Solder mask


Introduction

In the first part of this unit we looked at a range of laminate materials. Now, as we consider a key polymeric material which contributes to the ease of assembly and reliability of the board, we are more fortunate, as there are relatively few materials and processes available.

In this unit, we are restricting ourselves to the main materials issues, and there will be more about application methods in PCB fabrication processes.

Activity 1

Rather than just tell you about solder mask, we want you to do some of the thinking!

Base your answers to the questions below on what you know about the assembly process, about polymers, and about adhesion and flow.

  1. Most of the boards you will have seen have a green coating covering most of the surface. This is the solder mask. What functions might such a covering have?
    [Hint: our list has nine entries]
  2. What properties does this solder mask material need to have?
    [Hint: our list has six entries]
  3. How might you set about applying such a solder resist? Are the methods you can remember (or envisage) equally capable of doing an adequate job?
  4. What might be the design and quality issues of which you as a designer should be aware?

If you have access to fellow-students or interested colleagues, you could tackle each of these in 4-5 minutes of focused discussion.

Compare your answer with the discussion below.


Solder mask functions

Solder mask (also known as ‘solder resist’) is used on the vast majority (97.7%) of boards (IPC 1996 survey), and has many functions. Our first list, expanded from Wallig’s contribution to Coombs Printed Circuits Handbook, contained seven items:

The most crucial ideas relate to the environmental protection and the control of solder. How many of these did you identify as you went through the activity?

The items we missed out first time around were:



Solder mask properties

In the words of a British Standard, solder mask is a ‘permanent polymeric coating’, and it has to be permanent despite the harsh nature of the application and environment.

A ‘permanent polymeric coating’ that cracked up!

A ‘permanent polymeric coating’ that cracked up!

One list of the properties required of a solder mask that we used to publish was that a solder mask must:

How many of these did you identify as you went through the activity? And did your list have any more? [Apart, that is, from sordid commercial considerations such as low price and ready availability!]

Based on a memory of how solder masks are applied, you might have tied this into the polymer and flow information, and listed some qualities associated with the intended application method, such as:

But these aren’t all the qualities a solder resist needs. When you think of the requirements of the fabrication process, you will realise that:

One thing no solder resist has is the ability to remain firmly attached and unwrinkled if it is applied to a solder surface which is subsequently reflowed.

Resist crinkle due to remelting of underlying solder

Resist crinkle due to remelting of underlying solder

The unsatisfactory visual result is one of the reasons that the SMOBC (solder mask over bare copper) sequence is generally preferred: more about this in Conductor finishes: tin-lead.


Practical solder resist

Panel preparation

In order to give adequate adhesion and reliability, solder resist has to be applied to a clean surface. The aim is to remove organic and ionic contaminants, surface oxides and intermetallic compounds. The process typically starts with chemical cleaning. Finishes other than HASL then allow an intermediate stage of mechanical scrubbing. Finally, boards receive an extended wash in deionised water before being dried by baking.

Solder resist application

Perverse as it may seem in the light of the discussion in the last section, it is reported that many board fabricators will first decide on the method of application and cure, and then on the equipment to be used, and only at the very end choose a particular material which is compatible with method and equipment! However, whilst there are many formulations, most of these are epoxy-based.

For low-cost work, solder resist patterns can be applied directly by screen printing. However, for high density boards, such resists are inadequate owing to ‘bleeding’ onto pads, to poor coverage between closely packed pads or conductors, and to poor registration accuracy. SM designs in particular tend to have fine surface detail, with narrow spaces between tracks and pads, and the change to surface mount encouraged a move away from direct screen printing. The 1996 IPC survey showed that most solder mask used was photoimaged, except for single-sided consumer products, where screened solder mask was used for reasons of cost.

The majority of resists used for fine line work are, therefore, photopolymers, in either dry film or liquid form, using a photographic exposure and developing process to define the required pattern in the solder mask. There are two approaches, based on the form in which the mask is supplied.

Dry film solder mask

The earliest photoimageable solder masks were ‘dry film’ types, which can in theory be rolled over the board surface in the same way as photoresist is applied. But photoresist is applied to a very flat surface: the difficulty with dry film solder mask lies in ensuring that the film is in intimate contact at all points with densely packed tracks, without leaving gaps filled with air which would expand and cause delamination during wave-soldering or solder levelling.

Vacuum laminating gives a better result, but needs different equipment. The resist is held slightly away from the board while a vacuum is created in the machine. Atmospheric pressure is then allowed to force the film into contact with the board. This process can be carried out on one side, or on both sides simultaneously.

A resist film supplied at 75-100µm thickness will meet the common specification requirement for a minimum of 25µm coverage over the top of conductor traces.

Section showing thinning of resist over track

Section showing thinning of resist over track

Liquid photopolymer solder mask

Liquid photopolymer resists were developed first as etch resists for multilayer production, remaining on the board after etching, and reacting with the epoxy resin of the prepreg during lamination, saving several process steps. However, suitable formulations can also be used as solder masks, where they are described as LIPSM (occasionally LIPSR), an abbreviation for Liquid Photoimageable Solder Mask (Resist).

