X-Ray Inspection: This is quite often integrated in the equipment line, like automated optical inspection (AOI), especially for BGAs.
13.14.1 Good Quality Solder Joints
A good quality solder joint is a shining and smooth surface with an intermolecular bond of less than 1mm and more than 0.5 mm. The shape of the solder joint should be concave formed by a small wetting angle. The angle should be less than 90°. However, it is preferable to have it less than 40°.
Solder surface should be smooth and finely grained and should not show blow holes, voids, inclusions or cracks. The placement of the SMD components should be accurate so that more than 75 per cent of the component termination cover the land or solder pad.
13.14.2 Common Soldering Faults
The standard “IPC-A-610-C, Acceptability of Electronic Assemblies” provides the basis for solder joints that are acceptable or otherwise. The common soldering faults are detailed below.
13.14.2.1 Inaccurate Placements/Misalignment
The guidelines for inaccurate placements for different kinds of SMD components are provided in the standard IPC-A-610-C. For different shapes of component terminations, the following criteria applies (Figure 13.36):
Soldering, Assembly and Re-Working Techniques 523
Fig. 13.36 Acceptance criteria for inaccurate placements and misalignments
a It is a defect for class 3 if the side overhang A is larger than 25 per cent of component termination width W or 25 per cent of land width P, whichever is less.
a It is a defect for Class 1 and 2 if the side overhang A is larger than 50 per cent of component termination width W or 50 per cent of land width P, whichever is less.
From the above, it may be concluded that a solder joint is rejected if the solder covers < 75 % of the edges of the SMD terminals resting on the pad for Class 3 and for Classes 1 and 2, it should be
< 50 %.
However, these rules for misalignment are only valid for the side overhang A and the end joint widthW. They are not valid for the axially situated side joint length. Any axial side joint length is acceptable if all other joint parameter requirements are met, but the terminations of ‘Rectangular or Square End Components’ must not overhang the land.
13.14.2.2 Non-wetting
Non-wetting or poor wetting occurs when the solder does not wet the PCB completely. Therefore, the board must be inspected in its entirety, and not joint by joint. There can be a number of causes for non-wetting, of which the most typical in wave soldering are:
a Presence of contamination such as oil, grease, etc. on the surface to be soldered, which may prevent the flux from coming in to direct contact with the surface;
a Inadequate solderability of the base metal;
a Unsuitable flux for the surface to be soldered; and
a Improper soldering conditions such as improperly controlled time, and temperature cycle during the soldering process.
In hand soldering, non-wetting occurs due to insufficient heating of the joint, improper lack of flux and lack of solderability of the surfaces.
In fact, non-wetting is a serious defect and calls for stopping the manufacturing process, if a significant number of joints (say 5 per cent or more) are found to be defective.
13.14.2.3 De-wetting
De-wetting is a condition in which the molten solder wets the complete pad/land and because of low adhesion, it forms an irregular film which may be every thin or very thick at places. It implies that the molten solder withdraws from the base metal after initial wetting and forms irregular droplets. De-wetting is generally caused by certain types of contaminants on the surface of the base metal, for example, the contaminant may be embedded in the cleaning abrasives. Similarly, metallic impurities, present in sufficient concentrations in the solder bath can also result in de-wetting. Another cause of de-wetting is the use of wrong flux. De-wetting is acceptable only if at least 75 per cent of the land size meets the solder joint criteria and the angle between the solder and the thin coated area is less than 90° for 75 per cent of the circumference (Figure 13.37). Re-soldering a board with dewetting on wave soldering equipment usually does not improve the situation.
The only way to re-work on such a joint is to mechanically remove it from the surface to be soldered with a fine sand paper down to the copper and then resolder it.
Non-wetting and de-wetting are mostly caused by surface oxides during improper storage or by deposits of other contaminants as well as by large crystallites of the inter-metallic layers which grow on the solder surface and cause the solder to recede. Non-wetting of SMDs can also occur by clogged solder paste screen or wrong squeegee pressure.
13.14.2.4 Bridging
Bridging is a short which occurs when an excess of solder makes an unwanted electrical connection between two adjacent conductors, or two leads or one lead and a conductor as shown in Figure 13.38. Bridging is a major defect and is usually not accepted, except when it shorts two conductive parts which are otherwise electrically connected with one another on the PCB. In wave soldering, the cause of bridging is quite often a too low temperature or insufficient flux. The too low temperature is generally associated with the speed of the conveyer belt, with the contact time as well as with the temperature of the pre-heating zone. Usually, boards with a large area of copper or a high density of terminal areas and terminations tend to act as heat sink which may cause bridging. Other factors, such as the form of the wave and the angle at which the assembled board approaches and leaves the molten solder during wave soldering may also have a strong effect on the tendency for bridging to occur.
In manual soldering, bridging is due to either a lack of skill on the part of the operator or the use of improper equipment with a too large iron bit.
Bridging can also take the form of a web of solder joining the legs and adjacent conductors.
In webbing, non-metallic surfaces can even be involved and many conductors may be thus shorted together.
