MICROSECTIONING GUIDE

This document was originally edited from material produced for training course run by the ICT Institute of Circuit Technology now part of the PCIF. The ICT traininmg workshops were originally presented by many of the engineers working in the PCB industry in the United Kingdom.

Introduction

Microsections are an essential tool in the quality evaluation and control of PCB manufacture.  It is very important that the people who produce and assess these microsections are well trained and supervised by a person who is well versed in the methods of PCB manufacture.

Considerations

The majority of microsections are taken from test coupons, test boards or scrap boards.  Test coupons are manufactured due to the prohibitive cost of loading spare test boards.  There are drawbacks with test coupons, their proximity at the edge of the board and differences in make, e.g. number of layers at different hole positions, must be taken into consideration.

These are other variations which  should be evaluated on each design.  Once the variables are known test coupon results may be used with more confidence.

Before taking  samples for microsectioning, the following should be considered;

1)	What are we looking for?  this may seem an absurd consideration but the parameter being checked will determine which part of the board/test coupon should be sectioned.

2)	If plating is to be assessed; avoid areas in the proximity of jigging fixtures.  Choose holes as near as possible to the board centre.  Section holes in "x" and "y" axis.

3)	Drilling and plating evaluation; choose an area where a range of hole sizes and inner pad configurations occur.  It is good practice to arrange for the first and last holes drilled to be adjacent to each other, this allows evaluation of both with one section

4)	Decide whether the sample requires overplating and with what. 

Overplating

This acts as a support to the plated deposit under evaluation, preventing the outer edge of the deposit from deformation and profiling during polishing.

The overplated metal shall not chemically react with the plated deposit or be softer.  Overplating should be carried out where possible prior to the pattern being etched.  The samples shall be degreased and chemically cleaned to allow the surface to "water wet" prior to overplating.  Without the pre-clean the overplate will peel away and possibly give misleading results.

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In most cases overplating is unnecessary if adequate care is taken in the preparations and assessment of the microsection. 

Sample removal

Thin laminates of 0.25mm (0.010") or less.  A hammer and chisel with the sample supported on a firm base, or scissors, will very successfully remove the requisite sample.  Thicker materials may be cut with a variety of tools, from dental burr to fly press.  Use of punches or presses to remove thicker samples is not recommended unless carried out under very controlled conditions.  Some methods are listed below:-

Punch/Press	May cause deformation to the sample.  Accurate fixturing is essential.  Methods is dust free.

Dental burr	Slow but accurate.  Very little sample damage likely to be incurred. Very fine dust produced.

Abra file/Jigsaw	Awkward, can cause vibration damage.  Some dust.

Drill route	When fitted to an engraver allows fast sample removal.  Accurate fixturing not required. Variable sample size removed.  Does generate dust.

Whichever method of sample removal is adopted ensure it is carefully evaluated and the shortcomings known.  Never cut too close to the sample and where possible use control samples.

Sample Encapsulation

Encapsulation of the sample prior to microsectioning is necessary for two main reasons:

1)	To prevent the break-up of the plating under evaluation which can occur when it is not supported, thus preventing an accurate assessment of the sample.

2)	To make handling of the sample easier.  The media used to encapsulate the sample is dependent on the finish of the board from which the sample is taken and the sample make up.  

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Hot Pressing using Acrylic Thermoplastic

This is a very quick and efficient method of encapsulating samples.  There are several hot presses on the market, some manual and some automatic.  Typically a temperature of about 180oC and a pressure of 290 1bs/in2 is used.  Their operating parameters limit the used of the method to samples which will not be affected by temperature and pressure.  Methods of encapsulation; a measured quantity of the resin is poured into the press barrel.  The sample which shall have been degreased and dried is placed onto the resin, and identifications label (see later paragraph) is also placed in the resin,  push both the sample and the label lightly into the resin.  This will prevent the sample moving during the heating and pressure cycle.  Pour a second measured amount of resin onto the sample/label, secure the press top.

When using a manual press  

Raise the pressure again.  After heating for 10 minutes turn off the heater and allow the press/sample to air cool for a period, then turn on the water cool.  When the sample has been cooled sufficiently (10 minutes) release the pressure and remove the sample.

With automatic presses the heating and cooling times are pre-set, also the pressure.

Use of thermosetting resin

These may be two part liquids, one of which is the hardener, or liquid/powder mixes.  Of the medias we have evaluated the liquid/powder mixtures give the harder resin finish.  Soft resins should be avoided as profiles edges will occur.  The resin cures due to oxothermic reaction at a temperature of about 70oC.  The time it takes to cure is dependent on the temperature of the room in which it is left to cure.  The sample itself will affect the rate of cure and temperature reached during polymerisation.  Most samples will be unaffected by the temperature but the drawback with these resins is the critical mixing.  Good or bad results will be attained depending on how the resin is mixed and under what conditions it is allowed to polymerise.  Excessive stirring and air temperature will result in aerated encapsulations, into which debris will collect during grinding and polishing.

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Sample Identification

For traceability each sample shall be given a unique number possibly with the last two digits being the year.  The batch number and board number should also be stated on the label.  A simple log book with the microsection numbers in numerical order, date and sample description completes the traceability.  Any paperwork will carry the relevant microsection number.

Deciding on which method of encapsulation to use

Table 1 indicates which samples can satisfactorily be "hot-potted" and which should be "cold-potted".

