Micro Joining and Assembly Technology -Metal mask cleaning-

Introduction

Application of solder paste in electronics packaging can be divided into dispensing, stencil printing, and pin transfer.¹⁾Stencil printing is the most popularly used among them because of its highly productive efficiency. Recent trend of high-density packaging with finer pitches of circuit patterns and downsizing electronic components, namely the installation of 0603 chip or 0402 chip, places new demands that stencil openings in printing process become ever thinner and smaller. Cases increased with this trend in which residual solder paste and small solder balls on the aperture edges cause trouble in printing or packaging process, once again highlighting the importance of stencil cleaning.
In the first half, we will describe requirements for stencil cleaner and features of their each existing type. In the latter half, we will move on to explain cleaning methods that are commonly used in stencil cleaning.

Object

Object of cleaning is solder paste residue on repeatedly used stencils. Stencils must be cleaned not only after everyday operation but also after certain times they go through the printing process, removing the solder paste accumulated on the openings thus to maintain the quality in printing and of the stencils themselves（Fig. 6.1.1）.

The solder paste in use has improved its performance; its thixotropic agent shifted to that of higher melting point in order to prevent heat slump as it goes lead-free, and its activator demonstrating better wettability and stability in reflow process. Moreover, even better in their performance and increased in the amount, the thixo agent and activator adjusted to the recent stream of halogen-free and finer stencil pitches in often cases make cleaning much more difficult. Therefore one should reexamine the performance of the cleaner when switched to the newer types of solder paste even if no trouble has occurred in removing the residue in the conventional way.

Purpose

The purpose of removing solder paste from stencils is to avoid the following problems that may occur during the printing process;

i. Inappropriate application of solder paste
Clogged apertures due to dried out solder paste cause insufficient application or of too much with dried paste being mixed into, making it unable to apply the proper amount of solder paste.

ii. Unnecessary application of solder paste
Clogged solder paste attached underneath the stencil (exposed to the substrate) may spread to end up causing soldering defects such as bridging.

iii. Contamination of lead into lead-free solder paste
If not properly cleaned, residual lead may contaminate lead-free solder paste even after the lead-free kind is installed.

Requirements

In order to prevent the above listed problems, stencil cleaner should meet the following requirements and conditions;

i. High dissolvability of solder paste
Some manufacturers hand-wipe clean the stencils using brushes and rags with Isopropyl alcohol (hereafter IPA). In such cases stencils are seemingly cleaned because of the physical impact of hand wiping, even though IPA has low dissolvability of solder paste. However, with finer and smaller stencil apertures, the physical force becomes inadequate for covering the low dissolvability of the cleaner, causing inconsistent cleaning performance namely the solder paste residue on stencil openings.
The same can be said in cases when cleaned in cleaning equipment. Low soluble cleaner may deteriorate the cleaning performance or take longer cleaning time than necessary.

ii. Low impact on components
Stencil is a stainless or nickel plated metal tightly taped with a metal frame made of aluminum and such via combination.²⁾ Except when the stencils are washed in cleaning equipment where only the openings are exposed to the cleaner, the cleaner is supposed to meet the contradictory conditions of high dissolubility and low impact on the combination or on the adhesive layer between frame and fixing tape. When the cleaner affects the adhesive layer between frame and fixing tape, deteriorated adhesiveness causes separation of the stencil or printing defects with lowered tension of the stencil.

iii. Human and environmental friendliness
As cleaner should be human- and eco-friendly as possible while satisfying the above-mentioned conditions, each chemical composing the cleaner other than water has a smell unique to its own. Although in general the working environment prefers cleaning chemicals with no or less smell, they may not necessarily be better for human or the environment. One should reconsider using especially those with no smell in terms of safety.

iv. Inapplicability to the related laws
Concerned parties shall adhere to the related laws including Ordinance on Prevention of Organic Solvent Poisoning in accordance with the Enforcement Order of the Industrial Safety and Health Law (hereinafter the Ordinance) and Law concerning Pollutant Release and Transfer Register (hereinafter PRTR Law) when applicable. The Ordinance requirements such as installation of local exhaust ventilation put additional costs to that of cleaning itself. Thus non-applicable cleaners are better in terms of cost effectiveness.

v. Total cost effectiveness
Total cost should be evaluated based on the liquid exchange frequency and recyclability considering the lifetime of the cleaner, determined by lower dissovability due to contamination, in addition to the price of the cleaner.