£90°
£A/2
A De-wetted area Fig. 13.37 Acceptance criteria
for maximum extent of de-wetting on a land if the de-wetted area is properly tinned (redrwan af- ter Leonida, 1989)
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(a) (b) (c)
Conductor Conductor
Fig. 13.38 Typical examples of bridging (a) between two leads (b) between lead and a land (c) reduction of clearance between a lead and a conductor due to bad assembly
A special form of bridging when the short is caused by a thin and relatively long mass of solder is called ‘whisker’. This defect is usually difficult to detect on visual inspection. As it usually appears where the clearance is small, sometimes it may be required to revise the board design.
13.14.2.5 Disturbed Solder Joints
Disturbed solder joints are often mentioned as “cold joints” because in manual soldering, they are often caused by inadequate heating. The ‘disturbed solder joint’ results from any movement of the solder during solidification, which may cause an irregular surface that appears, at least partially, rough and wrinkled. The cold joint also sometimes manifests itself as a crack in the joint. If cold joints are a continuous problem, it may be due to the working of vibration of boards on the soldering machine, due to the working of a poor conveyor. This may easily be corrected by providing a smooth transfer of the printed board during solidification (freezing). Besides vibration of the conveyor, any other movement of the solder during freezing like outgassing should be examined. It may however, be remembered that a cold joint causes a serious doubt about the integrity of the joint and the joint should be re-worked.
13.14.2.6 De-lamination/Blistering/Measling
De-lamination, blistering and ‘measling’ are problems of the laminate caused due to excessive exposure to heat and may also be combined with entrapped moisture or any stress during the temperature above the TG-value. They are briefly defined below.
Delamination: This implies the separation occurring between adjacent layers of the laminate or between the resin and the copper foil. De-lamination of the adjacent layers starts from the edge of the board or from the holes. De-lamination is generally not acceptable at all.
Blistering: This is a localized swelling and separation occurring only locally between adjacent layers of the laminate or between the base material and the copper foil. It looks like air bubbles inside the laminate and is acceptable if they are far from plated holes and the imperfections are non- conductive and no conductor from inner layers is affected.
Measling: It is an internal condition occurring in laminated base material in which the glass fibres are separated from the resin at the weave intersection. It appears in the form of small white singular
spots or crosses internal to the laminate and is due to the separation of the filaments of the glass fibres. It is acceptable if present to a limited extent only and if all the white spots are still covered with resin. Glass fibre must not be exposed to the surface by the laminate.
If the white spots are not singular, and the condition is in the form of connected white spots or crosses below the surface of the base material, this kind of laminate damage is called ‘crazing’. It is usually related to mechanically induced stress.
The causes for the above mentioned defects are either excessive exposure to heat during soldering (including curing) or due to handling (stress) when laminate temperature exceeds the glass transition temperature (TG-Value), which for FR4-Material, is normally 135 °C.
In PTH boards, it must be ensured that the solder has risen in all the plated holes and has fully wetted the walls of the holes. There shall be no non-wetting or exposed base metal on any plated through-hole. If this is not achieved, the joint is defective.
A hole may be considered filled if there is a minimum of 75 per cent vertical fill of the hole. A maximum of 25 per cent depression, including both primary and secondary sides is permitted.
Figure 13.39 shows such a condition in a through-hole.
Fig. 13.39 Acceptability criteria for vertical fill of the hole
If there are problems with the vertical fill of the hole, the contact angle relative to the hole wall may be checked. If the contact angle is larger than 90°, the bare PCB is out of standard and must be rejected.
13.14.2.7 Solder Fillet Extends onto the Component Body
The solder fillet height may sometimes exceed the component termination. However, it must be ensured that it does not extend further onto the component body.
13.14.2.8 Outgassing/Blowholes/Pinholes
A blow hole is a small spherical deep cavity in the solder fillet of the joint. It occurs when moisture or flux, which may be trapped in the board, is vaporized by the hot solder and blows out through the joint as the solder is cooling. If blow holes are caused by emission of a gas through the solder fillet, the defect is termed as outgassing. The problem can usually be alleviated by baking and/or by
Soldering, Assembly and Re-Working Techniques 527
increasing the pre-heating parameters. However, the temperature in the pre-heating zone as well as the baking temperature (100-110 °C) is kept less than the TG-value of the laminates.
Excessive flux or insufficient evaporation of moisture and/or flux solvent before soldering can give rise to blowholes and internal porosity in the joint.
The moisture may be from water vapour absorbed by the boards during storage. Tests have shown that even after 24 hours of baking, the moisture content again gets drastically increased when the boards are exposed to high relative humidity. That is why such boards should be soldered as soon as possible.
Blowholes, which are very small cavities in the solder fillet of the joint are called “pinholes”.
The major causes of blowholes on PTH boards are:
a Hole being too large as compared to the lead diameter;
a Incorrect insertion of the component;
a Organic residues like inks, photo-resists, solder mask, which vaporize upon heating;
a Moisture or other liquids absorbed by the plated walls of the hole;
a Excessive flux application;
a Thermal profile for preheating zone not optimal; and a Too quick freezing of the solder fillet.
