Lead-free process for SMT

 

In recent years, efforts to develop alternatives to lead-bearing alloys used in the electronics assembly have increased significantly. These efforts had begun as a response to the international legislation banning or restricting the intentional use of the lead (Pb) in electrical and electronics devices. In the European community, the restriction on Hazardous Substances (RoHS) directive will become effective on July 1, 2006 while in Japan take-back legislation for a variety of domestic product is in effect since 2001. The European RoHS legislation could represent to original Electronics Manufacturers (OEMs) and Electronic Manufacturers Services (EMS) provides a barrier for international commercialization if a lead-free manufacturing process is not developed, qualified and implemented in timely manner.

 

Approach

 

Lead-free Assembly Process Assessment

One of the biggest concerns regarding assembling large, dense, high complexity PCAs using lead-free solder was the potential incompatibility of the current equipment set with the new processing technology. Currently most of the microelectronic industry support the SnAgCu family of alloys to replace Sn-Pb solders. Although, the SAC family of alloys has many positive attributes, there are several concerns related to their high melting temperature ranging form 217⁰C to 224⁰C. This represents an increase of about 34 to 41⁰C just the melting temperature compared to the eutectic Sn-Pb alloy

 

The assembly process followed is described on Figure 1.

 

Figure 1: Double-sided SMT Assembly Process Flow

 

Only SnAgCu lead-free interconnect material was introduced in the process. All process specifications remained unchanged with the exception of the reflow profile, which was modified to accommodate the requirements of the new solder alloy. The units were assembled and inspected at every station along the process. Current process parameters were used at the stencil printing. A no-clean lead-free Type III solder paste was used on this operation. Solder paste brick definition was monitored particularly on 0.020

in. pitch components and compared against the same land-pattern design using Sn-Pb solder paste. No significant differences were observed as shown on Figure 2.

Figure 2: Definition of a SnAgCu No-Clean Solder Paste Brick

 

Automated optical inspection Equipment was used to inspect the components after the placement process. No defects were observed. Reflow profile was developed taking into consideration the properties of the new interconnect Material as well as the complexity of the PCBs. Following the recommendations provided by the Solder paste supplier, a linear heating ramp up to a peak temperature of 250°C monitored at the joint of The most thermally challenged component on the board was used to develop the thermal profile. A

dwell time of 75 sec. over the alloy melting temperature was achieved. The characteristic curve Of the thermal profile is described on Figure 3.

 

Figure 3: Lead-Free Thermal Profile

 

The boards were reflowed using a ten heating zone oven, and then visually inspected. Excessive

Amounts of flux residue and some level of discoloration on plastic packages were observed.

The solder joint appearance was to an extent different from the Sn-Pb joints. The new joints looked dull and rough. It was also noticed that the wetting ability of the new solder paste was Significantly reduced as shown on Figure 4.

Figure 4: Lead-free Gull-Wing Solder Joint

 

The results obtained from the initial investigation established that current SMT equipment sets can be

Used to assemble medium and high complexity PCAs without compromising their short-term integrity.

 

Adopted :

http://www.efsot-europe.info/servlet/is/251/19%20EddieHernandez-Sosa.pdf?command=downloadContent&filename=19%20EddieHernandez-Sosa.pdf

 

:: Quoted by Next-Star, 18^th September 2009.