Power dissipated by a circuit starts as electrical energy, but becomes heat, and this heat needs to be removed in order that the circuit may continue to operate correctly. Where a substantial proportion of the heat is dissipated by just a few parts, or by a single component such as a microprocessor, the designer will often provide a ‘heat sink’, to conduct the heat to a larger surface that may more effectively be kept cool by transferring energy to an external medium. Both natural and ‘forced’ (fan-assisted) convection are common, using the surrounding air to carry heat away; for some high-power systems, the heat sink may be water-cooled.
Unless the sources of heat are many and distributed widely across the board, such strategies are not normally applied to the smaller generators of heat. For these, the thermal energy will be lost by convection to the surrounding air, and by conduction (mostly through the leads) into the board.
Unfortunately, compared with metals, most laminates are very poor conductors of heat: the approximate thermal conductivities of FR-2 and FR-4 are 0.24 W/(m•K) and 0.27 W/(m•K) respectively, against 390 W/(m•K) for copper.
The consequences of this are that:
If you would like to check this, there is an interesting article and calculator in the library at the CoolingZone magazine site.
Note that providing extra copper for heat sinking should not be done at the expense of having the areas of copper evenly distributed, and of balancing the thicknesses of foil used, so that the board is symmetric about its centre plane. If balance is not achieved, this will lead to internal stresses and board warping.
In the last part we considered both operating temperature and the closely-related glass transition temperature: in this section we are looking at what happens when the laminate is taken above its normal operating range.
At high temperatures, thermoset resins will decompose. There is no specific temperature at which this happens, but degradation of characteristics starts near soldering temperatures, with oxidation of the resins leading to darkening. This is the reason why boards used for profiling reflow processes gradually darken in colour, as do boards that have become trapped in a reflow oven.
From the point of view of reliability, however, perhaps the more critical factor is the loss of adhesion which occurs, leading to delamination within the structure. A test procedure that is often quoted is the ‘time to delamination’ test. This measures how long a material will resist blistering or delamination, using a thermomechanical analyser in which the sample is heated to a specified temperature. The most common temperature used has been 260°C (the T260 test), but the higher T288 test is becoming more common with high Tg materials intended for operation with lead-free solders.
Table 1 gives the T260 time for a range of different materials. Broadly speaking the T260 time to blistering/delamination increases with Tg, but there are many differences both between materials and between the same material from different manufacturers.
|material||Tg||T260 (min)||decomposition temperature
(°C for 5% weight loss)
The loss of adhesion associated with delamination and blistering also affects the adhesion between foil and laminate, and at high temperatures the adhesion of foil to the laminate reduces. Such delamination is a frequent occurrence during rework processes, especially when soldering irons are used.
Time to delamination measures the bond between the components of the laminate: in order to assess the actual physical degradation of the resin system that is indicated by colour changes, we use thermogravimetric analysis (TGA). This measures the mass of a sample as it is gradually heated and decomposes. The decomposition temperature reported is the temperature at which 5% of the mass of the sample has been lost through decomposition. Table 1 also gives indicative values for decomposition temperatures: again there is both a relationship between glass transition temperature and decomposition temperature, and some spread of results.
Many of the standards in this area result from concerns from the insurance industry about the consequential damage caused by electrical fires, and the work carried out to qualify materials by Underwriters Laboratories in the USA. The key factors for UL’s sponsors are that a material should self-extinguish (that is, not keep any fire alight), and should not spread the fire by means of flaming droplets.
The UL94 test standard is a generally-used indicator of the acceptability of plastic for general use with regard to its flammability. There are three different tests, and several different levels, as indicated schematically in Figure 1 and Table 2.
The testing for the 94V-0 rating involves two separate applications of a flame for 10s to each sample from two sets of five specimens, and meeting the criteria that:
Most professional laminate materials, such as FR-2 and FR-4, have the highest (94V0) classification for vertical burn tests.