Figure 13.40 shows blow holes defect in a solder joint.
Fig. 13.40 Blowholes: a defect caused by the trapping of liquid or vapour inside the joint as the joint is forming
Blowholes usually occur on the solder side because the solder cools when rising into the hole and starts freezing from the top. The gas escapes through the path of least resistance, which is usually towards the side, where the solder is hotter.
13.14.2.9 Minimum Side Joint Length
The entire fillet for soldering should be properly wetted along the full length of the lead. The joint is not at acceptable, if the side joint length D is less than the lead width. The lead width is measured from the toe to mid-point of the heel bend radius as shown in Figure 13.41. The joint is acceptable if minimum side joint length (D) is at least equal to the lead width or 75 per cent of the lead length, which ever is less.
Fig. 13.41 Minimum side joint length — the joint is not acceptable if 'D' is less than the lead width or 0.5 mm, whichever is less
13.14.2.10 Solder Balls/Splashes
Solder balls/splashes are commonly caused by incomplete curing of the solder resist or in PCBs, without solder mask by not fully cured resin of the laminate. The defect often disappears when the board is re-soldered. However, the presence of non-soluble contaminants in the solder bath, sputtering due to use of wrong fluxes, or lack of cleanliness of the working area are likely to cause this kind of defect.
There should be no evidence of solder balls on the printed circuit assembly.
If the splashes are not entrapped or encapsulated and the adhesion is low, the splashes may detach and cause erratic shorts.
13.14.3 Solder Joint Defects and their Common Causes
A wide range of defects is observed in the soldered boards along with their common causes.
Table 13.4 shows the most common troubleshooting summary for soldered boards.
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Contaminated surface by improper storage Contaminated surface by fingerprints Contaminated surface by other contaminates Solder temperature too low Solder temperature too high Temperature application not homogeneous Conveyer speed too low Conveyer speed too high Preheating temperature too low Preheating temperature too high Flux insufficient or contaminated Flux application too less Excessive flux application Vibration of conveyor during solidification Table 13.4 Common Solder Joint Defects and their Causes (Braun, 2003)
Bridging/ ÷ ÷ ÷ ÷ ÷ ÷ ÷
Icicling
Delamination/ ÷ ÷ ÷
Blisters
Disturbed ÷ ÷ ÷ ÷ ÷
Joints/
Cold joints
Insufficient ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷
Solder flow
Non-wetting/ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷
De-wetting
Outgassing/ ÷ ÷ ÷ ÷ ÷ ÷ ÷
Blowoles *
Solder Balls/ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷
Splashes
Tombstoning/ ÷ ÷ ÷ ÷ ÷
Lifted Component
Warpage/Twist ÷ ÷ ÷ ÷ ÷
Solder wave uneven Solder contaminated Board not seated correct Symptom
Cause
Samsami (1990a) summarizes the causes of various fine pitch soldering defects. Examples of commonly encountered assembly faults are shown in Figure 13.42(a).
a. Solderbridging b. Tombstonedcomponent
c. Lifted lead d. Insufficient solder
e. Misalignedcomponent f. Unclipped lead
g. Missingcomponent h. Misaligned solder paste
Fig. 13.42 (a) common types of assembly faults (a) solder bridging (b) tombstoned component (c) lifted lead (d) insufficient solder (e) misaligned component (f) unclipped lead (g) missing component (h) misaligned solder paste (after Samsami, 1990b).
Soldering, Assembly and Re-Working Techniques 531
A lot of tools are available for hand soldering and repair work worldwide. Some are excellent and some are not very suitable for modern electronic re-work. A good address for soldering tools is www.ersa.com with its “Soldering Tools and Inspection Division”. For assembling of modern electronic items, tools such as de-soldering systems like Microprocessor Controlled Soldering Stations, SMD Soldering and Repair Systems, BGA-Placement and Re-work Systems, Optical Inspection Systems, Quality Assurance and Process Control Software are available from M/s ERSA.
Of course, everybody has to select the most suitable equipment for his shop (on the basis of price, performance and service), that best meets his need.
It is with experience that one learns the difference between a good or bad soldered joint. However, the following points should be kept in mind:
(a) The solder should be uniformly distributed over the elements and base metal. All solder joints, particularly in the high voltage circuit paths, should have smooth surfaces. Any protrusions may cause high voltage arcing at high altitudes.
(b) The quantity of the solder should be only so much that it does not obscure the shape of the element.
(c) No residue such as flux or oxide should be left on the surfaces.
(d) No solder should reach the shield of the wire.
A good solder connection will be quite shiny, not dull gray or granular. If your result is less than perfect, re-heat it and add a bit of new solder with flow to help it re-flow. The examples of bad solders are given in Figure 13.42(b).
< <
(a)Tooless solder (b)Toomuch solder (c) Preferredoroptimum quantity of solder Brokenstand
Charred insulation
Spilled solder Flux or solder
splatteredon surface Insulationgap
toolong
Fig. 13.42(b) Bad soldering examples