The earliest of these, Probimer 52, introduced by Ciba-Geigy in 1978, originally found favour (despite its high capital cost) primarily in markets which required its excellent corrosion resistance. It was not until surface mount demanded better definition that this process became more popular.

Applying LPISM

One option used is to screen print material over the whole board, and equipment allows this to be carried out simultaneously on both sides of the board. However, it is still common to apply liquid photo masks by ‘curtain coating’, the original Probimer process, which is shown schematically in Figure 1.

Figure 1: Curtain coating of solder resist

Figure 1: Curtain coating of solder resist

Practical machines are more complex than indicated, because they also need a means of collecting the material that falls round the side of the board and in the gaps between boards, filtering this to remove foreign bodies which might otherwise lodge in the coating head and cause an uneven curtain, and recirculating the material.

After coating, the solvent in the layer of resist needs to be dried under controlled conditions ready for imaging. Only one side can be coated at a time, so the board also has to be dried before coating the second side. As with printing on two sides separately, this means that there will always be slight differences between the two sides in the cure state of the solder mask.

Where both sides are coated in a single continuous process, integrating two curtain coaters and two drying ovens creates a long and expensive machine! However, all types and shapes of substrate can be coated without the need for special tooling.

Exposure and development

After the photopolymer has become tack-dry, it is exposed to ultraviolet light through a negative artwork, polymerising the exposed areas, so that they become insoluble in the developer. Exposure times are typically of the order of 20–90 seconds, depending on the material and exposure unit used.

Most of the original liquid solder masks were developed in 1,1,1-trichloroethane, but the use of that solvent is now discouraged because of environmental concerns. Current Ciba-Geigy Probimer materials use a mixture of nonchlorinated biodegradable solvents; most of the remainder use aqueous-based developers (typically sodium carbonate solution).

Tenting holes

Most of the boards designed with fine pitch surface mount devices use liquid resists, as the assembly industry generally prefers the ‘gasket seal’ between stencil and pads which can be achieved by liquid resists: in contrast, dry films typically create ‘wells’. However, dry film resists are still often favoured for ‘tenting’.

Section showing tented via hole

Section showing tented via hole

Tenting is the term used for deliberately putting a mask coating over the top of a via hole. The requirement to tent (or even fill) via holes comes from the fact that closely-spaced holes can trap flux during soldering, leading to bridging.

Unfortunately, not only are dry film resists expensive, but they also do not flow enough to fill the valleys between closely-spaced conductors. DuPont’s VALU system aims to meet this problem by combining a thin coat of liquid mask with a dry film laminated on top.

Tenting can also be accomplished with liquid mask systems, but needs a separate filling stage, where epoxy inks are screened onto the board either before or after the liquid mask is applied.

Curing solder mask

Having applied the solder mask, it needs to be cured. This can be carried out by heat or ultraviolet light or a combination of the two, depending on the chemistry of the resist. Curing removes any remaining volatiles and completes the polymer cross-linking process, making the coating sufficiently tough to withstand its application.

Curing processes that are not sufficiently controlled, especially under-curing, are the Number 1 cause of solder resist failure. What is Number 2? Adhesion failure, due to inadequate cleaning before mask application!

Poor adhesion of solder mask, showing as patches of lighter colour

Poor adhesion of solder mask, showing as patches of lighter colour

Self Assessment Questions

Explain to your assembly process engineer the materials and process options available for applying solder mask to a board with fine-pitch parts.

Compare your answer with this one


Solder mask design basics

Standards

Information on the use of solder mask are contained in IPC-SM-840C Qualification and Performance of Permanent Solder Mask. The specification is intended to facilitate evaluation of solder mask by a vendor using a standard board system, and to enable designer, manufacturer, and user together to qualify a production board process.

The test methods and conditions in IPC-SM-840C are based on end use and environmental reliability requirements for two classes2 of user:

2 Older IPC documents refer to Class 1, Class 2, and Class 3. For all practical purposes there is no Class 1 solder mask; Class 2 is equivalent to Class T; Class 3 is the equivalent of Class H

T – Telecommunication (includes computers, telecommunication equipment, sophisticated business machines, instruments, and certain non-critical military applications.) Solder mask on boards in this class is suitable for high performance commercial and industrial products in which extended performance life is required, but for which interrupted service is not life-threatening.

H – High Reliability/Military (includes that equipment where continued performance is critical, equipment down-time cannot be tolerated and/or the equipment is a life support item). Solder mask on boards of this class is suitable for applications where high levels of assurance are required and uninterrupted service is essential.

IPC-SM-840C divides responsibilities between materials supplier, board fabricator and board user. The user’s task is to “monitor the acceptability and functionality of the completed boards”. Note particularly that IPC-SM-840C specifically does not “determine the compatibility of solder mask materials with post-soldering products and processes” – if you want to conformal coat the final product, it becomes your responsibility to check for compatibility.