Note that the materials are allowed to burn when the flame is applied, but not to spread a fire (no ‘flaming particles’) or to support continued combustion (must be ‘self-extinguishing’).
There are similar methods in the IPC-TM-650 Test Methods Manual: Methods 2.3.9 for prepreg and thin laminate, 2.3.10 for the flammability of laminate, and 184.108.40.206 for the flammability of solder mask on printed wiring laminate. The tests are comparable, but there are some interesting observations which help shed light on this topic:
1 While correct for most laminate materials, UL94 should be used for thermoplastics.
2 Flaming particles are detected by placing a pad of dry absorbent surgical cotton 3–5mm below the test specimen – if it catches fire, this gives a very good visual indicator of non-compliance!
Despite the fact that the IPC test is better targeted at the requirements of the PCB industry, UL wins hands down in terms of universal acceptance, because the specification applies to so many of the materials used for purposes such as enclosures.
Provided that sufficient copper had been etched away, it used to be straightforward to see the classification of a laminate from the marking that appeared on one of the inner layers of the board, as a UL logo in combination with the manufacturer’s logo. A red logo indicated that the material was classified UL94 V-0, so the laminate was likely to be FR-2 or FR-4; a blue logo corresponded to a UL94 HB rating, which would indicate either XPC (phenolic) or G10 (glass-epoxy). Unfortunately, this useful feature has more or less disappeared from present-day laminates.
|Surface Burn tests|
Burning must stop within 60 seconds after five applications of five seconds each of a flame to a test bar, and there must be no burn-through hole. The flame is larger than that used in Vertical Burn testing.
This is the highest (most flame retardant) UL94 rating.
|UL94 5VB||As for 94 5VA, but a burn-through hole is allowed.|
|Vertical Burn tests|
|UL94 V-0||Specimens must extinguish within 10s after each flame application and a total combustion of less than 10s after 10 flame applications. No samples are to drip flaming particles or have glowing combustion lasting beyond 30s after the second flame test.|
|UL94 V-1||Specimens must extinguish within 30s after each flame application and a total combustion of less than 250s after 10 flame applications. No samples are to drip flaming particles or have glowing combustion lasting beyond 60s after the second flame test.|
|UL94 V-2||Specimens must extinguish within 30s after each flame application and a total combustion of less than 250s after 10 flame applications. Samples may drip flame particles, burning briefly; and no specimen will have glowing combustion beyond 60s after the second flame test.|
|UL94 VTM||Thin material version of the vertical
burning test applied to thin or flexible materials which may
distort, shrink or flex during the 94V test. A specimen 200
× 50mm is rolled longitudinally around a 12.7 mm diameter
mandrel and taped on one end. When the mandrel is removed the
specimen forms a cone shape, which gives it longitudinal rigidity.
Has the same three classifications as 94 V. Differences are that a flame is applied twice for only three seconds, and no specimens may have flaming or glowing combustion beyond a point 125 mm from the bottom of the specimen.
|Horizontal Burn test|
Slow horizontal burning on a 3 mm thick specimen with a burning rate is less than 75 mm/min or stops burning before a mark 125 mm away from the point of flame application. HB rated materials are considered ‘self-extinguishing’.
This is the lowest (least flame retardant) UL94 rating
Your Purchasing Manager has noticed that the paperwork relating to the laminate he/she is now buying refers to compliance with UL94 HB, whereas with most of the material your company uses the reference is to UL94 V-0. Explain the significance of the difference, and the reason for the restricted uses to which the new material can be put.
So far, we have considered the physical, thermal, mechanical and electrical characteristics of a laminate. Before reading further, make as complete a list as possible of any other aspects that you would expect to be covered in a specification for a laminate material.
Review your answer as you read further.
As you have discovered so far, laminate materials will be supplied to a specification that includes physical, mechanical, and thermal characteristics, and dielectric parameters such as surface and volume resistivity, dielectric constant and dissipation factor. All these parameters will be fairly generic to the resin/reinforcement selected.