Classification^3）

Stencil cleaner can be classified into following categories;

i. Glycol ether-based

Most widely used is glycol ether based stencil cleaner.
Fig6.1.2shows the structural formula of glycol and glycol ethers. These chemicals contain both lipophilic alkyl, lipotropic ether structure and hydroxyl group in their molecules. They have appropriate dissolvability in both nonpolar (lipophilic) compounds including rosin in soldering flux and polar (lipotropic) compounds of activator etc.

Generally speaking, ethylene glycol is widely used because of its lesser smell among others. However, some kinds of ethylene glycol with a low boiling point are strictly controlled in terms of their allowable concentration or under PRTR Law due to their possible reproductive toxicity and teratogecity in metabolism⁴⁾ (Table 6.1.1). Propylene glycol has increased demands from the market despite of its slightly stronger smell, for its lesser reproductive toxicity and teratogecity compared to ethylene glycol. Dialkyl glycol acts as solubilizer in spite of its higher dissolvability among the three and low smell, since it is rather expensive.⁵⁾

Primarily the Fire Services Act categorizes glycol ether-based cleaners as hazardous because of their having a flash point, yet those with a certain ratio of water canceling the flash point can be classified as nonhazardous.

Many of those classified as hazardous dry relatively fast with the boiling point of 120-170 °C (248-338 °F) and can be used solely.

The nonhazardous cleaners that have no flash point because of the added water have a larger proportion of chemicals with a high boiling point, making them dry at a slower rate. Therefore they require adequate rinsing in order to dry out. Cleaning chemicals compounds with a high boiling point remains without volatilization in an adhesive layer of the stencil frame, undermining the adhesiveness to cause separation of the frame. Although some cleaners with chemicals with a low boiling point do not require rinsing, they have less dissolubility since they contain a good deal of water to set off the flash point. It is important to control the water concentration of the cleaners as even the nonhazardous may be classified as hazardous under the Fire Services Act once the water vaporizes to uncancel the flash point.⁶⁾

Fig6.1.1　"Glycol Ether Based Solvents with Strict Concentration Control in the Working Environment"

Chemical & Structural Formula	Alias	Allowable Concentration stated by Japan Industrial Association (2010 Ed.)	TWA* (2010 Ed.)	PRTR Law	Ordinance
Ethylene glycol monomethyl ether Ch2－O－CH2－CH2－OH	2-Methoxyethanol, Methyl cellosolve	0.1ppm	0.1ppm	Type 1 Designated Chemical Compounds	Type 2 Organic Solvent
Ethylene glycol monoethyl ether C2H5－O－CH2－CH2－OH	2-Ethoxyethanol, Cellosolve	5ppm	5ppm	Type 1 Designated Chemical Compounds	Type 2 Organic Solvent
Ethylene glycol monomethyl ether acetate Ch2－O－CH2－CH2－O－CO－Ch2	2-Methoxy ethyl acetate, Methyl glycol acetate	0.1ppm	0.1ppm	Type 1 Designated Chemical Compounds	N/A
Ethylene glycol monoethyl ether acetate C2H5－O－CH2－CH2－O－CO－Ch2	2-Ethoxyy ethyl acetate, Cellosolve acetate	5ppm	5ppm	Type 1 Designated Chemical Compounds	Type 2 Organic Solvent
Ethylene glycol monobutyl ether C4H9－O－CH2－CH2－OH	2-Butoxy ethanol, Butyl cellosolve	N/A	20ppm	N/A	Type 2 Organic Solvent

*8-hour Time Weighted Averages (TWA) are an average value of exposure over the course of an 8 hour work shift.
A worker may be exposed to a level higher than the TWA for part of the day as long as he is exposed to levels below the TWA for the rest of the day.
(Recommended by The American Conference of Governmental Industrial Hygienists)

Cleaning Method

Stencils can be cleaned hand-wiped with highly volatile cleaners such as IPA or with manual or automated cleaning equipment near printing process of packaging line.