TABLE 1

FINISH                                         ENCAPSULANT  

Copper through hole plating                 
Tin nickel/gold finish   
Gold on copper (overplate recommended)          
"Hot-pot" method
Drill evaluations when sample is plated
Thermal shock 
Tin lead plated samples
Etched samples for evaluation
Fused/S.C.L. finishes
Cold-pot method
Solder 
Drilled unplated 
Solderability assessment 
Sample grinding

The encapsulated samples are ground using silicon carbide papers either on a rotary wheel or flat bed strip grinder (see illustrations). A range of paper grades should be used starting with the coarse one and progressing through to the finest one.  A continual flow of water of adequate flow rate to cover the paper and prevent clogging of the papers by ground off debris shall be maintained.  Unlike the dry method of grinding, the wet method allows pressure to be placed on the sample to speed microsection preparation without more and deeper deformation of the sample.  Deformation will be at its greatest with new papers and so the newness should be removed before critical grinding.

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Three general rules during grinding:

1)	The pressure on the sample shall be as high as is comfortable.

2)	The paper should be slightly worn.

3)	The time on each paper during grinding should be at least twice the time for removing the scratches left by the coarser paper. When microsectioning holes the coarse paper should be used until most halfway through the holes.  Then progress on to the finer grades until half the hole is exposed and the scratches are at a minimum

Polishing

Using a cloth impregnated with diamond paste on a rotary wheel 5 and 2.5 um pastes will give suitable finishes.  (BS 9760 calls for 1 um).  A lubricant is fed on to the cloth during polishing.  When not in use the cloth shall be kept covered to prevent dust contamination.

The polishing stage should be kept to a minimum to avoid rounding of the outer plated edge. 

Thoroughly rinse the sample and dry with a soft tissue or cloth. 

State of the polished sample

For photomicrography and critical evaluation of such attributes as intermetallics a much high degree of finish will be required than for routine plating thickness measurements.  The finish should be of such a degree that the person looking at the sample can draw the right conclusions.  It will be found that the results of a highly finished microsection viewed by a lesser experienced operator will equate with results found by highly trained operator looking at a lesser finished sample.

Etching the sample

The etch selected should provide the clearest possible contrast between deposits.  The etch removes any slight traces of metal spread over the surfaces during polishing

TABLE 2

FINISH                FEATURE BEING EXAMINED 

Copper    Grain boundaries                1      
Resin smear at interface between T.H.P. and inner lands.  
Drill quality
Tin Nickel on 
Boundaries between copper and  
copper tin nickel.                     1      
Tin lead on copper  
Boundaries between and tin   lead                            1                                             Fused tin lead     
Boundaries between copper fused tin, no attack on tin    lead                            2     Soldered joint        When soldering on gold will show up gold/tin dendrites           3     

ETCH MAKE UP AND USE

Ammonium Chloride  3 gms. The solution which will keep for
Sodium Dichromate  5 gms. longish periods as a stock      
Sulphuric acid     5 mls. solution, is applied by brush   
Nitric acid        5 mls. or immersion of the sample into 
De-ionised water 100 mls.    it.  The etch time is dependent  on etch use.  Rinse thoroughly  after use, dab dry with soft   
225% Ammonia in de-ionised    Solution has to be made up  water.                     freshly when required.  It  should be used in a fume   cupboard. Dip sample for several  seconds then rinse well. Dab dry with soft tissue
3Concentrated Hydrochloric  Sample is held over the acid    acid.                        vapour for several seconds. Rinse and very carefully dab dry

Preliminary examination of the microsection prior to recording of results. This should be read in conjunction with diagram page.

1)	Ensure holes if sectioned are as near as possible, are cut half way.  Under or over grinding will give false plating thickness results.  (Plating will appear thicker than it really is

2)	Ensure the hole sample has been ground at 90o0 to the surface.

3)	Check for resin shrinkage and separation between sample and encapsulant.

4)	Ensure sample has not been squashed.  Normally due to  insufficient resin being used.

5)	Check for deep scratches which may give the appearance of voids.  The tell tale sign is that the scratch will be evident in the resin as well as on the sample.

6)	Ensure the sample was not damaged during removal. Results of board manufacture

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7)	Plating Thickness.  Results to plating shop and chemist. At least three readings should be taken per finish i.e. both surfaces and the hole.  These readings plus the hole size and board thickness allow the results to be compared from day to day, and thus show up any plating deficiencies.

8)	Nailheading or drill burrs. Results to drill shop (and chemist)* Various causes for this defect.  A few are drill condition, feed and speed, resin cure, backing material not flat.  Not usually detrimental unless accompanied with resin smear*. Which can cause high resistance connections or even open circuits.

9)	T.H.P. Void. Results to the chemist and plating shop. Due to many variables some of which are grease on drills, poor hole conditioning, electroless failure, resin state etc.  Any degree of voiding to be notified.

10)	Print, Plate and etch Voids. Results to chemist and photomech, department. Due to surface contamination such as resist residues preventing plating of the etch resist finish e.g. tin lead, tin nickel gold

11)	Barrel Cracks as a result of thermal-shock. Results to chemist. Caused by "z" expansion of the base material exceeding the expansion rate of the copper.

12)	Corner cracks as above.

13)	Etch .Characteristic. Results to the etch department and chemist. Related to etch time.

14)	Misregistration of inner layer lands. Results to drill shop, lamination shop or photomech depending on the cause. May be due to inaccurate printing, laminate shift, artwork, drill tape error etc.

15)	Material defects. Results to material goods inspector or lamination department. May be bought in material defect or pre-preg fault.

16)	Severe back etch. Notify department concerned and chemist. Cause excessive time in solution.

17)	Preferential resin attack during de-smearing. Notify department concerned and chemist. Cause incorrect solution make up or with plasma operation excessive time.

Further information on PCB Defects, PCB Manufacture and Design for Manufacture may be obtained from video tapes, CD-ROM and Poster Sets available from the Bob Willis Internet Web Page www.bobwillis.co.uk