But specifications will also contain much quality and reliability information. In particular, they will give details about the foil that has been used in building the laminate. In the typical commercial foil specification which is given in Table 3, notice that much is made of the way in which the foil is suitable for use both with different materials and different processes. Similar documents from other sources will stress aspects such as bond strength with high Tg resin systems and chemical compatibility.
|minimum adhesion strength of 1 oz. copper at room temperature on conventional epoxy with 130–140°C Tg||9.5 lb/in = 1.66 N/mm|
|high temperature oxidation resistance||180°C for at least 60 min|
|solderability||passes IPC specification|
|UV-curable resist ink adhesion||excellent|
|solder resist ink adhesion||excellent|
|etchability||excellent with cupric chloride, ferric chloride, alkaline, and ammonium persulphate|
|resistance against laminate staining and microvoids||excellent|
|solder float test (288°C, 10 seconds)||<2% adhesion strength loss|
|thermal ageing test (150°C, 10 days)||<15% adhesion strength loss|
Even though the adhesion between foil and laminate also depends on factors other than the foil itself, adhesion strength is a key aspect of the specification. The emphasis reflects both the importance of the parameter and the fact that the surface finish3 and treatment of the foil strongly affect the outcome.
3 In foil manufacture, as well as creating a bottom side with high surface area that will couple mechanically to the resin in the prepreg, it is common to apply a thin passivation layer to protect against corrosion and laminate stain, and thus increase shelf life. On the upper (shiny) side of the foil, a stabiliser is applied to reduce oxidation during lamination and post-baking operations and facilitate chemical cleaning
The standard test for adhesion is to measure the resistance exerted on the test equipment when a long strip of foil of defined width is gradually peeled from a laminate. The value quoted will be the minimum load applied divided by the width of the foil strip. The test is defined in IPC-TM-650 Method 2.4.8, and there is also a ‘keyhole method’ variant (Method 220.127.116.11) for thin laminates. Because the measured peel strength is thickness-dependent, tests are normally carried out on 1 oz (35 µm) foil.
Tests are carried out not only on laminate ‘as received’, but also after tests that simulate storage in different conditions, treatment with solvents and simulated electroplating treatment. Usually results4 are little affected by exposure to process chemicals, but deteriorate slightly after thermal stressing (such as floating on molten solder), and reduce by up to 30% at elevated temperatures close to Tg.
4 See Table 8.4 in Section 8.2.6 of Coombs 2001. As always, you can expect to come
across both metric and imperial units. As a guide, 11–12 lb/in = 1.93–2.10 N/mm.
In Properties of laminates, we explained that the Z-axis expansion of a laminate increases substantially above Tg, that stresses from changes in temperature can lead to fatigue failure and that in consequence the plated layer in the through-holes needs sufficient ductility. However, the foil itself needs reasonable ductility to withstand CTE mismatch, especially where it is constrained from moving, as in the vicinity of a through-hole. Stress on the rest of the inner layer away from the hole can cause a fracture of the copper layer within the board. This is why high-temperature elongation (HTE) foil was developed in the early 1980s. HTE foil has higher elongation at the temperatures typical of soldering processes (Table 4), and its use for inner layers is particularly important in maintaining yields for complex multilayer circuits.
|description||unit||product name||typical values|
|½ oz||1 oz||2 oz|
|tensile strength (RT)||MPa||JTC-HTG||360||325||310|
As you will perhaps have noticed when reviewing the properties of other metals, tensile strength and elongation are inversely correlated, and both parameters depend on the crystal structure, and hence on the method of manufacture and subsequent heat treatment. Table 5 illustrates how ductile a typical electrodeposited (‘Type A’) copper foil can be compared to a rolled (‘Type B’) foil, until that rolled foil has been annealed. The rolled foil will typically only be used for making flexible circuits (a topic we will return to in Technology awareness).