Although hand wiping is inexpensive and easy to be done, it is highly problematic both in terms of cleaning performance and working environmental safety.

As for the cleaning performance, it is hard to completely clean spots namely micro aperture edges by hand, resulting in quality discrepancy largely depending on who to clean them. Moreover, hand-wiping may damage the shape of apertures or the stencil surface. Besides, it may cause working environmental problems since workers must operate in the chemical-filled atmosphere.

Some users who want cleaning performance without bringing in gigantic cleaners adopt cleaning spray, which cleans the stencils with chemical dissolvability and pressure of the spray, or small-sized handy ultrasonic cleaners (Fig 6.1.3)

As for the cleaning equipment, the market has several kinds depending on the cleaning methods; shower, soak into the cleaner and irradiate ultrasonic, and directly irradiate ultrasonic. Cleaning equipment must be able to provide a high cleaning performance while being eco-friendly and cost effective. The less cleaner the better since it cannot fulfill these conditions if a large amount of cleaner is necessary. Also the longer cleaning time causes more volatilization of the cleaner and harsher impacts on the frame due to longer exposure to the cleaning product.

Thus cleaning equipment must be designed to enable fast cleaning with a little amount of cleaner.

The mechanism of cleaning differentiates in each method. The most suitable cleaner must be chosen for the particular cleaning equipment. Followings are the representative cleaning methods;

iii. Non-immerseble ultrasonic cleaning (Fig 6.1.4)⁷⁾

In this method cleaner acts as a liquid membrane as applied on the front and back of the stencils, transmitting ultrasonic onto the stencils. The amount of cleaner can be much less for it is only necessary for covering the stencil surface. This method of direct irradiation offsets the influence of distance on ultrasonic that its effectiveness weakens in inverse proportion to distance. Thus less than half the electricity brings about the same effect as in the immerseble ultrasonic cleaning. Moreover, it has other advantages including less impact on the stencils, less consumption of the cleaner and electricity, and less running costs.

Drying Method

Air blowing is popular in drying after cleaning or rinsing. Emission volume of volatile organic compounds is of the issue since much of the stencil cleaner compounds have the boiling point below 200°C while emissions are released via local exhaust. Cleaners should curtail volatile organic compounds, along with their emission control regarding the equipment.

Conclusion

Stencil cleaning asks for inclusive consideration not only on the cleaning performance of the subjected solder paste but also on the impacts on the stencils as well as the environment, and stencil quality after cleaning. It is not hard to imagine that higher cleaning performance along with finer pitches and less environmental burden will be in greater demand. One should considerately choose appropriate cleaner and cleaning equipment in order to meet these requirements.

Reference

1）Micro-Soldering Education Division, Japan Welding Engineering Society, Standard Micro-Soldering Technology 3rd ed. (Business and Technology Daily News, 2011), 114.
2）Japan Institute of Electronics Packaging, Dictionary of Electronics Packaging Technology (Kogyo Chosakai Publishing Co., Ltd., 2009), 609.
3）Japan Industrial Conference on Cleaning, Industrial Cleaning Checklist 2009 ed. (Japan Industrial Conference, 2009)
4）Kasuke Nagano et al, “Testicular Atrophy of Mice Induced by Ethylene Glycol Mono Alkyl Ethers,” Journal of Occupational Health (1979), 21. 29-35
5）Shigeo Hori, Ready-to-Use Cleaning Technology (Kogyo Chosakai Publishing Co., Ltd., 2001), 273.
6）Nahoko Imori, Fabricated Materials for Electronic Components April ed. (2000), 105.
7）Akio Inoue, Basics and Application of Electronics High Quality Screen Printing (CMC Publishing, 2011), 175.