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.

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