|mass per unit area||minimum tensile strength||minimum elongation percentage|
|Type A (electrodeposited) copper foil|
|152||0.5||10 500||15 000||2||5|
|305||1.0||21 000||30 000||3||10|
|610||2.0||21 000||30 000||3||15|
|Type B (rolled) copper foil, as rolled|
|152||0.5||35 000||50 000||0.5|
|305||1.0||35 000||50 000||0.5|
|610||2.0||35 000||50 000||1|
|Type B (rolled) copper foil, rolled and annealed|
|152||0.5||10 500||15 000||5|
|305||1.0||17 500||25 000||5|
|610||2.0||17 500||25 000||10|
Whilst foil thicknesses are often quoted, foil is usually specified in terms of nominal mass per unit area and percentage deviation from that nominal (typically ±5% or ±10%), with the thickness and thickness deviation given for information only. User preference for the units used in specifications differs between the USA (mostly ounces per square foot) and Europe (mostly grams per square metre), though even in Europe the ½ ounce/1 ounce/2 ounce measure is often to be heard, as it is very convenient.
The inherent conductivity of the foil material may be quoted in absolute SI units (µW m) or relative to the purest possible copper, expressed as the International Annealed Copper Standard (IACS). The BS lower limit for ½ ounce foil is 0.0184 µW or 93.7% IACS, and 0.0179 µW or 96.6% IACS for 1 ounce foil. Such differences in characteristics between thin and thicker foils are to be expected: in the former, a greater percentage of their volume is affected by the material at the surface.
More usually, the specification will quote the maximum resistance of the foil (Table 6). The tests used, whether IEC or IPC, all use a 150 mm section of a longer 25 mm wide foil. The resistance measurements employ four-terminal techniques, because of the low ohmic values, and are corrected to 20°C.
|mass per unit area||maximum resistance|
|152 g/m2 (½ oz/ft2)||7.0 mW|
|305 g/m2 (1 oz/ft2)||3.5 mW|
|610 g/m2 (2 oz/ft2)||1.75 mW|
In order to reach these low resistance values, the copper used must be pure, and some specifications will put limits on the purity of the material. BS EN 60249-3A, for example, demands a minimum purity of 99.8% copper for electrodeposited foils and a minimum of 99.9% for rolled foils. In each case, any silver content is regarded as copper, and the percentage excludes any ‘as shipped’ surface treatment.
Mechanically, typical foil specifications will set standards for the maximum surface roughness, and ask that the foil surfaces should be free of:
The specifications for laminates are similar to those for foil, with limits for inclusions, indentations, bumps, wrinkles and blisters, but with the additional requirement that the surface shall be free of resin, as laminate processing may cause this type of defect. On surface marks, they are arguably less severe: for example, BS EN 60249-2-5 (for FR-4) allows scratches of depth up to 10 µm, or 20% of the nominal thickness of the copper foil, whichever is the lower. However, there is an added limitation of 1 m per square metre for the total length of scratches deeper than 5 µm. Care in handling is needed at every point during laminate manufacture and use!
Under the surface, with the copper etched away, the base material has to be substantially free from pits, holes, scratches, porosity and foreign inclusions (including pre-cured resin particles). Although a small amount of irregular variation of colour is permissible, the base material also has to appear substantially uniform in colour.
The solderability of both copper foil and laminate is also important. Typically the tests carried out evaluate both wetting and de-wetting properties of the foil by simulating wave soldering, rather than using a wetting balance. Samples are first fluxed and then moved at constant speed so that the test face makes contact with the molten solder.
Most of the mechanical aspects have already been considered in Properties of laminates. However, laminate specifications will also set limits for dimensional tolerances, including thickness, and for surface waviness (typically £5µm) as well as warp and twist.
Dimensional stability is important, and this is addressed in BS EN 60249-2-5 by using the test given in BS EN 60249-1 Method 3.11, in which board dimensions are measured before and after exposure to the category temperature for 30 mins, followed by recovery. For FR-4 laminate, the test temperature is 150 ±2°C, and the requirement is that the change in dimension should be less than 0.3 mm/m for boards thicker than 0.8 mm (0.5 mm/m for thinner boards).