When you discuss the materials and requirements for solder resist with your fabricator, you will have to take into account a number of factors:

Layout

Figure 2 shows in schematic a typical solder mask application. Notice the very definite three-dimensional nature of the board surface

Example of resist side walls

Example of resist side walls

It is important that:

Exposed tracks due to over-size resist aperture

Exposed tracks due to over-size resist aperture

Figure 2: Solder resist on a printed wiring board

Figure 2: Solder resist on a printed wiring board

Solder mask artwork should be oversized (a minimum of 75µm is suggested) to allow for misregistration or slump, which can obscure or contaminate a pad surface, causing excessive solder balls or defective joints.

Excessively thick solder mask can cause drawbridging where used between component pads, and omitting mask between chip component pads is sometimes recommended to reduce this effect, and also make cleaning easier. This of course is not possible when tracks are run in the spaces between pads. The two different types of solder mask window needed are shown in Figure 3, from which it is clear that pocket windows need solder mask that is much better defined and in register than gang windows.

Figure 3: Solder mask window types

Figure 3: Solder mask window types

The gang solder mask window will normally be able to be manufactured by screen printing, using a 0.38 mm spacing; however, the pocket solder mask type will require clearances of 75–125 µm, and must therefore be made with photoimageable resist.

By the way, don’t assume that putting fingers of resist between pads on an otherwise bare board might help overcome bridging, because most bridging occurs between component leads above the board. You are just adding to the cost by using a more expensive process, and gaining little.

IPC-2221 Generic Standard on Printed Board Design and IPC-SM-782 Surface Mount Design and Land Pattern Standard contain a number of recommendations and comments on other issues, some of which are summarised below:

Self Assessment Questions

What aspects of solder mask design will impact on the manufacturing costs and quality of the assembly you are laying out?

Compare your answer with this one


Temporary solder resist

There are a number of wave-soldered designs when one needs to prevent solder pickup on parts of the board. For example:

This requirement is commonly met by specifying temporary solder masks, which mask off areas of tracking and pads from the solder wave, although physical barriers or custom pallets are often used, and there is an increasing number of selective soldering solutions.

Temporary solder mask approaches

There are three approaches to providing a temporary solder mask:

  1. Soluble coatings. These are usually applied by painting or dabbing onto the surfaces to be protected. The materials can be selected to be water washable or solvent soluble, so that they are able to be removed during the cleaning process.
  2. Peelable tapes, such as self-adhesive polyimide materials, which are pressed into position. These can be applied manually or by machine, and by the board fabricator or the assembler. Often used for protecting gold fingers, they are most useful when the areas to be masked are few and rectangular.
  3. Peelable liquid masks, which are cured after they have been applied, typically by screen printing at the board fabricator, or by dispensing at the assembler. The majority of peelable masks are latex-based, but the range also includes vinyl, silicone and urethane based chemistries.

Note that board fabricators sometimes use peelable solder masks as part of their internal processing. Examples are: in Hot Air Solder Levelling, to keep solder away from areas such as gold contacts; in selective plating applications, such as protecting gold electroplated areas during electroless gold plating.

Temporary solder mask issues

Peelable masks should peel off easily from the surface and the holes, but there is often a problem in achieving complete removal: a dotted line of residual mask may be seen around the perimeter of the peeled area; material may be left in via holes.

Removing peelable solder mask

Removing peelable solder mask

The nature of such adhesive residue is critical: it must either not be detrimental to circuit function, or else be dissolved completely in a cleaning process. Inspectors are rightly suspicious of residues in plated holes: even if non-conductive, they may trap flux and contamination during subsequent assembly. The greatest problems are seen with liquid masks, as these penetrate the holes and vias to give a mechanical key.

Unfortunately, some resins can become very difficult to remove once they have been subjected to reflow temperatures, so it is important to make sure that the mask material has been designed to cope with the double soldering process. Improved results have also been reported from buying boards with the peelable mask a little under-polymerised, so that it reached the right physical properties only after assembly.

An alternative is for the assembly house to apply a selective mask post-reflow/pre-wave. This can be done by hand, or in volume by using selective coating equipment (dispensing or spray).

Peelable masks vary widely in their ionic properties and flux absorption characteristics, but should always be removed either before or during the cleaning operation. Not only may the mask itself degrade, but flux tends to become trapped underneath. Whether the resulting contamination originates from the mask or the flux, corrosion during life may be the outcome.

Quote

“A point I would like to make is that people don’t pay much attention to materials that are not part of the final assembly. Why worry about the latex mask? I’m just going to throw it away anyway. Well, they can have detrimental effects. The same thing applies to other ‘temporary’ materials like water soluble solder mask, water soluble temporary spacers, and water soluble tapes. Just keep in mind that every material has an effect, and every material has to go somewhere.”

Doug Pauls on TechNet, 23 August 2000

Self Assessment Questions

You are designing a wave-soldered product that contains a small number of non-wets. What are the assembly process options for keeping the through-holes clear? Identify and discuss any options for which you will need to involve your fabricator.

Compare your answer with this one