The EN specifications we referred to in the previous part are either for foils or for finished laminates, where there is no requirement for specifying the construction of the laminate (which needs to be negotiated with the vendor). IPC-4101A Specification for Base Materials for Rigid and Multilayer Printed Boards covers both finished laminates and prepreg. A large number of ‘slash sheets’ attached to the basic document spell out the specific properties for different classes of materials.
Of course, when we buy prepreg there are subtly different considerations to be borne in mind. Here the prime information required is:
Having a standardised test for the last of these is important, even though the actual flow in any situation will depend on factors such as the bonding between the many different layers. The fabricator must have consistent flow characteristics in order to ensure repeatable manufacturing performance.
The standard four-ply flow test (Test MF) involves cutting pieces of prepreg to a specific size, weighing them and pressing them at a controlled temperature and pressure. The resin flows under the test conditions and afterwards a smaller section is cut from the centre, reweighed and normalized to the original area. The ‘flow' is calculated as the loss in weight between the original material and the final weighed ‘flow biscuit’, expressed as a percentage.
A rose by any other name . . .
So far we have mentioned three standards authorities, so perhaps it is time to look at their role in Electronic Design Realisation.
What are standards?
“Standards are documented agreements containing technical specifications or other precise criteria to be used consistently as rules, guidelines, or definitions of characteristics, to ensure that materials, products, processes and services are fit for their purpose.
For example, the format of the credit cards, phone cards, and ‘smart’ cards that have become commonplace is derived from an ISO International Standard. Adhering to the standard, which defines such features as an optimal thickness (0,76 mm), means that the cards can be used worldwide.
International Standards thus contribute to making life simpler, and to increasing the reliability and effectiveness of the goods and services we use.”
from the ISO web site
So, who is making life simpler? Regrettably too many groups, as Table 7 demonstrates!
ANSI American National Standards Institute
BS EN standards are adopted by ISO, the EN standing for EuroNorm
BS(I) British Standards (Institution)
CECC CENELEC Electronic Components Committee
ENELEC (Comité Européen Normalisation Electrotechnique) European Committee for Electrotechnical Standardisation
IPC Founded as Institute for Printed Circuits, then Institute for Interconnecting and Packaging Electronics Circuits, and now IPC with an accompanying ‘identity statement’ as ‘IPC – Association Connecting Electronics Industries’
ISO International Organization for Standardization5
JIS Japanese Industrial Standard
NEMA National Electrical Manufacturers Association, producers of one of the first classifications for printed circuit boards
5 ISO is a worldwide federation of national standards bodies from more than 140 countries. If you want to know why it isn’t called IOS, visit http://www.iso.ch/iso/en/aboutiso/introduction/whatisISO.html.
When it comes to specifying laminate, you have to be aware that:
For the two materials we have reviewed during this document, Table 8 gives some alternative descriptions. The NEMA and BS 4584 type numbers may technically be obsolete, but you will still find them used, especially the original NEMA names . . .
|NEMA||IPC 4101A||BS 4584||BS EN 60249 Part 2||JIS|
|full form||short form|
So far we have confined our attention to phenolic-paper and epoxy-glass laminates which come under the generic NEMA descriptions of FR-2 and FR-4, and looked only briefly at the range of desirable properties of a laminate.
Before you start the next part, we would like you to review and extend your knowledge in this area. Restrict the scope of your survey to the prepregs and laminate (base laminate plus copper) that are supplied to the fabricator – we shall be looking at board finishes later.
Be aware that, in many cases, it may not be possible to get information on FR-2, whereas equivalent information on FR-4 can be found. Don’t worry about this: even our extended searches didn’t provide answers to everything!
When you have spent an hour or so on this activity, proceed to the next part, where the answer will be discussed.
6 Please check that the link in your email works! Some sites don’t identify specific pages, and you may need to explain how to drill down from the home page in order to access the problem.